/tory behind the /tory Dalton's Atoms or Dalton's Molecules? Roman Mlerzecki University of Warsaw PL-02-093 Warszawa. Poland
I t is a common opinion t h a t the notion of indivisible atoms was introduced int; modern chemistry by John Dalton at the beginning of the 19th century. Partington in his textook on the history of chemistry ( I ) argues t h a t t h e atomic theory can be found "explicite or implicite" in Dalton's notebook of September 6, 1803. Partington ascribes t o Dalton the conclusion: "Atoms are indivisible, cannot be created or destroved." T h e careful analvsis of the development of these t e r d s in Dalton's notebook as published by Roscoe and Harden (2). however. cast some doubts on such a n opinion. In t h e l f t h and 18th renturies the term "atom" was very seldom used for the smallest part of matter. T h e term "molecule" introduced by Gawendi was found in chemical papers of that time. T h e term "atom" was really promulgated hy Dalton, but it was nor till the First International Chemical Conyress in Karlsruhe in 1860 when the two notions "atom" and "molecule" were definitely differentiated. T h e terms "atom" and "element" were a l w often w e d interchangeably. Even in the IYthcentwyan element was the limit ofanalylical possibilities, and not a s a n absolutely simple hody. 'This can be seen. for instance. in the foreward tn the Lavoisier's ' l i o l r i
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in 1808 Dalton writes not only about the single atoms but also about binarv. .. ternarv... and uuaternarv atoms. Thus the problem arises how Dalton himself romprehmded the terms "atom" and "indivisible." Lurkilv we can answer with Dalton's own words (6): I have chosen the word atom to signify these ultimate particles, in ref ere nee to particle, molecule, or any other diminutive'krm, because I conceive it is much more expressive; it includes the notion of indivisible, whieh the other terms do not. It may perhaps be said that I extend the applicationof it too far, when I speak of compound atoms; far instance, I call an ultimate particle of carbonic acid a compound atom. Now, though thimtom may be divided, yet it ceases to be carbonic acid, being resolved by such division into charcoal and oxygen. Hence I conceive there is no inconsistency in speaking of compound atoms, andmy meaning cannot be misunderstanded.
It seems t h a t in the early years of t h e 19th century Dalton could not and had notintroduced the notion of atoms as the smallest quantity of a n element. T h e meaning of "atom" corresnonds to~the contemoorarv ~ - rather ~ ~ . " term "molecule." His great and important contribution to chemistry is the formulation of a notion t h a t was called by him "weight of ultimate atom" (1803) (7), or "weight of ultimate particle hydrogen heine- 1" .(1804). (8). . . Dalton considered these numbers t o be characteristic for each particular substance. In connection with this discovery we often read that Dalton has introduced into chemistry the concept of "atomic weights!' In light of the above analvsis of the meanine of the term "atom" Dalton was really writing about "molecu~arweights." I t should also he noted t h a t during the whole 19th century the ideas of a n indivisible atom was never accepted without doubts by many distinguished chemists. For example in 1816 William Prout suggested that all atoms are composed of the atoms of hydrogen. In 1867 Kekulk differentiated between the "physical atom," i.e., the unknown ultimate Particle of the matter, and the "chemical atom" that acts as a whole, though it can be composed of smaller parts (9).
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. . . if by the term of element we mean to express those simple and indivisible atoms of whieh matter is composed, it is extremely probable we know nothing about them (3); however, if the name of elements or principles of the bodies we refer to the idea of the ultimate members achieved by the analysis, all substances which we cannot decompose by anyway, are for us elements. . . . . ..from our point of view they act as simple bodies and we cannot mnsider them as mmpased, till it will be not proved by the experiment and observation (4). The idea of an'ahsolute indivisible oartide can be found in manv Literature Cited papers uf the 17th and lRrh centuries, and the existence of such a was a problem of faith. I t war clearly expressed by Isaac (1) Psnington.J.R..Sci@n&,49,221(1955);"Historyof Chemistry."Vol. 3, MaeMillsn, Newtunin hisRlst queryat theendofthe~~rondeditlun~~fhis"OpLondon, 1964. (2) Roe-. H. E.. and Hmde".A.."A New virluon the miainofDdton~aAtomieThww," tics" ( 5 ) ~~~
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It seems probable to me that God in the beginning formed matter in solid, massy, hard, impenetrable, movable particles. . . even so very hard as never to wear or to break in pieces; no ordinary power being able to divide what God Himself made One, in the first creation. Contrary to Partington's statement ( I ) , Dalton has not defined an atom a r indivisible in any of his writing. It i 3 known that in his "New SystemofChemical Philosophy"puhlished
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MacMiUao, London,1896. (3) Ref. (21, p. 112 (tran.lstion by J. Dalton from: A. Lavoiaier. "Trait4 elOmanfaire dc
(1) Ref. w , p . 41. (8)Ref. (2). p. 65.
(9) Kekul€.A..Labomlory.1.303 i1867):Aeeountin"ClaaaiealSdenti6cPapr8-Chsmiafry." Second Seriea, American Elsevier, Mills. Boon Ltd., NY, 1970.
