222 MODERN SCIENCE READER 



chemists early directed their attention to the problem of 

 molecular constitution. Berzelius was led to this problem 

 by his electro-chemical theory. But in the twenties facts 

 became known which made its study an imperative necessity 

 also from a purely practical standpoint. Not small was 

 the surprise of chemists when Gay-Lussac and Liebig found, 

 in 1823, that silver fulminate had precisely the same com- 

 position as silver cyanate. Two years later, Faraday dis- 

 covered a volatile liquid hydrocarbon that had precisely 

 the same composition as ethylene gas. Berzelius first 

 thought it unwise to abolish, on the evidence of a few facts, 

 what had seemed an axiom viz. that two different com- 

 pounds cannot possibly have the same composition. But 

 when he discovered that racemic and tartaric acids, too, had 

 the same composition, he realized that the character of a 

 substance must depend not only on its composition, but also 

 on its constitution i.e., not only on the kind and number, 

 but also on the arrangement of the atoms in its molecule. 

 Thus was born that great problem of modern chemistry 

 to determine the constitution of substances from the stand- 

 point of the atomic hypothesis. 



In 1832 Liebig and Wohler made an important discovery. 

 A series of compounds allied to benzoic acid were trans- 

 formed by them into one another, and through all the trans- 

 formations a group of atoms (made up of carbon, hydro- 

 gen, and oxygen) which they named the "benzoyl radicle" 

 remained unchanged ; the molecules of benzoic acid, benzal- 

 dehyde, benzamide, and benzoyl chloride, contained that 

 radicle in common, as if it were a single atom of some ele- 

 ment. The discovery of benzoyl was followed by Liebig 's 

 discovery of ethyl, a radicle common to ordinary alcohol 

 and ether, and by Bunsen's discovery of cacodyl, which is 

 possessed in common by several compounds of arsenic. The 

 discovery of radicles was obviously the first step toward a 

 knowledge of the constitution of compounds. But almost 

 from the beginning the idea of radicles became associated 

 with certain other ideas that could not be maintained in 



