( 331 ) 



XXV. — On the Classification of Chemical Substances, by means of Generic 

 Radicals. By Alexander Crum Brown, M.D., D.Sc. 



(Read 5th February 1866.) 



The idea of chemical structure, as founded on that of atomicity (or the equiva- 

 lence of atoms), enables us to divide any molecule, whose chemical structure is 

 known, into radicals. The number of ways in which this may be done increases 

 with the complexity of the molecule. Each of these modes of division corre- 

 sponds to a series of conceivable reactions, some of which have been observed. 

 Any one of these series may be made the basis of classification ; but it is obviously 

 most convenient to select for this purpose the most characteristic reactions, and 

 those which are common to such substances as form natural groups. In studying 

 these, we find that each series implies the presence of a particular radical, 

 within which the reactions in question take place. We may call such series of 

 reactions the Generic reactions, and the corresponding radicals Generic radicals. 

 These are sometimes residues of double decomposition, but very frequently this 

 is not the case, and this may account for the fact, that the importance of these 

 generic radicals has been very much overlooked. 



I shall consider some of the cases in which this principle of classification is 

 already, to some extent, recognised, before proceeding to apply it generally, and 

 examine first those examples furnished by groups of bodies which are referred to 

 the types H 2 and NH 3 . 



I. A large number of the substances referred to the type H 2 0, are formed by 

 the replacement of one atom of H in each molecule of H 2 0, by a radical, as, — 



\0 C 2 H 5l C 2 H 3°1 A) So}°&c 



V}0, H 2 )0, 



The part common to all such bodies is the radical (HO) 7 or jj j- 0, and the reactions 



common to the group affect this radical alone. These reactions are — 1st, The 

 replacement of (HO)'; 2d, The replacement of H in (HO/; and 3d, The 

 replacement of in (HO)'. Thus (HO) may be replaced by CI, Br, &c, or (HO), 

 by ; (HO)' may become (KO)', (C 2 H 5 0)', &c. by the replacement of H, or (HS) , 

 (HSOJ', &c. by replacement of 0. (HO)' is therefore the generic radical of this 

 large genus. 



II. An extensive series of substances referrible to the type NH 3 have only 

 , one atom of H in each molecule of ammonia replaced. 



VOL. XXIV. PART II. 4 u 



