258 The late Dr. Thomas Thomson. 



R. Thomson to superintend a new edition, completing it with a memorial, 

 such as he might write, of its illustrious author, they would produce one 

 of the most interesting works that could be given to scientific men. 



Instead of Dalton's term "atom," which Thomson adopted, Davy 

 always used the word "projmrtion," and WoUaston "equivalent," which 

 was much better ; but whatever term we employ, now that the thing itself 

 is understood, there can be no doubt that the use of the word " atom," 

 (which conveys at once the idea of an ultimate indivisible particle,) 

 greatly contributed to the reception of the doctrine of definite proportions. 

 In 1808 Mr. Dalton published a volume of his own, in which not more 

 than five pages, widely printed, and one plate with explanations, were 

 devoted to the announcement and illustration of the atomic theory. This 

 treatise, if such it can be called, is little more copious than that which 

 had been given the year before from Dr. Thomson's notes. 



In 1809, Gay Lussac made known his theory of volumes. In 1810, 

 as I understand, Berzelius first published his ''E>say on the Cause of 

 Chemical Proportions," and it was translated by Dr. Thomson for the 

 Annals of 1813-14. It contains a determination of the constituents of 

 many important bodies. It shows that when a sulphate is formed from a 

 sulphite or a proto-sulphuret, the sul[ihate is always neutral, and that in 

 compounds of acids and bases, containing oxygen, the acid contains 2, 3^ 

 4, 5, &c. times as much oxvgcn as the base. In 1813, Dr. Thomson 

 commenced in the Annals an elaborate treatise on the Daltonian theory, 

 and appended to it an extensive list of atomic weights. This he was 

 enabled to do by comparing together the results of experiments which had 

 recently been undertaken by Davy, Gay Lussac, Biot and Arago, 

 Berzelius, Wollaston, and others, on the specific gravities of gases and 

 the composition of solid bodies. The numbers for the elementary bodies 

 are exceedingly near the truth. They attracted the notice of Dr. Prout, 

 and in November, 1815, that chemist announced anonymously his cele- 

 brated doctrine, that the atomic weights of all bodies, solid as well as 

 gaseous, are multiples of the atomic weight of hydrogen. 



The view taken by Dr. Prout was not an arbitrary one, guessed at 

 from the accidental approach to it of a few of the elements. The 

 discovery of Gay Lussac, that gases unite together in exactly equal or 

 multiple volumes, and the circumstance that the specific gravities of 

 several of the gases bad been already ascertained (before 1815) with 

 great precision, led directly, as far as these gases were concerned, to that 

 conclusion. There were means also for taking the specific gravities of 

 solid bodies in the gaseous state, as carbon in the state of carbonic acid, 

 sulphur in sulphurous acid, &c., and the application of the principle to other 

 solid substances, such as potassium, calcium, or iron, although none of their 

 compounds can be produced in the state of gas, was apparently inevitable. 

 Round numbers were thus obtained for several of the elements. 

 Carbon, for example, had an atomic weight six times that of hydro, 

 gen. Oxygen, as it united with twice its volume of hydrogen to form 



