232 HISTORY OF CHEMISTRY. [LECTURE XII. 



a glimpse of the operation of this law amongst the more com- 

 plex organic groups, which led Laurent and Dumas to the 

 enunciation of the theory of types ; and had not those distin- 

 guished chemists extended their views beyond the point to 

 which they were well supported by then existing facts had they 

 not assumed, that the properties of an organic compound are 

 dependent upon the position and not upon the nature of its 

 single atoms, that theory would undoubtedly have contributed 

 to the development of the science to a still greater extent than 

 it has already done ; such an assumption could only have 

 been made at a time when the data upon which it was founded 

 were few and imperfect : and, as the study of the phenomena 

 of substitution progressed, it gradually became untenable, and 

 the fundamental principles of the electro-chemical theory again 

 assumed their sway. The formation and examination of the 

 organ o-metallic bodies promise to assist in effecting a fusion of 

 the. two theories which have so long divided the opinions of 

 chemists, and which have too hastily been considered irre- 

 concilable ; for, whilst it is evident that certain types of series 

 of compounds exist, it is equally clear that the nature of the 

 body derived from the original type is essentially dependent 

 upon the electro-chemical character of its single atoms, and 

 not merely upon the relative position of those atoms." 41 It is 

 then pointed out, in conclusion, how " Stibethin furnishes us, 

 therefore, with a remarkable example of the operation of the 

 law of symmetrical combination above alluded to, and shows 

 that the formation of a five-atom group from one containing 

 three atoms, can be effected by the assimilation of two atoms, 

 either of the same, or of opposite electro-chemical character." 42 

 Frankland, consequently, gives up the idea of coupling, 

 and now regards cacodyl as sulphide of arsenic in which both 

 sulphur atoms are replaced by methyl. He had now adopted, 

 although in a somewhat different form, the theory of types, 

 and if he considers that he really differed from the decided 

 adherents of this theory, inasmuch as he did not assume, with 



41 Phil. Trans. 1852, 441 ; Annalen. 85, 368. 42 Phil. Trans. 1852, 

 442 ; Annalen. 85, 371. 



