48 HISTORY OF CHEMISTRY. [LECTURE IV. 



give an account of the chemical researches which rendered the 

 assumption of the atomic theory necessary, and which were 

 brought to a conclusion by means of the theory. 



In the preceding lecture I dealt with one of these regu- 

 larities ; that is, with the law of definite proportions. This 

 alone, however, would not have sufficed for that theoretical 

 conception, and another law was also necessary, namely, the 

 law of multiple proportions. The latter was propounded by 

 Dalton in 1804, that is to say, prior to the close of the con- 

 troversy regarding the constancy of the proportions by weight 

 in which substances combine. I have intentionally departed 

 from the chronological sequence of the facts, in order to secure 

 their logical arrangement. The law of multiple proportions 

 has no meaning, so long as the law of constant proportions is 

 not proved. It includes the latter within it, and can only stand 

 along with it. We may well be surprised that it should have 

 been propounded at the very time when doubts were enter- 

 tained as to the accuracy of the law of constant proportions. 

 The explanation probably lies in the fact that Berthollet and 

 Proust lived in France, whereas Dalton made his discovery in 

 England, and withheld the publication of his investigations 

 until 1808; whilst prior to that date the scientific world only 

 obtained a short statement of the results of Dalton's experi- 

 ments from Thomson's System of Chemistry, in which the 

 investigations are mentioned. Dalton's theory, which very soon 

 gained favour, undoubtedly exercised a decisive influence on 

 the views of the chemists of the period with respect to con- 

 stant proportions; and it is partly to be attributed to the 

 labours of Richter, of Dalton, and of Wollaston, that Berthollet, 

 if he did not exactly withdraw his previous assertions, at least 

 did not further endeavour to bring about their acceptance. 



It may be that Dalton, as his biographer, Smith, asserts, 1 

 had no knowledge, or only a very incomplete knowledge, 

 of the work of Richter, which might otherwise have contri- 

 buted materially to the establishment of the atomic theory. 



1 Memoir of John Dalton and History of the Atomic Theory, 214. 



