102 PROGRESS OF SCIENCE IN THE CENTURY. 



salt it is more important to recognise its two constitu- 

 ents than to know the quantitative proportions of its 

 component elements. This may suggest the idea, 

 which has been of enormous importance in organic 

 chemistry, that in the usually complex substances in- 

 volved there exist groups of elements which because 

 of their stability of union, may be said to play the 

 part of an element. Such a group is called a com- 

 pound radical. To take a concrete case, in their re- 

 searches on bitter almond oil and the allied com- 

 pounds, Wohler and Liebig ^' showed that we may as- 

 sume the existence, in these substances, of an oxygen- 

 ated group which remains unchanged in the majority 

 of the reactions, and therefore behaves like an ele- 

 mentary substance. On this account, they called it 

 the radical of bitter almond oil.'' * 



In 1837, Liebig wrote : " We call cyanogen a 

 radical (1) because it is a non-varying constituent 

 in a series of compounds, (2) because in these latter 

 it can be replaced by other simple substances, and 

 (3) because in its compounds with a simple sub- 

 stance, the latter can be turned out and replaced by 

 equivalents of other simple substances." The idea 

 may seem to the outsider far off and theoretical, but 

 there can be no doubt that the formulation of the 

 radical theory not only introduced new clearness into 

 chemistry, but was most provocative of research, some 

 of the results of which have had no small influence 

 on practical human affairs. 



Summary. — Just as it had been sliown (Ampere, 

 1816) that the salts of ammonia can he conveniently 

 discussed and studied by regarding them as salts of 

 a compound element (NH"^) so Berzelius, Dumas, 

 Wohler, Bunsen, Liebig and others sought to work 

 * Ladenburg, 1900, p. 109. 



