LAW OF DEFINITE RELATIONS 



and Ampere established the facts that the number of 

 molecules in equal volumes of all gases are the same. 

 This served as the points of departure for the dis- 

 covery of the relation between the gaseous state and 

 volume of the elements, and their chemical and atomic 

 relations, that in the last forty years have changed 

 the nomenclature as well as the theories of chemical 

 affinities. The various systems by which it was shown 

 that the molecules were grouped viz : the Dualistic, 

 the theory of Radicals, the Substitution theory of 

 Gerhardt and Laurent, and Gerhardt's theory of 

 types each contributed in turn to the advance of 

 science, being complemented finally in the theory of 

 the Valences of the elements. By this it is shown 

 how chemical combinations are formed; new mo- 

 lecular groups with their distinctive properties may 

 be predicated, and at the same time it is made evident 

 why the atoms cannot enter into every desjred com- 

 bination, and why certain grouping of molecules must 

 be impossible. 



Of the cause, or rather of the nature of chemical 

 affinity, we have no conception. We know that cer- 

 tain elements or molecules have the quality of unit- 

 ing with certain other molecules with great energy ; 

 that they displace other molecules to do so ; but what 

 this affinity is, we know not. 



The combinations of atoms into molecules, or the 

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