Manchester Memoirs, Vo/. x/vz'i. (igOT,), No. 11. ii 



was far beyond the range of their knowledge. That the 

 distinction is not ahvays borne in mind by physicists is 

 illustrated by the fact that in Clerk Maxwell's article 

 " Atom," in the Encyclop(£dia Britannica, Dalton is 

 not even mentioned, and that the phenomena there 

 selected for discussion are molecular only. Maxwell was 

 surely not ignorant of the difference between atoms and 

 molecules, but his knowledge had not reached the point 

 of complete realisation. His thought was of molecules, 

 and so Maxwell unconsciously neglected the real subject 

 of his chapter, the atom. Of late years many essays upon 

 the atomic theory have been written from the physical 

 side, and few of them have been free from this particular 

 ambiguity. At first, a similar error was committed by 

 chemists, who paid small attention to Avogadro's law, 

 and so the latter failed to exert much influence upon 

 chemical thought until more than forty years after its 

 promulgation. The relation discovered by Dulong and Petit 

 in 1 8 19, that the specific heat of a metal was inversely 

 proportional to its atomic weight, was more speedily 

 accepted ; but even this law did not receive its full appli- 

 cation until many years later. To apply either of these 

 laws to chemical theory involved a clearer discrimination 

 between atomic weights and equivalents than was possible 

 at the beginning. A long period of doubt and controversy 

 was to work itself out before the full force of the physical 

 evidence could be appreciated. Mitscherlich's researches 

 upon isomorphism were more fortunate, and gave imme- 

 diate help in the determination of atomic weights and the 

 settlement of formula;. For the moment we need only 

 note that the chemical atom was the underlying concep- 

 tion by means of which all these lines of testimony were 

 to be unified. 



From Dalton and Gay Lussac to Frankland and 



