44 EEPQET 1881. 



dni'Ing the past fifty years ; but the theory whicli along with the radical 

 theory has exercised most influence on the development of the views now 

 held, is the theoiy of types, first stated by Dumas (1839) and developed 

 in a diff'erent form and amalgamated 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 jirevailing theory of atomi- 

 city we possess a generalisation which, while greatly extending the scope 

 of chemical science in its power of classifying known and predicting 

 unknown facts, includes all that was valuable in the generalisations 

 which preceded it. The study of the behaviour 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 thej can combine, 

 and demonstrated 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 

 employment 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 begun 

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

 nition, and was not completed till the subject was taken up by Cannizzaro 

 in 1858. The law of atomicity lias given to chemistry an exactness 

 which it did not previously possess, and since its discovery and recog- 

 nition 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 constihition 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 laboratorj-, while the latter could only be produced in the 

 living bodies of animals or plants, — requiring for their construction not 

 only chemical attraction, 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 Hofmann, 

 Wurtz, Berthelot, Butlerow, and others, has made great strides. In- 

 numerable natural compounds have thus been produced in the labora- 

 tory — ranging from bodies of relatively simple constitution, such as the 

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

 cular structure as alizarin (the principal colouring matter of madder), 

 coumarin (the odoriferous principle of the tonqua bean), vanillin, and 

 indigo. The problem of the natural alkaloids has also been attacked, in 

 some C8/Ses with more than partial success. Methylconine, which occurs 

 along with conine in the hemlock, has been recently prepared artificially 

 by Michael and Gundelach, this being the first instance of the synthesis 



