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In Search of El Dorado: John Dalton and the Origins of the Atomic Theory

Posted on: Tuesday, 10 May 2005, 03:00 CDT

THE HISTORIAN OF THE CHEMICAL ATOMIC THEORY EXPERIENCES AN embarrassment of riches-or just embarrassment-when he turns to the origin of the tale. He has a hero-the English Quaker and self-made natural philosopher John Dalton-as well as a birth date for the theory: Dalton's September 6, 1803, entry in his laboratory notebook. Beyond these apparently secure fixed points lurk historiographie monsters; and sometimes the bewildered historian is led to wonder how fixed even these apparently secure points are.1 Son of a Cumbrian weaver who lived near Kendal, John Dalton (1766- 1844) was first a schoolteacher, but in 1793 was hired as a professor of natural philosophy at the dissenting New College, Manchester. In 1800 he resigned his position (it was about to disappear, anyway, due to financial problems at the college), and thereafter earned his living in Manchester as a private lecturer and tutor. His most famous contribution was the atomic theory of matter.

In what follows I review the diverse accounts of Dalton's conceptual route to atoms, and attempt to clear a pathway through the morass. With so many sharply differing stories, virtually all derived from Dalton himself, there is little likelihood of resolving all the anomalies. Nonetheless, there is reason to hope that one may make progress toward a probable picture of how Dalton arrived at his atomic theory. We will see that his investigative pathway often appeared to move straight ahead, but Dalton was not always moving in the direction he thought he was. As an early Dalton obituarist proclaimed, "like Columbus, who missed an El Dorado, but found an America, he discovered something better."2 Thus, on one level this is a case study of a big mistake leading to a big breakthrough, parallel to Arthur Koestler's famous depiction of Johannes Kepler's "sleepwalking" performance en route to the three laws of planetary motion. However, we will see that a close examination reveals many complications in this apparently simple plot line. The proper view is a more interesting one, comprising not only a kind of "thematic pluralism" written of by Gerald Holton, but also involving a multiplicity of circumstances and influences-both theoretical and empirical (Koestler, 1959; Holton, 1956: 340-51; 1986: 26).

We should first be clear what the stakes are. A generation ago Henry Guerlac justly called Dalton's contribution the first successful example of scientifically probing the world of the invisibly small, and he characterized this as the origin point of a "Molecular Revolution" as momentous as that of Newton (Guerlac, 1968: 70, 85). Indeed, Dalton's work called forth a vigorous international research program that has a continuous history from 1803 to the present-nanoscience long before the word became vogue. Almost from the start-we will examine the "almost" presently-that research program utterly transformed the science; moreover, it was atomistic theory that enabled chemistry, two generations after Dalton, to become the earliest example of a welldeveloped science- based theory acquiring the practical power to transform the world of technology and commerce.

Here is a brief discussion of how the theory works. Dalton knew (we use his data from the year 1810, but current definitions of the words "atom" and "molecule") that water consists of 87.5 percent oxygen and 12.5 percent hydrogen by weight; that is exactly seven times as much oxygen as hydrogen. If one assumes, with Dalton, that the invisibly small water molecule consists of an atom of hydrogen united to an atom of oxygen, then every oxygen atom must weigh seven times as much as every hydrogen atom, for under these circumstances it is obvious that the weight ratio of the atoms must match the composition of the compound. But we notice that this procedure requires us to assume a formula for water. In contrast to Dalton, Humphry Davy and Jacob Berzelius assumed (in 1812 and 1814, respectively) that the formula for water was (as we would write today) H2O rather than Dalton's HO. In this case, two atoms of hydrogen must make up the same quantity that one did in the previous exercise, so the hydrogen atom must weigh 0.5 on the scale of the oxygen atom weighing 7; or alternatively, if H = 1, then O = 14. (These are, of course, only relative weights, the absolute weights remaining fully unknown-hence the omission of any particular weight units such as grams or ounces.)

But let us not for the moment be diverted by the notions of others, and proceed further along Dalton's particular path. Taking the molecule of the previously known substance "carbonic oxide" to be CO (today we call this gas "carbon monoxide"), contemporary analytical data led Dalton to assign 5.4 as the relative weight of a carbon atom (on the scale of oxygen as 7). From early in 1803, Dalton knew of no fewer than four oxides of nitrogen. Taking one of these oxides, "nitrous gas," as NO, Dalton's data indicated a weight for nitrogen of about 5, again relative to oxygen as 7. Dalton could now cross-check some of his assumptions by taking different inferential lines to his various atomic weights. Namely, let us assume with Dalton that ammonia gas (the only then-known compound of hydrogen and nitrogen) is NH. The elemental composition of this substance indicates that there is indeed about five times as much nitrogen by weight as there is hydrogen. The same atomic weight for nitrogen had now been deduced through two separate chains of evidence. Again, on Dalton's assumption that the molecule of "olefiant gas" (today known as ethylene) is CH, the calculated weight of the carbon atom approximately matches that calculated from the oxide of carbon, obtained above. The cross-checks worked, and so gave Dalton greater confidence that he was on solid ground with his assumptions regarding formulas.

All this is reasonably clear. But historical problems arise when one inquires about under what circumstances and proximate inducements Dalton began down this path, and what gave him the courage to continue in the face of difficulties. Dalton's notebook provides only meager help. The earliest annotations like the ones just discussed appear suddenly in his laboratory notebook under the date September 6,1803, with little to indicate the context or reasoning that led him to pursue these ideas. One dominating background influence, however, was without doubt that of Isaac Newton.

NEWTON, DALTON, AND RANSOME'S TALE

In Proposition 23 of Book 2 of his masterwork Pnndpia (1687), Newton explored mathematically the properties of a hypothetical elastic fluid (that is, a gas) composed of self-repulsive stationary particles. He demonstrated that if one starts with the proposition that the gas obeys Boyle's law (that volume and pressure are inversely proportional), then it is necessary that the repulsive force between any two particles be inversely proportional to the distance between them; and conversely, that if one starts by supposing a hypothetical fluid, the particles of which repel each other by a force which is inversely proportional to distance, then the resulting gas will obey Boyle's law. He than added an important proviso: "Whether elastic fluids consist of particles that repel each other is, however, a question for physics. We have here mathematically demonstrated a property of fluids consisting of particles of this sort, so as to provide natural philosophers with the occasion to treat that question." More concretely and with less reserve, Newton averred 30 years later in the thirty-first and last "Query" of his Opticks that "All Bodies seem to be composed of hard Particles." He continued, "it seems probable to me, that God in the Beginning form'd Matter in solid, massy, hard, impenetrable, moveable Particles, of such Sizes and Figures, and with such other Properties, and in such Proportion to Space, as most conduced to the End for which he form'd them; and that these primitive Particles being Solids, are incomparably harder than any porous Bodies compounded of them; even so very hard, as never to wear or break in pieces; no ordinary Power being able to divide what God himself made one in the first Creation."3

It is well known that eighteenth-century British science was thoroughly imbued with Newtonian influence. The experimental and theoretical sides of Newton's legacy became equally central to British science at both the professional and popular levels. In the science of chemistry, many leading figures attempted to apply Newton's precepts by exploring the possible quantification of the force of chemical "affinity," but this proved an arduous and frustrating task. On the popular level, Newtonian science was effectively disseminated in Britain by textbooks and other works of popular science, by tracts of natural theology, and by the increasingly prevalent tradition of itinerant lecturers on natural philosophy (Schofield, 1970; Thackray, 1968,1970; Guerlac, 1968).

Dalton was heir to these more diffuse Newtonian traditions, but with a north-country Quaker twist to the story. The Cumbrian Quakers were notable for the high value they placed on reading, mathematics, natural philosophy, and other intellectual pursuits. "To all appearances," wrote Robert Angus Smith a do\zen years after Dalton's death, "he was like those around him, born to be a clodhopper," but appearances were deceiving; Dalton "was a student of nature from his cradle" (Smith, 1856: 4). Moreover, although west Cumberland was surely a backwater in most respects, the local community of Friends was well integrated with more cosmopolitan Quaker centers, and Dalton made good use of such connections. Dalton was interested above all in meteorology and the physics of gases, and was late coming to the science of chemistry. His first book, Meteorological Observations and Essays (1793), was written largely in ignorance of chemistry, his first careful introduction to the subject apparently little predating his own teaching of it at New College Manchester.

Meteorological Observations and Essays reveals Dalton's habits of mind. As historians of science have amply demonstrated, it was commonly and usually implicitly assumed by eighteenth-century natural philosophers that substances are composed of fundamental smallest particles, that chemical combination of two substances occurs by one-to-one association of their smallest particles, and that compounds have constant and definite proportions of their constituent elements (Thackray 1966a: 37-39; Mauskopf, 1969). Following usual notions of the day, philosophers viewed gases as collections of normally stationary particles that repelled each other, creating the elasticity of an "elastic fluid." Some of the new ideas of the Meteorological Observations betray Dalton's adherence to these underlying notions.

However, few eighteenth-century chemists gave much thought to the consequences of these assumptions; on the theoretical level, as mentioned, they were much more engaged in attempting to quantify the short-range forces of chemical affinity than in playing number games with presumed atoms (Thackray, 1970: 238-44). Adhering as he did to the more popular Newtonian textbook and lecture tradition and influenced by both a materialist metaphysics and Scottish common- sense philosophy, Dalton was distant from this dominant professional chemical tradition. Therefore, when he "set to work to combine [his] atoms on paper" (Roscoe and Harden, 1896: 14), as he wrote autobiographically in 1810, Dalton never questioned his own assumptions. In the series of connected papers on the constitution of mixed gases in which he announced his first theory of gases, he mentioned no fewer than five times Newton's association of Boyle's law with the particulate force-law gas model (Dalton, 1802).

In approaching his future chemical atomic theory, the physicist Dalton had several real advantages over any professional chemist. The number of known chemical elements was rapidly increasing at this time (it reached about 33 by 1812), and this proliferation increased the suspicion of many chemists that none of these were elements in the absolute sense. By contrast, Dalton simply accepted A. L. Lavoisier's list of elements uncritically, thus unknowingly transforming Lavoisier's provisional elements into ontological ones. second, in the absence of a well-developed set of relations of elemental combining weights, it was not clear at the time what benefit for chemical theory an atomic theory could provide. Dalton, whose first foray into atomic theory was clearly motivated by physical rather than chemical reasoning, would not have been concerned by this worry.

Finally (and crucially), in Dalton's day chemistry was unable to determine what are the formulas of the compounds on the basis of which any atomic theory must rest. But Dalton the physicist unconsciously sidestepped this difficulty, deriving his formulas on the basis of a Newtonian physical conception. Namely, since Dalton thought that like atoms repel when approaching each other, he concluded that chemical combination must result in molecules that contain as few atoms as possible-for instance, he proposed that they are always binary whenever two elements were known to form only one compound, so that the formula for water must be HO rather than H2O. Without this physical conception there would have been no unambiguous route to assigning formulas-no warrant, for instance, for the claim that water was HO rather than H2O, and so no warrant for stating that the weight of an oxygen atom was 8 rather than 16 or almost any other number. In this way, this piece of popularized- strictly speaking, mistaken-Newtonian physics was, in the context of Dalton's day, a sine qua non for developing the atomic theory.

Dalton clearly wished to adopt the Newtonian mantle in a very personal sense. In his autobiographical narrative to which we previously referred (a lecture given to the Royal Institution on January 27, 1810), he wrote that "Newton had demonstrated clearly, in the 23rd Prop, of Book 2 of the Printipia, that an elastic fluid is constituted of small particles or atoms of matter, which repel each other by a force increasing in proportion as their distance diminishes." This is, of course, a very different matter from Newton's careful statement that if a hypothetical gas were thought to be constituted of such particles exerting such forces, then the gas would obey Boyle's law.

In his next Royal Institution lecture three days later, Dalton referred to Davy's more reductionist vision of elements (without mentioning his rival's name), but then stated, "From the notes I borrowed from Newton in the last lecture, this does not appear to have been his idea. Neither is it mine. I should apprehend there are a considerable number of what may be properly called elementary principles, which never can be metamorphosed, one into another, by any power we can control." Dalton was referring to Newton's statement from Query 31 cited above, but this too is a misreading of Newton's ideas.4 In a letter to Berzelius of September 20,1812, Dalton explicitly analogized himself to Newton: "The doctrine of definite proportions appears to me mysterious unless we adopt the atomic hypothesis. It appears like the mystical ratios of Kepler, which Newton so happily elucidated" (Roscoe and Harden, 1896:159).5 More superficially, Dalton took pride in his putative physical resemblance to Newton, and told a variation of a Newtonian anecdote about himself: "IfI have succeeded better than many who surround me, it has been chiefly, nay, I may say, almost solely from unwearied assiduity" (Henry, 1854: 223, 235).6

After Dalton's death his younger friend (and personal physician) Joseph Ransome wrote the biographer W. C. Henry a long letter recounting a conversation he had with Dalton 30 years previously (ca. 1820) during a day-long hike in the Lake District. Ransome wrote that Dalton had told him that "the germ of [the atomic theory] presented itself to him, when still young, before he had undertaken the special study of chemistry. It had occurred to him that if the ultimate particles of matter were hard and indestructible, as there appeared to be many possessing different qualities, recognized as simple elements by chemists, so might they vary in size or weight or in both. At this stage of our conversation he illustrated the subject by picking up from the road a piece of limestone, of about a pound weight...." Dalton then engaged his younger friend in Socratic dialogue, leading him in his imagination to the ultimate atoms of the stone. He explained that he chose binary compounds such as water as the basis on which to build his theory, and supposed that such compounds are formed by one-to-one association of their elements. "He concluded with a few remarks on the ternary compounds, and alluded to the peroxide of hydrogen as one; pointing out that in these the principle of MULTIPLE PROPORTIONS existed, for as he said, with great navet, THOU KNOWS IT MUST BE SO, for no man can split an atom.'"7

Henry remarked at this point, in a footnote to Ransome's anecdote, "This is a striking illustration of the a priori tendencies of Dalton's mind"; in another place he described those tendencies as "exclusively to meditation and abstract reasoning" (Henry, 1854: 222n, 235). Both judgments are generally just, though require some qualification. Dalton was not a sophisticated mathematician-he never learned calculus, for example-but he was exceedingly clever and adept with figures and mathematical ideas, and always strove to apply those ideas to nature. He also had an extraordinary pictorial imagination, and was easily able to combine his mathematical manipulations with his mental imagery. Plain- spoken and plain-thinking, he adopted without qualm-really, apparently, without thought-what he erroneously took to be Newton's microcosmology: hard, irreducible, spherical atoms, a characteristic kind of atom with a characteristic weight for each chemical element. He was probably drawn to studies of gases at least partly because this approach seemed to reveal the properties of matter in the most primitive state, the atoms or molecules of a gas repelling each other by some version of Newton's inverse force law.

Important empirical elements of what later became atomic theory long predated Dalton. Broad, widely shared assumptions about the presumed atoms of substances have already been mentioned. L. J. Proust had explicitly propounded the law of definite proportions. Carl Wenzel and J. B. Richter had established acid-base equivalents. Bryan and William Higgins had come so close to atomic theory that after 1810 the latter mounted an unsuccessful priority claim against Dalton (a claim that Davy to some degree defended). However, none of these figures had quite the mental apparatus of Dalton. Smith justly wrote,

We find no scientific man holding the idea [of atoms] with such firmness; to others it was a theory, to Dalton it was a fact, which he could not conceive otherwise.... We appear to be entirely removed from the region of speculation when reading his words; although he leads us farther than the most fant\astic speculator had done, the road is made so clear before us, that we find no difficulty, either physical or metaphysical.... Some persons would apply to [his descriptions] the word material, and still more the word mechanical. It was by following rigidly the mechanical properties of his atoms that he arrived at his results. To those who read his works, it will be clear that his mind became gradually more confirmed in this course (Smith, 1856:231-32).

So far this seems to be smooth sailing, historically speaking. From this evidence, Dalton appears to have pondered chemical atoms long before he uncovered any direct evidence for them (curiously, the Utopian industrialist Robert Owen is another witness for this chronology).8 Applying elements of Newtonian microcosmology and a Newtonian mathematical proposition on gases, both of which he had misunderstood-making them incorrect in Newton's own terms-led him nonetheless to one of the greatest breakthroughs in the history of science.

But the sailing turns out to be not nearly so smooth as we thought, for Dalton told other narratives about how he had arrived at chemical atoms. We need to examine these alternative origins before proceeding further, and we need to do this with some care. Only then will we be in a position to provide a moral to our story.

OTHER ORIGIN STORIES: INDUCTIVE ROUTES

If one compares the compositions of olefiant gas (CH in Dalton's terms) with "carburetted hydrogen" (marsh gas, today's methane) by inquiring in each case how much hydrogen combines with a given weight of carbon, one may observe an integral multiple proportion: just twice as much hydrogen is found in marsh gas than in olefiant gas, holding the carbon weight constant. This striking result is masked if one simply compares the percentage compositions of the two substances. This fact also can be adduced as evidence for an atomic theory of matter: if the molecule of olefiant gas is CH, then the marsh gas molecule will be the "ternary" (three-atom) molecule CH^sub 2^, and the analytical data then make perfect sense.

The Glasgow chemistry professor Thomas Thomson, who was the earliest proponent of Dalton's atomic theory, first became acquainted with Dalton, and Dalton's theory, during a visit to Manchester on August 26,1804. In his History of Chemistry (1831), he stated that on this occasion "Mr. Dalton informed me that the atomic theory first occurred to him during his investigation of olefiant gas and carburetted hydrogen gases, at that time imperfectly understood...." In 1825, Thomson made a similar but somewhat more qualified statement about the origin of Dalton's ideas. Thomson was in effect asserting that the empirical discovery of integral multiple proportions in the two known hydrocarbons inductively called forth an explanation, Dalton's atomic theory (Thomson, 1830- 31, vol. 2: 289-92; 1825, vol. 2: 9-11).

But this account cannot be literally accurate. It is known that Dalton did not investigate the hydrocarbons until just three weeks before his first meeting with Thomson (Roscoe and Harden, 1896: 62- 63; Thackray, 1966a: 43; 1972: 80), while his own laboratory notebook attests that the theory existed as early as September 1803. It would seem that Dalton's investigation of the hydrocarbons was a case of seeking further support for a theory already in existence.

But perhaps there was another instance of integral multiple proportions that better fit the inductive path for the origin of the theory? Humphry Davy, who published an analysis of three different oxides of nitrogen in 1800, later wrote: "It is difficult to say how [Dalton] gained his first notion of atoms, but I strongly suspect that [my published analyses], and perhaps Cruikshank's discovery of gaseous oxide of carbon gave him his first ideas. He always referred to those labours in his early papers, but afterwards seems to have forgotten them" (Henry, 1854: 217).9 Davy's suspicion of the origin of the theory in the nitrogen oxides is supported by Thomas Thomson's later flat statement (apparently contradicting his testimony cited above) that "Mr. Dalton founded his theory on the analyses of two gases, namely, protoxide and deutoxide of azote [that is, nitrous oxide and nitrous gas]..." (Thomson, 1850:140).

This origin story is also supported, apparently, by some of Dalton's own retrospective accounts. In 1811, Dalton wrote, "I remember the strong impression which at a very early period of these inquiries was made by observing the proportion of oxygen to azote as 1, 2, and 3 [sic, for 4], in nitrous oxide, nitrous gas, and nitric acid, according to the experiments of Davy."10 A manuscript of a lecture that Dalton presented to the Literary and Philosophical Society of Manchester on October 15, 1830, contains the remark, "So far back as the year 1803 I had resolved in my mind the various combinations then known of azote & oxygen, & had determined almost without doubts, that nitrous gas is a binary compound and nitrous oxide a ternary...." And in another lecture five years later, Dalton stated, "Nitrous oxide is composed of two particles of azote and one of oxygen. This was one of my earliest atoms. I determined it in 1803, after long and patient consideration and reasoning."11

Dalton's "long and patient consideration and reasoning" may be followed in the pages of his published papers and of his laboratory notebook in the days and months preceding September 6, 1803, but only with limitations. The limitation connected with his published papers is the circumstance that the relevant papers appeared in the Memoirs of the Literary and Philosophical Society of Manchester (hereafter Manchester Memoirs and "Lit & Phil") with a two- or three- year delay between oral presentation and publication. Dalton was known occasionally to update the text of his articles in the interim, so it is not known precisely what a paper may have contained on the date it was first read. The limitations connected with his notebook include Dalton's casual dating of pages and the unreliable chronological sequencing of pages. More importantly, the notebook, along with about 75 percent of all the Dalton papers held by the Lit & Phil, was destroyed as the result of an air raid on December 24,1940; as a consequence, the only records of the notebook that survive are (fortunately in some cases rather detailed) descriptions of it in the work of Roscoe and Harden.

In a paper read on November 12, 1802 (published November 1805), Dalton reported experiments that showed that nitrous gas reacts with the oxygen in common air in two different proportions, depending on reaction conditions, and that these two proportions bear an integral relation, namely two to one. The "theory of the process," then, is that "the elements of oxygen may combine with a certain portion of nitrous gas, or with twice that portion, but with no intermediate quantity" (Dalton, 1805a: 249-50). The exact data cited in this paper are found in the notebook, on a page that was written probably between October 10 and November 13,1803, so the published paper must have been altered after it was presented. It is just as clear that "the theory" and mention of an exact integral multiple refer to chemical atoms, but it is unlikely that these words occurred in the paper as first read (Notebook, I, 305 in Roscoe and Harden, 1896: 35). Therefore, from this evidence alone we can trace Dalton's understanding of multiple proportions in the nitrogen oxides only back to October 1803 at the earliest, which postdates his atomistic notebook notations of September 6. This nitrogen oxide work, then, is still the context of confirmation, not the context of discovery.

However, there is an earlier passage in Dalton's notebook that suggests that Dalton may have recognized multiple proportions in the nitrogen oxides before September 6. Roscoe and Harden cite an entry for August 4, 1803: "It appears, too, that a very rapid mixture of equal parts com. air and nitrous gas [above water], gives 112 or 120 residuum. Consequently that oxygen joins to nit. gas sometimes 1.7 to 1, and at other times 3.4 to 1" (Notebook, 1,132 in Roscoe and Harden, 1896: 38, 58-59). Dalton already had the figure of 1.7 nitrous gas to 1 oxygen by volume, which forms nitric acid. The cited figure of 3.4 nitrous gas to 1 oxygen to form nitrous acid emerges from just one of the two stated results (112 unreacted parts by volume remaining), and this circumstance suggests that Dalton was seeking or expecting an integral multiple. Such apparently deliberate focus on integral multiple proportions does not appear in his first annotations regarding the nitrogen oxides in March 1803 (Notebook 1,122, dated March 21,1803, in Roscoe and Harden, 1896: 34, 57). These indications might enable us to date Dalton's "great idea" to the summer of that year, if, as this line of thought suggests, it was inductively prompted by his chemical experiments on nitrogen oxides. But the story gets more complicated as we look into it further.

OTHER ORIGIN STORIES: DEDUCTIVE ROUTES

Retrospectively associated with the discovery of integral multiple combining proportions is the even earlier discovery of equivalent proportions by the obscure German chemist J. B. Richter. Indirect evidence suggests that Dalton twice implied that Richter's work had led him to the atomic theory, but there is compelling reason to discount this claim. The evidence, pro and con, was first discussed by Dalton's disciple and early obituarist, W. C. Henry. In the end, Henry concluded that Dalton's theory grew not from inspiration from Richter, but deductively out of "a general physical conception from the study of matter in the ariform condition" (Henry 1854: 62-63, 84-86; Smeaton, 1977,1978).

Henry came to this conclusion by doing the obvious: he examined Dalton's investigative pathway leading up to his first atomic weight calculations by reading Dalton's published papers in ch\ronological order. He also had the advantage of communicating directly with George Wilson, the author of what Henry characterized as "beyond comparison the ablest and justest appreciation that has yet appeared of Dalton's philosophical character and discoveries" (Henry, 1854: x- xi). In a lengthy 1845 obituary, Wilson had argued emphatically and well that Dalton's theory arose "in the course of a purely physical inquiry into certain of the properties of a single class of bodies, the gases.. ." (Wilson, 1862: 333).

Dalton was an informally educated natural philosopher whose primary interest throughout life was meteorology and the physics of gases.12 One of the earliest conundrums he sought to solve was the question of the constant composition of the atmosphere, namely, how the four gases that were then known to make up common air (nitrogen, oxygen, carbonic acid, and water vapor) remain so thoroughly and continuously mixed. The prevailing scientific opinion of the day was that these gases were, in effect, dissolved in each other. That presupposes a chemical interaction between the gases. Dalton found reason to oppose this view, and suggested that there must instead be some purely physical or mechanical reason for the homogeneity of the air (Dalton, 1793:132-35).

In 1801 he proposed one. The particles of any gas, he wrote, repel their own kind, but have no interaction with the particles of any other gas-that is, oxygen particles repel each other, as do nitrogen particles, but there is no repulsion or attraction between an oxygen and a nitrogen particle (Dalton, 1802). It is obvious that such a supposition would result in homogeneity in any gaseous mixture. It amounts essentially to what we know today as the law of partial pressures, even though the generally accepted model of a gas was substantially altered after about 1860.

Dalton's theory of mixed gases did have some testable consequences, and he spent the next few years attempting to defend and develop the idea. One of the central issues was carbonic acid gas, an atmospheric constituent that is substantially soluble in water. Dalton found that the amount that dissolves in water is always proportional to its gaseous pressure. His friend William Henry, who had at first been opposed to Dalton's theory, further found that in any mixture of gases the solubility of one component of the mixture was determined by the partial pressure of that single component. No amount of oxygen, for instance, could keep any carbonic acid in solution, and vice versa. The mechanical character of this phenomenon seemed to be a perfect analog, if not a direct consequence, of Dalton's theory of mixed gases, and thus provided important support for it.13

However, if the solubility of a gas is purely a physical or mechanical phenomenon, how is it that different gases display different solubilities? In a paper read to the Lit & Phil in the fall of 1803, Dalton posed this question, and then wrote: "I am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the several gases: those whose particles are lightest and single being least absorbable and the others more according as they increase in weight and complexity. An enquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new: I have lately been prosecuting this enquiry with remarkable success. The principle cannot be entered upon in this paper; but I shall just subjoin the results, as far as they appear to be ascertained by my experiments." There follows the first published table of relative atomic weights of six elements, along with molecular weights (implying their assumed respective formulas) of fifteen compounds. It was the public debut of the atomic theory, but devoid of the reasoning that led him to those particular numbers (Dalton, 1805b: 286).

This paper was read publicly on October 21,1803, six weeks after the date of his first atomistic notations in his notebook, but it did not appear in print until November 1805.14 Some time in that two- year interval (probably in October 1805) Dalton took the trouble to emend two of his atomic weight values, to add two new compounds to the original list, and to provide a new footnote stating that: "Subsequent experience renders this conjecture [of a solubility/ complexity correlation] less probable." But having made these alterations, he chose not to elaborate further on his cryptic reference to an "entirely new enquiry" into the relative weights of the ultimate atoms; that is, he did not tell the reader his means of deciding on these weights.

It is now clear why so many Dalton biographers, including two of the earliest (Wilson and Henry), have concluded that Dalton was led to his chemical atomic theory deductively from physical concerns, especially the physics of gas solubilities. We have just seen that Dalton himself stated this explicitly, in the very paper that first announced to the world a set of relative atomic weights. But once again Dalton had a trick up his sleeve.

In December and January of 1809-10, Dalton presented a course of 20 lectures at the Royal Institution in London. Roscoe and Harden reproduced the verbatim text of the last six of these, from a notebook in Dalton's hand, since destroyed. Lecture 17, entitled "Chemical Elements" and read January 27,1810, contains a first- person account of the genesis of his atomic theory. Dalton described the line of thought that led to his 1801 theory of mixed gases, but omitted any mention of gas solubilities or of his 1803/05 paper discussing the subject and introducing atomic weights. Instead, he simply emphasized that he was never able to eliminate a disturbing improbability in his 1801 theory of mixed gases. Why should there be a different repulsive force inhering in each species of matter, none of them identical with the familiar repulsive force of caloric (heat)?

"Upon reconsidering this subject," he wrote, "it occurred to me that I had never contemplated the effect of difference of size in the particles of elastic fluids"-by which he meant, he quickly added, not their weights but their volumes. If the atomic volumes of gaseous elements and compounds were all different,

then on the supposition that the repulsive power is heat, no equilibrium can be established by particles of unequal sizes pressing against each other. This idea occurred to me in 1805. I soon found that the sizes of the particles of elastic fluids must be different. . . . Hence the suggestion that all gases of different kinds have a difference in the size of their atoms; and thus we arrive at the reason for that diffusion of every gas through every other gas, without calling in any other repulsive power than the well-known one of heat. . . . The different sizes of the particles of elastic fluids under like circumstances of temperature and pressure being once established, it became an object to determine the relative sizes and weights, together with the relative number of atoms entering into such combinations. ... Thus a train of investigation was laid for determining the number and weight of all chemical elementary principles which enter into any sort of combination with each other (Roscoe and Harden, 1896:14-17).

This makes eminently good sense taken in isolation, but it raises some difficult questions. Dalton seems here to locate the origin of his theory in 1805, rather than 1803 when he first wrote of atoms in his notebook and presented a paper referring to these ideas to the Lit & Phil; moreover, he provides a different rationale for the investigation than that contained in the 1803/05 paper. What are we to make of this obvious conflict in dates and origin stories derived directly from the protagonist himself?

Having published both the September 6, 1803, notebook annotations and the 1810 manuscript lecture for the first time, Roscoe and Harden had to reconcile the apparently irreconcilable. Their solution was to accept the 1810 narrative in toto, simply arguing that Dalton must have meant to write 1803 in his lecture notes rather than 1805 (Roscoe and Harden, 1896: vii-ix, 25-26, 49-51). They dismissed earlier suggestions that Dalton had derived his atomic theory inductively from purely chemical data. However, their approach does not resolve the problem that the 1803/05 paper explicitly states a route to atoms that differs from the 1810 narrative; moreover, there is no evidence that Dalton's second theory of mixed gases was devised as early as 1803, which is required by their suggestion.

Whereas Roscoe and Harden took the 1810 narrative as gospel, Nash preferred the 1803/05 gas solubility story, and buttressed his case with convincing argumentation from Roscoe and Harden's transcriptions from Dalton's notebook. As Roscoe and Harden had, Nash rejected all suggestions of an inductive chemical route from multiple proportions, and proposed that Dalton had devised his atomic theory in 1803 deductively as a limited gambit, specifically and solely to resolve his questions about gas solubilities. According to Nash, Dalton only realized the wider chemical importance of his theory following his work with the hydrocarbons and Thomson's visit, both in August 1804. Nash also dated the second theory of mixed gases to the same month, "close enough" to 1805, he thought, so as not to create too much of a conflict with the date in Dalton's 1810 narrative (Nash, 1956).

More recently, Thackray presented compelling evidence that the second theory of mixed gases did indeed derive from 1805, rather than 1803 or 1804, just as Dalton had said. Thackray's solution to the conundrum, like Nash's, was to hypothesize a two-stage process, by which Dalton created the theory in 1803 merely to solve an isolated problem in the physics of gases, and only gradually came to realize the importance for chemistry of a wider application of the theory. For Thackray, th\at realization came in 1805, whence the outline of the 1810 narrative (Thackray, 1966a: 38-47; 1972: 39-41, 61-88,107-14).

WHEN WAS THE CHEMICAL ATOMIC THEORY FIRST DEVELOPED?

So let us accept the proposition, so ably argued by Thackray, that Dalton's second theory of mixed gases was devised only in 1805. Did this theory lead, as Dalton appeared to claim five years later, to his earliest ideas and calculations regarding chemical atoms? The answer, I suggest, must be no: in this section I will argue, contra Nash and Thackray, that Dalton's first self-conscious development of the theory came in 1803, at the time of its first introduction, and that chemical development was far more than just a perfunctory exercise for a physical purpose. As an initial conjectural exercise toward resolving the obvious contradictions in the stories provided by Dalton himself, it is important to note that both the 1803 gas solubility story and the 1805 mixed gases story have a common arc, which starts from the physics of mixed gases and ends in a motivation for determining atomic and molecular weights. It is also curious that the gas solubility paper, although deriving from events in 1803, was revised and published in the same year in which Dalton stated that the mixed gas theory was devised-1805-the two events occurring possibly within a few weeks of each other.15 So the second theory of mixed gases could well have given Dalton not the initial incentive, but simply further incentive to determine atomic weights, which is possibly what he may have intended to imply in his 1810 narrative. Given their commonalities, might not both stories reflect true events, and might Dalton simply have conflated the two in his mind? This conjecture does not conflict with the views of Nash and of Thackray.

However, I want to argue that Dalton developed the theory in the fall of 1803 much more thoroughly and deliberately than either Nash or Thackray believe. There is much evidence in Dalton's notebook of his considerable interest in self-consciously creating, developing, and testing his incipient theory right from the start. On September 6 his rubric upon the first introduction of atomic ideas in his notebook was "Observations on the ultimate particles of bodies and their combinations." Five weeks later (October 12) the heading had changed to "New theory of the constitution of the ult. atoms of bodies" (my emphasis; Roscoe and Harden, 1896: 26, 45). Over the course of six weeks and several long entries in his notebook, Dalton had pursued what he clearly regarded as a new and potentially valuable theory. The theory may well have been devised initially to support his current interest in the solubility of gases, but his wording noted here suggests that he did in fact recognize the broader novelty and importance of what he was doing from the start.

We can argue this case from more than just wording. From the notebook we know that in September and October 1803, Dalton privately derived atomic weights for six elements (hydrogen, oxygen, nitrogen, sulfur, carbon, and phosphorus), and molecular weights and formulas for no fewer than 18 compounds (water, 10 oxides, 3 hydrides, 1 salt, and 3 organic substances).16 In August 1804 he added two more atomistically analyzed compounds to the list, olefiant gas and marsh gas. When Dalton finally did publish a proper description of his theory, in the first part of his New System of Chemical Philosophy (1808), his list of elements and compounds was larger, but not radically so-20 elements (6 of which were soon thereafter shown to be compounds) and 17 compounds (Dalton, 1808, vol. 1: 211-20).

As we have seen in the first section of this paper, in order to deduce an element's atomic weight Dalton needed two things: an assumed molecular formula for one of the element's common compounds, and analytical data on that compound. Dalton could have invented formulas to his heart's content, but he must have been aware that his peers would have considered such a process a mere tissue of conjectures if he did not also adduce empirical support or tests to show that he was on the right track. Even as early as September 1803, we can tell that Dalton pursued two different kinds of tests.17 One was to examine the analytical data on known examples of multiple proportions to see whether the different proportions were integral or not. On September 6 he looked at respected contemporary analyses of the four known oxides of nitrogen, the two oxides of carbon, and the two oxides of sulfur, and was satisfied that in all three cases the multiple proportions were very close to small integers, as required by the atomic theory. He also made a first rough attempt to examine three organic substances-alcohol, ether, and sugar-to check whether the analyses of these substances could also be legitimately interpreted atomistically. On October 12 he added a fifth multiple-proportions test, the two known oxides of phosphorus. In August 1804 he successfully interpreted the two hydrocarbons-olefiant gas and marsh gas-as yet another case of integral multiple proportions. Thus, of his six multiple- proportions tests, five seemed to provide clear evidence for the atomic theory (the case of alcohol, ether, and sugar was too confused, and remained unpublished). The fact that he was mostly using analytical data that was already in the literature must have made his argument seem even stronger, since he could argue that he was making sense of already known facts, and could not be accused of cooking the data to fit his model.

In these early notebook entries one can also discern a second and more indirect kind of empirical test, which may be called "reticular" evidence.18 Dalton was able to deduce each of his atomic weights through two separate lines of inference; each line required assuming a formula, but if the numbers derived through the two independent lines agreed with each other, the assumptions at the heart of the process could be seen as more probable. Dalton derived an atomic weight for nitrogen through its hydride (ammonia), and another value separately through the oxides; the two weight determinations appeared to agree, within reasonable limits. He also derived each of the atomic weights for sulfur and phosphorus in a double fashion, through the respective hydride and through the respective two oxides. By August 1804 he could do the same double test for carbon. The oxygen-containing organic substances alcohol, ether, and sugar could have provided even a further cross-check, since they could reticulate three elements simultaneously; however, analyses of these compounds were very uncertain at that time.

The picture I am drawing of Dalton's activity in September and October 1803 is of a resourceful scientist vigorously developing what he views as an important new theory, and having some significant success at the task. But so far, all of this effort was private. On October 21, as we have seen, he presented to the Lit & Phil his paper on the absorption of gases in water, and communicated his table of 21 atomic and molecular weights with the appended comment: "An enquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new: I have lately been prosecuting this enquiry with remarkable success" (Dalton, 1805b: 286). But, as noted, he did not reveal how he had arrived at these numbers.

That Dalton was eager to communicate his atomic theory to a wider audience is suggested by the fact that in the fall of 1803 he applied to the managers of the Royal Institution of London that he be hired to give a course of 20 lectures there. His proposal was accepted, and the lectures took place between December 22,1803 and January 25, 1804.19 The (fragmentary) surviving information on these lectures suggests that they were comprehensive, covering the entire range of natural philosophy; his new atomic theory must have comprised a very small part of the whole. Nonetheless, it is clear that he did explain the new theory to the London audience. One anonymous reviewer of the lectures wrote, "Mr. Dalton seems to be of opinion that bodies always combine in the same degree of intimacy, if they combine at all, and the nature of the combination varies only when the proportions of the constituents vary. The very curious theory of atoms, which this philosopher explained last winter in his lectures in the Royal Institution, seems indeed, to lead irresistibly to this conclusion. Into this theory we do not at present enter; nor, indeed, would it be decorous to do so, as Mr Dalton has not yet thought proper to give it to the world."20 And four years later Dalton himself stated that "In 1803 [I] was gradually led to those Primary Laws which seem to obtain in regard to... chemical combinations.... A brief outline of them was first publicly given the ensuing winter in a course of Lectures on Natural Philosophy, at the Royal Institution..." (Dalton, 1808, vol. 1:v).21

It is also known that Dalton took this occasion to explain his atomic theory to the young Humphry Davy, who had been appointed professor of chemistry in the Royal Institution 18 months previously. This is the earliest expert transmission of the theory that we can document. Dalton reported on his conversations with Davy in a letter to a friend, wrote an entry in his notebook regarding Davy's nitrogen oxide analyses on the very day of his first London lecture, and reminisced about those conversations many years later. In the middle of his London sojourn, Dalton wrote that Davy was "a very agreeable and intelligent young man, and we have interesting conversations of an evening; the principal failing in his character as a philosopher is that he does not smoke." They had "frequent [conversations ... in which we] discussed the merits of the atomic [theory]." Dalton was "the more happy in this as [Davy's nitrogen oxide] results formed some of the most exc\ellent exemplifications of the [atomic] principles." Davy gave Dalton valuable counsel about how to lecture more effectively, and even coached him. Dalton was highly pleased with the apparent success of these lectures. But there is also evidence of friction between the two young natural philosophers. In 1830, Dalton reported that to Davy at that time, "the [atomic] speculation appeared rather more ingenious than important." Seven years later Davy used almost exactly the same words ("more ingenious than correct") in a letter to Jacob Berzelius.22

Dalton also later stated that while giving these lectures he prepared a brief written outline of his views on atoms, and upon his departure from London this document "was left for publication in the Journals of the Institution; but [Dalton was] not informed whether that was done" (Dalton, 1808, vol. 1: v). It was not done, and Davy may have had something to do with the failure. Davy was undeniably brilliant, but also ambitious, both scientifically and socially, and not always welcoming to those whom he considered social inferiors. Dalton's unsophisticated north-country manners and speech patterns may have led the more cosmopolitan Davy to underestimate him. But there is also no question that Davy would have been viscerally opposed to the sort of realist-materialist theory Dalton was advocating, no matter who bore the message. Davy was a reductionist, an idealist, and a romantic, oriented toward a dynamical philosophy of nature, and opposed to all subtle fluids, microscopic mechanisms, and other detritus of Enlightenment materialism. About 1807 or 1808 Davy had a change of heart about chemical atomism more carefully (positivistically and dynamically) defined, and became an enthusiastic chemical atomist himself-but he never accommodated himself to Dalton's version of the theory, which seemed to him far too material and hypothetical (Knight, 1967,1978; Rocke, 1984: 55- 61).

On his return to Manchester about February 1,1804, Dalton went back to his teaching and other activities. On February 24 and again on August 17,1804, he presented lectures to the Lit & Phil whose content is unknown, but whose titles suggest the probability that atomic theory was part of the subject matter.23 He also contracted that summer to return to the Royal Institution to present new lectures in the coming winter, though for unknown reasons this never happened. In August 1804 he carried out additional experiments and atomistic analysis of the two hydrocarbons, and explained his atomic theory to an enthusiastic Thomas Thomson. (Thomson's account of this conversation formed the basis of the first publication of some of the details of Dalton's theory, in the 1807 edition of Thomson's textbook) (Thomson, 1807, 3: 424-31; 451-52). The following April and May Dalton presented a course of formal lectures on natural philosophy in Manchester, including many details of his "original ideas on the division of matter into elements and their composition" (Thackray, 1966b: 33-38). It is about this time that Dalton began to prepare his New System of Chemical Philosophy for publication, but the book, containing a thorough discussion of his atomic theory, did not appear until 1808 and 1810.

Taking all of this together, I suggest that the pattern demonstrates Dalton's eagerness to spread the word and provide many details about his theory in the 18 months after the first public hints of October 1803. After privately developing his theory as evidenced in his laboratory notebook, he submitted his first atomic weights in the October Lit & Phil paper for publication in the society's memoirs (he could not then know it would take two years to appear); three months later he left a manuscript outlining the details of his theory for publication in London (he could not then know that it would be permanently shelved); he privately explained his theory to two prominent chemists, Thomson and Davy; and in these months he presented public lectures touching on the atomic theory on at least three, and quite possibly four or five separate occasions.

Whether or not I am right that Dalton had a strong interest in promoting his theory from the beginning, there is no question that his pace picked up after 1805, a point strongly urged by Thackray. After Dalton presented a new round of public lectures in Edinburgh and Glasgow in the spring of 1807 and Thomson published a description of the theory that same year, many elite chemists began to talk about the theory, in the capital and elsewhere, and Dalton pushed hard to finish his book. In the fall of 1807 the universally respected London chemist William Wollaston joined Thomson in enthusiastic advocacy, and (according to an anecdote told by Thomson) succeeded in persuading Davy of the merits of the theory (Thomson, 1830-31, vol. 2:, 293-94). Then, with the publication of Dalton's New System, the theory was finally well launched.

SOME CONCLUSIONS

We began this essay with an examination of Dalton's understanding of physics, which had been formed in a British Enlightenment context of popular Newtonianism, and which involved at least two crucial misunderstandings of Newton. We have seen that these were fruitful mistakes, and Joseph Ransome's tale as well as other evidence suggests that they were constitutive elements of the pathway that led Dalton to his chemical atomic theory. But there were no fewer than five other accounts of the origin of the theory: inductively from multiple proportions in the hydrocarbons, inductively from multiple proportions in the nitrogen oxides, deductively from the influence of Richter, deductively from Dalton's first theory of mixed gases, and deductively from Dalton's second theory of mixed gases. All of these accounts derived from Dalton himself. Which should be given preference?

It should first be noted that not all of the stories are really incompatible. Some of them refer unequivocally to an origin point in time, but others refer more vaguely to "founding his theory upon" and other such language that does not necessarily indicate a chronological beginning. In other words, it is not always clear in each case what is being assumed to constitute the essence of the theory for a particular story. If the theory is identical to the basic assumptions about matter discussed in the preceding paragraph and nothing more, then, as Ransome's anecdote suggests, Dalton's theory might well have long antedated his first chemical work. If, on the other hand, the theory is only deemed to have been created when some of the consequences were worked out and tested, then we need to look more to chemical details. We must distinguish between motivations for beginning an investigation from the investigation itself. Nothing prevents us from accepting the notion that Dalton's theory was motivated by physical concerns having to do with gases, as well as believing that his chemical atomic theory was developed first through chemical studies.

Dalton thought atomistically from the start. As a physicist, not a chemist, he inadvertently avoided some mental stumbling blocks on the conceptual road to atomic theory that would have troubled any chemist. As an informally educated natural philosopher, one who apparently thought entirely in materialist-mechanist-realist terms, he misunderstood some of Isaac Newton's most central pronouncements about the nature of matter, but precisely in ways that also assisted his mental route to atomic theory. He began the investigation into chemical atoms in an attempt to prove a hypothesis (the solubility- complexity correlation) that he very soon thereafter found false. And finally, in his very first chemical investigation, he was led by well-performed but misleading experiments on nitrogen oxides to perceive the first "confirmed" instance of integral multiple proportions.24 It appears that he immediately understood these chemical results atomistically, and that this perception gave him the entre to a chemically interpreted atomic theory-especially when, shortly thereafter, he looked at other chemists' analyses of cases of multiple proportions. He may also have needed this incipient atomic theory to help him establish the hypothetical and only briefly held correlation between molecular size and gas solubility, but nothing prevents us from supposing that he at once saw-and developed-both of these possibilities, the chemical and the physical, and the preponderance of evidence supports just this supposition.

I cannot provide a perfect origin account, resolving every contradiction between the various stories. To me, it seems impossible to ignore or even to reduce the significance of the evidence of the September and October 1803 notebook annotations and of the 1803/05 published paper, especially considering what we know of his activities after October 1803. At first blush, Ransome's origin story would seem to be much more subject to doubt, deriving as it did from a single conversation with Dalton recounted three decades after the event. However, no evidence contradicts, and much implicitly supports, the idea that Dalton was thinking atomistically long before he developed a specific and testable theory of atoms, as he apparently averred to Ransome. Indeed, supposing that Dalton's atomic speculations were extraordinarily vivid and real to him would help to explain some of the multiplicity of origin stories. When once he began to look chemically rather than physically, and with the correctly focused mental eye, he found evidence for atoms in every project he took up. For Dalton, everything seemed to confirm an idea whose germ he had so long believed in, and as a consequence he had trouble sorting through his own filiation of ideas in the correct chronological order. There is a natural tendency for one unconsciously to impose retrospective order to a complex series of connected events, but that order may be different on different tel\lings, since everything had been so closely connected.

If the suggestions provided here are accurate, the story of Dalton's route to atomic theory appears in one sense to be an unusual one in the history of science. In chemical-atomic terms, Dalton would have been able to articulate from the start what the goal of the theory should be, but he, like everyone else, initially had no way to get to that goal, and therefore wasted little time or thought on the prospect. Moreover, he was a physicist, not a chemist, and would not necessarily have been engaged in chemical questions at all. However, having obtained experimental evidence for a case of integral multiple proportions in his very first chemical investigation, he realized that this instance not only demonstrated that atoms existed (he apparently never doubted this) but also gave him the idea of how to get down to that level-his "calculating system," as Thackray called it. That calculating system was the real theoretical innovation, along with the two different kinds of checks that confirmed the accuracy of the system (verification of integral multiple proportions, and formula reticulations).

Putting to one side all of Dalton's oversimplifications and misunderstandings of Newton that provided framework and motivation, once he figured out how to proceed by inventing his calculating system, his route to atomic theory was a short and straight line- sharply contrasting with the meandering, twisting investigative pathways of many major figures in history of science. The scholar who has done the most to follow and interpret the fine structure of the investigative pathways of great scientists is Frederic L. Holmes. In his monographic treatments of the work of Antoine Lavoisier, Claude Bernard, and Hans Krebs, for example, Holmes was able to delineate the complicated twists and turns in each case study, and show how important new results emerged in a piecemeal and often unexpected fashion, over long months and years of intensive experimental work and thought (Holmes, 1985,1974, 1991-93, 2004). By contrast, from the point when Dalton first turned to chemistry to the invention of his chemical atomic theory was apparently just a few weeks at most.

Such a case is rare, but it is not unique. Galileo and the Copernican theory may represent a parallel example of a short and seemingly direct route to a major discovery. Little is known for certain about the evidence that first convinced Galileo of the truth of heliocentrism. Like Dalton and atoms, heliocentrism apparently just seemed right to Galileo, and over the decades he used whatever tools came to hand in order to convince others. Some of these tools were brilliant theoretical novelties, such as his new ontology of motion, or important new data, such as sunspots or the phases of Venus. Galileo also used tools whose empirical bases, even in his own day, were weak-his circular concept of inertia, for instance, or his flawed argument from the tides. However, in the case of Galileo as with Dalton, the apparent simplicity of the story belies the underlying complexities of mental models, courses of reasoning, historical contingencies, and variety of evidence used over time to build the theory.

Like Galileo, once Dalton's theory was achieved, his basic notions never altered. To his death in 1844, Dalton retained his atomic weights of 1808/10, despite the gradual but very significant continual increase in the accuracy of gravimetric analyses. To his death, he retained the fiducial standard H = 1, despite the nearly universal adoption of Berzelius's standard 0 = 100. To his death, he retained nearly all of his assumed formulas of 1803/05, despite numerous revisions by most other atomists. To his death, he retained his cumbersome ideographic formula notation, in the face of the overwhelming popularity of the Berzelian abbreviations that had become the international standard by about 1830. All of this was characteristic of his personality and scientific style. Throughout his life, as Robert Angus Smith justly wrote, Dalton exhibited "a great rapidity of reasoning, a direct passage from premise to conclusion without fear, as if more than usually persuaded that true reason could not misguide him.... He drives on like a new settler, and clears the ground before him, leaving it rather rugged, it is true, nonetheless it is resolutely cleared" (Smith, 1856: 23, 50). Successors planted in the ground that he had so well prepared, and reaped full and rich harvests.

NOTES

I wish to thank Arien Mack and Melvyn Usselman for very helpful comments.

1. Some of the standard works that deal with the origins of chemical atomism are Henry (1854), Smith (1856), Harden (1896), Nash (1956), Thackray (1966a, 1972), Greenaway (1966), Cardwell (1968), Patterson (1970), and Rocke (1984).

2. Wilson ( 1862:340). This obituary originally appeared in British Quarterly Review 1 (1845): 157-98.

3. Quotation taken from the republished 4th ed. of the Opticks (Newton, 1952: 389,400). see also Newton (1999: 697-99; 1717).

4. Roscoe and Harden (1986: 13,112). see Thackray (1970: 273- 74), for a fuller view of Newton's understanding of corpuscular theory.

5. Unfortunately, when Davy, as president of the Royal Society, presented the Royal Medal to Dalton in 1826, he compared Dalton to Kepler; the "Newton of chemistry" had apparently not yet come. see Davy (1839-40, vol. 6: 97).

6. When asked how he had discovered universal gravitation, Newton is supposed to have responded simply, "by thinking on it continually."

7. Henry, reproducing Ransome's letter verbatim (date not mentioned, original typography retained) (1854: 220-22). In Dalton's terms, the molecule of water was HO and the molecule of hydrogen peroxide HO^sub 2^.

8. Owen stated that Dalton became an "intimate friend" during the period ca. 1795-99, and had many evening conversations with him. He wrote that Dalton "first broached his then undefined atomic theory" in one of those conversations. see Podmore (1907: 55-56).

9. Davy's manuscript notes of February 1829 were communicated to his brother John, who transmitted them to Henry.

10.

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