212 THE POPULAR SCIENCE MONTHLY. 



and its importance has increased with the progress of the science. It 

 would take too long to enumerate all the theoretical views which have 

 prevailed at various times during the past fifty years ; hut the theory 

 which along with the radical theory has exercised most influence on 

 the development of the views now held, is the theory of types, first 

 stated by Dumas (1839), and developed in a different form and amal- 

 gamated with the radical theory by Gerhardt and Williamson (1848- 

 1852). It is, however, the less necessary to refer in detail to these 

 views, seeing that in the now prevailing theory of atomicity we pos- 

 sess a generalization which, while greatly extending the scope of chemi- 

 cal science in its power of classifying known and predicting unknown 

 facts, includes all that was valuable in the generalizations which pre- 

 ceded it. The study of the behavior of organo-metallic compounds in 

 chemical reactions led to the conclusion that various metallic elements 

 possess a definite capacity of saturation with regard to the number of 

 atoms of other elements with which they can combine, and demon- 

 strated this regularity of atom-fixing power in the case of zinc, tin, 

 arsenic, and antimony. A serious obstacle, however, in the way of 

 determining the true atomicities of the elements was the general em- 

 ployment of the old so-called equivalent weights, which were by most 

 chemists confounded with the atomic weights. This difficulty was 

 removed by the rectification of the atomic weights, which, though be- 

 gun by Gerhardt as early as 1842, met for a long time with but little 

 recognition, and was not completed till the subject was taken up by 

 Cannizzaro in 1858. The law of atomicity has given to chemistry an 

 exactness which it did not previously possess, and since its discovery 

 and recognition chemical research has moved very much on the lines 

 laid down by this law. 



Chemists have been engaged in determining, by means of decom- 

 positions, the molecular architecture, or constitution as it is called, of 

 various compounds, natural and artificial, and in verifying by synthesis 

 the correctness of the views thus arrived at. 



It was long supposed that an impassable barrier existed between 

 inorganic and organic substances : that the chemist could make the 

 former in his laboratory, while the latter could only be produced in 

 the living bodies of animals or plants requiring for their construction 

 not only chemical attractions, but a supposed " vital force." It was 

 not until 1828 that Wohler broke down this barrier by the synthetic 

 production of urea, and since his time this branch of science in the 

 hands of Ilofmann, Wurtz, Berthelot, ButleroAV, and others, has made 

 great strides. 



Innumerable natural compounds have thus been produced in the 

 laboratory ranging from bodies of relatively simple constitution, such 

 as the alcohols and acids of the fatty series, to bodies of such complex 

 molecular structure as alizarine (the principal coloring-matter of mad- 

 der), coumarine (the odoriferous principle of the Tonka bean), vanilline, 



