68o POPULAR SCIA'NCE MONTHLY. 



Classification of compounds became imperatively necessary, and to 

 that all speculation was subordinated. In 1842 Schiel found tliat 

 the alcohols formed a regular series, with progressive variation in 

 their properties; Dumas observed a similar relation among the 

 fatty acids, and so something like order began to appear. 



In 1843 Charles Gerhardt proposed to use the law of Avogadro 

 as a basis for the determination of atomic weights. This involved 

 the doubling of many existing values, especially the atomic weights 

 assigned to oxygen, carbon, and sulphur. At first the proposition 

 was violently opposed, and even ridiculed, but by slow degrees it 

 managed to make its way, although it was not until after 1858 that 

 it began to find anything like general acceptance. In that year 

 Cannizzaro put forth his revision of the atomic weights, adjusted 

 to accord with physical laws, and a new era in chemistry began. 

 The modern theories of chemistry became possible, and the many 

 researches in which they had been foreshadowed received a clearer 

 meaning. Cannizzaro did not stand alone; his work was but the 

 capstone of a structure which had long been growing; Liebig, 

 Dumas, Laurent, Gerhardt, Wurtz, Graham, Williamson, and 

 Frankland were among the builders. But at last chemical and 

 physical evidence were brought into full convergence, and each 

 gave emphasis to the other. 



During the formative period of the new doctrines, between 

 1840 and 1858, many discoveries were made which helped toward 

 the final consummation. Even earlier than this the researches of 

 Graham upon the phosphoric acids had familiarized chemists with 

 the idea that different substances might have very different com- 

 bining powers, and other polybasic acids were found to exist among 

 organic compounds. The discovery by Wurtz, in 1849, that the 

 hydrogen of ammonia was replaceable by organic radicles, forming 

 the compound ammonias or amines, was a logical extension of the 

 theory of substitutions; and the recognition at about the same 

 time, by Hofmann, of ammonia as a distinct type upon which many 

 other substances could be modeled, w^as another long step forward. 

 In 1851 Williamson argued that nearly all inorganic and many 

 organic molecules could be represented as analogous in structure to 

 water, and a year later, as a result of his researches upon the 

 organo-metallic bodies — zinc ethyl, tin ethyl, etc. — Frankland ex- 

 pressed the belief that every elementary atom has a definite com- 

 bining power which limits the number of other atoms capable of 

 direct union with it. This was the theory of valence in its first 

 and simplest form, undeveloped to its consequences, but unmistak- 

 ably clear. To carbon compounds in general it was yet to be 

 applied. 



