698 



DR ALEXANDER CRUM BROWN ON AN 



raesoxalates, form a series in which the common difference is CO. In the series 

 — methylic alcohol, ethylenic glycol, glycerine, erythrite, the unknown body 

 C 5 H 5 (HO) 5 , and mannite, as also in the series tartronic acid, tartaric acid, the 

 unknown trioxypyrotartaric acid, and mucic acid, we have a common difference 

 of CH(HO). It is quite possible that relations may be discovered between the 

 successive terms of these and similar series, which will show that they are 

 successive functional series ; and indeed homology has been assumed upon less 

 evidence in cases where the common difference is the usual one CH 2 , as for in- 

 stance in the series of bases C„H 2re _ 5 N (pyridine, picoline, &c). 



It is instructive to compare successive functional series with the independent 

 functional series isomeric with them. Thus, ammonia, methylamine, dimethyla- 

 mine, trimethylamine, with ammonia, methylamine, ethylamine, propylamine : or 

 benzol, toluol, xylol, pseudocumol, &c, with benzol, toluol, ethylbenzol, propyl- 

 benzol, &c. Kekul£, to whom we are greatly indebted for clearness of conception 

 of the structure of the aromatic bodies, calls both of the last-mentioned series 

 "homologous;" as the word "homologous" has never, as far as I know, been 

 strictly defined, there can be no objection to this ; but if we call the series C 6 H 6 , 

 C 6 H 5 (CH 3 ), C 6 H 4 (CH 3 ) 2 , &c. homologous, we must apply the same term to the 

 series ammonia, methylamine, dimethylamine, &c. It will be observed, that while 

 no three successive terms of an independent functional series can belong to the same 

 successive functional series, any two successive terms may. For n] -« and <p" + l] -a 



and i_ 

 4> 



a, and these are the initial terms of the series r | a, 





a. 



•a. Thus, methylamine, dimethylamine, and trimethylamine, 



are (p ns -a, 



2 a cj) n 



are not in successive series, but methylamine and dimethylamine are the initial 



members of the series which continues methyl ethylamine, methyl propylamine, &c. 



If we restrict the term "homologous series" to successive functional series 

 having the common difference CH 2 , we see that one subtance may be the starting 

 point of several homologous series. In this sense, Noad's toluic acid and a toluic 

 acid are both homologues of benzoic acid. The one series being, benzoic acid, 

 methyl benzoic acid (Noad's toluic acid), ethyl benzoic acid, propyl benzoic 

 acid, &c. ; the other being benzoic acid, a toluic acid, the acid C 6 H 5 • CH 2 • CH 2 

 •COHO, &c. 



When <p in the expression cp-x involves x, it is obvious that there can be no 

 common difference in the series a, <p-a, cf> 2, a • • • • (p n, a. 



Such series have not been specially examined, but examples can easily be 

 suggested ; for instance, let <p be the series of processes by which acetic acid is 

 successively converted into acetone, acetonic acid, and isobutyric acid ; then (p 

 can again be applied to the last mentioned acid, giving isobutyrone, isobutyronic 

 acid, &c, thus giving rise to a series of acids, having the formulas, — 



