Manchester Meinoiis, Vol. liv. ( 1 9 1 o), No. y. 



VII. The Development of the Atomic Theory : (i) 

 Berthollet's Doctrine of Variable Proportions. 



By Andrew Norman Meldrum, D.Sc 



{Carnegie Research Fcllozv). 



( Communicated by Professor H. B. Z)ixoti, Jlf.A., P.J^.S.) 



Received a)id read November ■^oth, igog. 



Two of the burning questions in science, between the 

 years 1800 and 1810, were the theory of "mixed gases," 

 and the fixed or the variable composition of chemical 

 substances. On each of these questions Claude Louis 

 Berthollet and John Dalton were leaders of thought on 

 opposite sides, and in each case Dalton's ideas were 

 ultimately triumphant. Yet his rights in these directions 

 have never received full attention, the subjects having 

 been treated in an inadequate manner by the historians 

 of science. The present paper shows in outline how the 

 doctrine of constant proportions developed. 



The doctrine that chemical compounds have a con- 

 stant composition is not a discovery made in the XVIIIth 

 century by a certain man. Lavoisier, Wenzel, and 

 Richter were outstanding workers on the subject, and 

 other workers in the same field, and to much the same 

 general effect, were Cavendish, Bergman, Klaproth, Vau- 

 quelin, and Kirwan. Many more might be named, for 

 practically all chemists towards the end of the century 

 seem to have judged the doctrine to be a satisfactory 

 account of the facts of chemistry. 



But no doctrine can be regarded as established, or 

 even as rightly understood, until it has been called in 

 question and successfully defended. The challenge puts 

 the defenders on their mettle and compels them to con- 



Febrnary ist, igio. 



