364 Dr. H. Debus on the Genesis of 



The values of M/S=C are dependent on the specific 

 gravities, the chemical composition, and the theories about the 

 constitution of gases. The composition of water is, according 

 to Lavoisier, 85 pts. of oxygen and 15 pts. of hydrogen, 

 according to Gay-Lussac and Humboldt 87*4 pts. of oxygen 

 and 126 pts. of hydrogen. Lavoisier's numbers make the 

 atomic weight of oxygen =5*66, Gay-Lussac's and Hum- 

 boldt's = 7. The errors of observation in Dalton's time were 

 So considerable that he might have assumed for from 50 to 

 66 per cent, of the gases in the above list the same molecular 

 volume. The numbers obtained for M/S = C, therefore, left 

 it undecided whether the hypothesis, that equal volumes of 

 different gases contain the same number of molecules, is true 

 or not true. The probabilities are, perhaps, slightly in favour 

 of the hypothesis. This seems to have caused Dalton to 

 return to his old view, that the particles of gases are all of 

 the same size, or M/S = C This is my explanation of the 

 contradictory statements on two pages, 246 and 260, of his 

 Note-book, i. (R. 27, 42) (pp. 361, 362). The near agreement 

 of several of the numbers of M/S in the last table caused a 

 series of new experimental investigations. Dalton, who up 

 to this time had done very little practical work in chemistry, 

 now began with great zeal to determine the composition of 

 important substances, such as ammonia, marsh-gas, and ole- 

 fiant-gas. The results of these investigations, as far as they 

 concern gases, are collected in a table at the end of the second 

 part of the New System. 



Five out of 16 gases, or 31 per cent., and if errors of ob- 

 servation of 2 per cent, are allowed three more, or, together, 

 50 per cent, of the gases examined, possess the same molecular 

 volume. Dalton's experiments have not explained why 

 50 per cent, of the gases examined do not conform to the 

 hypothesis M/S = C, and they could not do so, the chemical 

 knowledge of the day not being sufficiently advanced. As a 

 matter of fact, it has required the practical work of half a 

 century to convert the hypothesis M/fc> = C into an empirical 

 law. 



Thomas Thomson, the author of the celebrated work " Sys- 

 tem of Chemistry/ himself a chemist of eminence, paid a visit 

 to his friend Dalton in August 1804. On this occasion the 

 latter communicated to him the principles of the new atomic 

 theory and the results obtained by their application. In 

 1807 a new edition of the i System of Chemistry ' came out, 

 in which Thomson, with Dalton's permission, published a 

 sketch of the new theory (vol. hi. p. 424) . Thomson's de- 

 scription is extremely clear and accurate, and Dalton has, as 



