LECTURE XII.] HISTORY OF CHEMISTRY. 23! 



and equivalent ; but still it also possesses over the latter 

 system, certain advantages which are to be found especially in 

 the greater importance that is attached to the formulae, and in 

 the breaking up of the radicals containing carbon into simpler 

 ones. 



I have just pointed out that, in Kolbe's opinion, the radical 

 (or element) with which a substance is conjugated has only 

 a subordinate influence upon the nature of the compound; 

 Frankland attacks this doctrine in i852, 40 and he succeeds in 

 convincing Kolbe that it cannot be maintained. 



Frankland justifies his views by reference, especially, to the 

 radicals containing metals. In the coupling of arsenic with 

 methyl, the former, according to Frankland, changes its saturat- 

 ing capacity. Whilst it possesses in the free state, the capacity 

 for uniting with five atoms of oxygen, the highest stage of 

 oxidation of cacodyl contains only three atoms of this element. 

 The remaining organo-metallic compounds give occasion to 

 similar considerations, and in consequence Frankland is led 

 to make the following important observations: "When the 

 formulae of inorganic chemical compounds are considered, 

 even a superficial observer is struck with the general symmetry 

 of their construction ; the compounds of nitrogen, phosphorus, 

 antimony and arsenic especially exhibit the tendency of these 

 elements to form compounds containing three or five equivs. 

 of other elements, and it is in these proportions that their 

 affinities are best satisfied ; thus in the ternal group we have 

 N0 3 , NH 3 , NI 8f NS 3 , P0 8 , PH 3 , PC1 3 , SbO 3 , SbH 3 , SbCl 3 , AsO 3 , 

 AsH 3 , AsCl 3 , etc. ; and in the five-atom group, NO 5 , NH 4 O, 

 NH 4 I, PO 5 , PH 4 I, etc. Without offering any hypothesis re- 

 garding the cause of this symmetrical grouping of atoms, it is 

 sufficiently evident, from the examples just given, that such a 

 tendency or law prevails, and that, no matter what the character 

 of the uniting atoms may be, the combining power of the 

 attracting element, if I may be allowed the term, is always 

 satisfied by the same number of these atoms. It was probably 



40 Phil. Trans. 1852, 417; Annalen. 85, 329. 



