. Manchester Memoirs, Vol. Hi. (1908), No. 10. 9 



developed by Dalton is at bottom describable as the 

 principle of the essential homogeneity of each pure 

 substance, that it is composed of molecules of only one 

 type, absolutely alike. Once it is postulated that only 

 one kind of aggregation into molecules occurs, e.g., that 

 in water there is only one way in which the hydrogen 

 attaches itself to the oxygen, the laws of definite and 

 multiple proportions are self-evident. The only way to 

 ascertain the truth of this hypothesis was to test the 

 consequences experimentally. In the hands of Lavoisier 

 it had become clear that in chemical transformation mass 

 does not to a sensible extent ever disappear or re-appear, 

 that chemical operations are not attended by dissipation 

 or destruction of matter. In the hands of Dalton it 

 became clear that each type of substance is characterized 

 by its own specific type of aggregation of constituent 

 atoms, by its own molecule. 



At that time neither principle could have stood out 

 in the full light in which we are accustomed to view it 

 now. Physical ideas had retrograded since Newton's 

 day. The heat which to his view seemed so obviously to 

 be vibratory motion, due to the clash of atoms under their 

 specific energies, had come to be regarded since Stahl, 

 aided perhaps by a misreading of Black's doctrine of 

 specific and latent heat, as a substance combined in 

 various proportions with different bodies, on the idea that 

 it is only something material that could be conserved. 



And, moreover, as in all fundamental advances, the 

 result attained was not so much the vindication of any 

 inflexible experimental fact, as the introduction of an 

 abstract guiding principle into the Science, fortified of 

 course by experimental support. For it is still a 

 legitimate aim of experiment to try whether any detectable 

 change, either in mass or in gravity, is produced by that 



