n THE PROGRESS OF SCIENCE 71 



such decomposition. The phenomena of iso- 

 morphism showed a relation between the units 

 ami crystalline forms ; certain units are thus able 

 to replace others in a crystalline body without 

 altering its form, and others are not. 



Again, the laws of the effect of pressure and 

 heat on gaseous bodies, the fact that they combine 

 in definite proportions by volume, and that such 

 proportion bears a simple relation to their com- 

 bining weights, all harmonised with the Daltonian 

 hypothesis, and led to the bold speculation known 

 as the law of Avogadro that all gaseous bodies, 

 under the same physical conditions, contain the 

 same number of units. In the form in which it 

 was first enunciated, this hypothesis was incorrect 

 perhaps it is not exactly true in any form ; but 

 it is hardly too much to say that chemistry and 

 molecular physics would never have advanced to 

 their present condition unless it had been assumed 

 to be true. Another immense service rendered by 

 Dalton, as a corollary of the new atomic doctrine, 

 was the creation of a system of symbolic notation, 

 which not only made the nature of chemical 

 compounds and processes easily intelligible and 

 easy of recollection, but, by its very form, suggested 

 new lines of inquiry. The atomic notation was as 

 serviceable to chemistry as the binomial nomen- 

 clature and the classificatory schematism of 

 Linnaeus were to zoology and botany. 



Side by side with these advances arose another, 



