CHEMISTRY AND ITS DEVELOPMENT 225 



and for a time it seemed that all organic compounds could 

 be grouped under these four simple types. It was soon, 

 found necessary, however, to introduce the ideas of " con- 

 densed types" like the condensed water type (H 2 0) 2 , 

 " mixed types" and the type marsh gas, CH 4 . In the 

 course of the fifties the type theory thus gradually became 

 less and less simple, and, hence, less and less valuable for 

 the purpose of correlating organic compounds. 



Meanwhile an idea of inestimable value had thrust itself 

 upon chemists. Inspecting the typical formulas of com- 

 pounds, they could not help noticing that certain radicles 

 (e.g. methyl, CH 3 , or ethyl, C 2 H 5 ) were capable of replac- 

 ing each a single atom of hydrogen; others were capable 

 of replacing each two atoms of hydrogen, etc. In other 

 words, some radicles were seen to be equivalent to an atom 

 of hydrogen ; others had double its combining capacity, 

 etc. Hence the idea of the valency of radicles and atoms. 

 Like most other general ideas, that of valency was not new. 

 In a vague and more or less specialized form it may be 

 found in the researches of Berzelius, Graham, Liebig, and 

 others ; and Frankland, who first clearly enunciated it, in 

 1852, justly points out that it was probably a vague recog- 

 nition of the valency of radicles, as exhibited by the facts 

 of substitution, that gave birth to the theory of types. 

 Frankland 's statements, however, attracted no attention. 

 In 1858 Kekule and Couper independently developed the 

 same idea, the latter proposing to symbolize the combining 

 capacity of different atoms by the dashes now generally 

 employed in graphic formulae. Kekule called attention to 

 the quadrivalence of the carbon atom, as shown directly by 

 compounds like the following: CH 4 , CH 3 C1, CH 2 C1 2 , 

 CHC1 3 , CC1 4 ; or indirectly by such compounds as C0 2 , 

 COC1 2 . In the former compounds a single atom of carbon 

 is seen to be equivalent to four atoms of hydrogen, and a 

 single chlorine atom to a single atom of hydrogen, which is 

 also shown by the formula of hydrochloric acid, HC1, In 

 a compound like COC1 2 , the oxygen atom must therefore 

 15 



