PEOGRESS IN CHEMISTRY. 247 



CLEAR IDEAS OF THE STRUCTURE OF THE MOLECULE. 



The first clear ideas of the structure of the molecule were, however 

 gained from the study of the compounds of car])on. It was difficult to 

 apply the dualistic theory to them. For few of them are electrolytes 

 and therefore their products of electrolysis, being- nonexistent, could 

 not be classified. Nevertheless, Gay-Lussac regarded alcohol, C.,H,.0, 

 as a compound of i\B.^, ethylene, and H.3O, water, and oxalic acid 

 (anhydrous), C.Og, as one of CO^ with CO. The discovery of " isomeric 

 compounds,'' i. e., of compounds which possess the same ultimate 

 formula and yet difi'er entirely in their properties, forced upon chemists 

 the necessity of attending to the structure of the molecule, for only bv 

 such a supposition could the difference between two isomeric l)odies )>e 

 explained. In 1823 Liebig discovered that silver fulminate and silver 

 cyanate both possessed the empirical formula AgCNO. In 1825 this 

 was followed by the discovery b}' Faraday that oil gas contains a 

 hj^drocarbon identical in composition with ethvlene, C.,H^, yet difiering 

 from it in properties; and in 1829 Wohler, professor in Gottingen 

 (1800-1882), discovered that urea, a constituent of urine, could be 

 produced by heating ammonium cj^auate, NH^CNO, a substance of the 

 same formula. It therefore became clear that the identity of a com- 

 pound must depend on some other cause than its ultimate composition. 



In 1838 Liebig and Wohler took an important step in elucidating 

 this question by their investigations on benzoic acid, an acid obtainable 

 b}' distilling a resin named gum benzoin. They showed that this acid, 

 C^HyOg, could be conceived as consisting of the group CIH.O, to which 

 they gave the name "benzoyl," in combination with OH; that benzoic 

 aldehyde, C\HgO, might be regarded as its compound with h3'drogen; 

 that it also formed compounds with chlorine and bromine and sulphur, 

 and replaced hj^drogen in anmionia (CjHgO.NHJ. They termed this 

 group benzoyl, a "compound element," or a "radical." This research 

 was followed by one by Robert Bunsen, professor at Heidelberg, born 

 in 1811, and recently (1899) dead, which bore reference to cacodyl, a 

 compound of arsenic, carbon, and hydrogen, in which the idea of a 

 radical was confirmed and amplified. 



The idea of a radical having thus become established, Je:\n Baptiste 

 Andree Dumas, professor in Paris (1800-1884). propounded the theory 

 of "substitution," i. e., that an element such as chlorine or oxygen 

 (which, be it noticed, is electro-negative on Berzelius's scale) could 

 replace hydrogen in carbon compounds, atom for atom, the resulting 

 compound belonging to the same "type" as the one from which it was 

 derived. And Laurent, warden of the mint at Paris (1807-1853), and 

 Gerhardt, professor at Montpelier and at Strassburg (1816-1856), 

 emphasized the fact that one element, be it what it may, can replace 

 another without fundamentally altering its chemical character, and 

 SM 19U(J 18 



