48 THE ADVANCE OF SCIENCE 



thus able to replace others in a crystalline 

 body without altering its form, and others 

 are not. 



Again, the laws of the effect of press- 

 ure 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 combining 

 weights, all harmonised with the Dalto- 

 nian hypothesis, and led to the bold 

 speculation known as the law of Avoga- 

 dro — 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 hy- 

 pothesis was incorrect — j:>erhaps it is n °t 

 exactly true in any form ; but it is hardly 

 too much to say that chemistry and mo- 

 lecular physics would never have ad- 

 vanced 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, 



