CONSTITUTION OF CHEMICAL COMPOUNDS. XXlii 



order that it may become saturated, and this is what we 



.H 

 understand by a monovalent group. If we divide N-H 



H, 



we obtain H and NH 2 , or H 2 and NH ; by the former 

 division there are left two monovalent, by the latter two 



TT TT 



bivalent factors. And so in the case of TT'C'TT if we divide 



H* *xi ; 



this formula, the following cases are possible : H and 

 CH 3 , H 2 and CH 2 , H 3 and CH, leaving in the first case two 

 monovalent, in the second, two bivalent, and in the third, 

 two trivalent factors. This principle may be carried out 

 further in connection with other and more complicated for- 

 mulae, and so are obtained the formulae of a great variety 

 of these so-called residues; in most cases, however, the 

 division made, and the residues resulting, may be com- 

 pared to the simpler forms described. We speak of a 

 water-residue, OH, which, on account of the exceedingly 

 important part it plays in the constitution of chemical 

 compounds, has received a distinct name, hydroxyl ; the 

 ammonia-residue, NH 2 , is called amide; the residue, NH, 

 is called imide ; the residue, CH 3 , of marsh gas, is called 

 methyl ; the residue, CH 2 , methylene, etc. etc. 



If we now operate with the groups mentioned instead of 

 with atoms alone, we shall find that we are able to build 

 up a larger number of formulae representing compounds, as 

 follows : 



.H .OH ^:0 .CH 3 .OH .CH 3 



N-H similar to N H, -OH, N -H P-OH P-CH 3 



H - H, -H, -OH, -CH 3 , 



etc. 



similar to ; c ; ;c: O:C: H N;C.OH, 



- 0:C:* etc.' ' ; 



Still further complications are introduced when, instead 

 of compounds consisting of atoms of different valence, we 

 have atoms of the same element, or of different elements 

 of the same valence, united together, forming chains. Ex- 

 amples of the first kind are met with particularly in the 

 case of carbon. If two carbon atoms unite in the simplest 



manner possible, we have a group -C-C-, which must have 



