144 HISTORY OF CHEMISTRY. [LECTURE VIII. 



acid; chlorine atoms equal in number to the hydrogen atoms 

 eliminated are substituted for the latter, so that the physical 

 and chemical properties of the original substance are not 

 essentially altered. The chlorine atoms thus occupy the space 

 left vacant by the hydrogen atoms. In the new compound the 

 chlorine to a certain extent plays the same part as the hydrogen 

 did in the original substance. 44 



Laurent endeavours to give expression to the observed facts 

 and to the hypotheses based upon them, in the so-called nucleus 

 theory. 45 This theory is of importance in our science (although 

 it never obtained any general recognition in it) because we have 

 adopted, even if in another form, many of the ideas embraced 

 by it, and also because it was adopted by Gmelin as the basis 

 of the organic portion of his excellent handbook. On this 

 account I shall state the chief points of Laurent's doctrines. 



According to Laurent, all organic substances contain certain 

 nuclei, which he calls either fundamental (radicaux fonda- 

 mentaux) or derived. The former are compounds of carbon 

 with hydrogen, in which the mutual proportion of the number 

 of the atoms is a simple one (i to 2, 3, 4, etc., 2 to 3, etc.). 

 For any definite proportion, several nuclei exist which are 

 polymeric amongst themselves. Besides, these fundamental 

 radicals are so chosen that the hydrogen and carbon atoms 

 contained in them occur in pairs. 



Subsidiary nuclei are formed from the fundamental nuclei 

 by the substitution of other elements for the hydrogen ; for 

 example, chlorine, bromine, iodine, oxygen, nitrogen, etc. 

 Laurent afterwards assumes replacement by radicals or groups 

 of atoms. In such reactions Dumas' rule always holds, that 

 the hydrogen turned out is replaced by equivalent quantities of 



44 I wish to recall the fact here, simply because a question of priority 

 arose with respect to it (compare Comptes Rendus. 10, 409 and 511), that 

 Liebig and Wohler, in their investigation of bitter almond oil, had already 

 assumed that during the formation of benzoyl chloride the chlorine takes 

 the place previously occupied by the hydrogen (compare p. 140). 45 Ann. 

 Chim. [2] 6l, 125 ; compare also Gmelin, Handbuch. 4, 16 ; E. 7, 18. 



