332 DR A. CRUM BROWN ON THE CLASSIFICATION OF 



As «£.}, , CAO| Nj a H,O,| Ni JjJH = : }N, fc 



•"■2 ) -"-2 J ' 



In these we have (NH 2 )', or jj j- N common to the group. The generic reactions 



are — 1st, Replacement of one or both atoms of H in NH 2 by radicals. 2d, The 

 addition of two monatomic atoms to each NH 2 , N'" becoming N v . 3d, The replace- 

 ment of N"' by 0" and (HO)' (by the action of nitrous acid), or what comes to the 

 same thing, the replacement of NH 2 by HO. All these affect the radical NH 2 alone, 

 which is therefore the generic radical. In the same way (NH)" is the generic 

 radical of the substances derived from ammonia, by the replacement of two 

 atoms of H. 



The best way of extending this method of classification generally, is to examine 

 the chemical structure of those bodies which form well-marked genera, and see 

 what group of atoms they have in common, and whether the common group (or 

 radical) is that part of the molecule, in which the reactions characteristic of the 

 genus take place. One very well-marked genus is that of the monobasic acids, 

 which have aldehydes and alcohols corresponding to them (Kolbe's " Monocar- 

 bonsauren"). As examples, we may take acetic acid, acrylic acid, benzoic acid, 

 and cinnamic acid. The chemical structure of these substances is represented, as 

 far as known, by the following graphic formulae : — 



® ^ (")(») («)<■) 



©-0-0-0 ©-©-0-9=© 0_0_0=© ©-<Q)-©-(^)=0 

 © © © © © ri'X© AAXKfi 



X X © © y uu @©y 



^^ ^^ (0 (in 



Acetic Acid. Acrylic Acid. Benzoic Acid. Ciunamic Acid. 



-0=0 



The part common to all these acids is obviously the monatomic radical 



(h) 



and the generic reactions take place within it ; the salts are formed by the 

 replacement of the H by metal, — chlorides, bromides, aldehydes, &c, by the 

 replacement of the (HO)', by CI, Br, H, &c. ; alcohols from the aldehydes, by the 



-©:© <? 



addition of H 2 X becoming -©-©-© ; amides, by the replacement of (HO)' 



© 

 by NH 2 ; nitriles, by the replacement of 0"and (HO)' by N"'; acetones, from two 

 molecules, by the loss of CO(HO) in one, and the substitution of the radical 

 thus produced for HO in the other. {(CO(HO)}' is, then, the generic radical of 

 these acids. As this radical is of very great importance, it is advisible to indi- 



