128 HISTORY OF CHEMISTRY. [LECTURE VII. 



means of a series of investigations, of which I have shortly 

 stated the most important. In doing this, I was at pains to 

 explain the development of the ideas, and I only desire now to 

 be permitted to define the meaning of the term as it eventually 

 became fixed in the minds of the chemists of the period. 



I begin with the celebrated definition of Liebig : 57 " We 

 call cyanogen a radical," he says, in 1837, in his criticism of 

 Laurent's theory, "(i) 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 substance, the latter can be turned 

 out and replaced by equivalents of other simple substances." 



These three requirements, of which, according to Liebig, at 

 least two must be fulfilled in order that an atomic group may 

 have any claim to the designation of radical, prove that only a 

 study of the nature of a compound could lead to a knowledge 

 of the radical contained in it. The behaviour of a substance 

 towards elements and compound bodies required to be known, 

 in order that its radical might be ascertained ; and from this 

 it may be gathered what significance such a determination 

 possessed. The choice. involved, to a certain extent, a resume 

 of the whole investigation, since the decomposition products 

 were known when the radical was known ; ihe latter was, of 

 course, composite itself, but with its decomposition, those 

 affinity relations ceased which connected with one another, 

 substances containing the same radical. That the radical 

 behaved like an element, had been confirmed over and over 

 again. Not only did it enter into combinations with elements, 

 but it could also be isolated from these combinations. How 

 far this comparison was carried, is shown by a quotation 

 taken from a joint paper by Dumas and Liebig : 58 " Organic 

 chemistry possesses its own particular elements, which some- 

 times play the part taken by chlorine and oxygen in inorganic 

 chemistry ; sometimes, on the other hand, the part of the metals. 

 Cyanogen, amide, benzoyl, the radicals of ammonia, of fatty 



57 Annalen. 25, 3. 58 Comptes Rendus. 5, 567. 



