O INTRODUCTION. 



how evolution acted, and no wonder that numerous 

 different opinions have been held on this fascinat- 

 ing subject. Even yet, we are far from a communis 

 opinio. 



We are best informed as to the non-living part of 

 the universe. Ingeniously worked out methods have 

 given us the certainty that all celestial bodies consist 

 chiefly of the same kind of substances, and we have also 

 ascertained that a comparatively small number of 

 primary substances — the so called elements — suffise 

 to compose alte those numerous substances which oc- 

 cur in nature or can be made in our laboratories. 



The discovery of the existence of these elements led 

 at first and had to lead, to the conception that elements 

 were absolutely independent primary substances, but 

 Prout, a London physician (obiit 1850), called already 

 in 1815, attention to the fact that the atomic weights 

 of all elements are invariably integer multiples of the 

 atomic weight of the element Hydrogen, and from this 

 knowledge deducted the hypothesis that all elements 

 might very well be derivatives, by a process of con- 

 densation, of Hydrogen. 



It has later been shown that the integrity of these 

 multiples is not perfect, but the ascertained deviations 

 were not sufficiently great, to force us to discard 

 Prout's supposition of the existence of some kind of 

 connexion between the different elements. 



On the contrary, Mendelejeff's periodical system ga- 

 ve new support to Prout's idea. 



This system arranges the elements in a series accor- 

 ding to their atomic weights. Part of the elements so 



