XX11 CONSTITUTION OF CHEMICAL COMPOUNDS. 



constitution as hydrochloric acid, i. e., it consists of one 

 monovalent atom united with another monovalcnt atom, 

 thus H.H and C1.C1 ; and, in order that the compound 

 H.C1 may be formed, it is necessary that the union of 

 atoms already existing be broken up ; which is accom- 

 plished by virtue of the stronger affinity of the hydrogen- 

 atom for the chlorine-atom, than of the hydrogen-atom for 

 hydrogen-atom, or the chlorine-atom for chlorine-atom. 

 Thus the affinities, as stated, are not free, but the instant 

 they become free they are taken up, neutralized, saturated 

 by those of other atoms present. 



In the simple cases which we have considered, viz., hy- 



H 

 drochloric acid H.C1, water H.O.H, ammonia N*H and 



H, 



TT TT 



marsh gas jj'.C^jj the constitution of the compounds is 



plainly indicated by the formulae. By replacing one or all 

 of the elements in the above formulae by other elements of 

 equal valence, we have the formulae of a number of known 

 compounds : 



H.C1 similar to H.Br, Na.Cl, K.Br, etc. 



H.O.H " K.O.H, Cl.O.H, C1.O.C1, H.S.H, etc. 



.H .K .H .01 .H 



N-H " N-H P-H P-C1 As-H etc. 



H -H, -H, -01, -H, 



H. H Cl H Cl .01 Cl 01 C1. S ..C1 t 

 H--H H- C -H, H--H, Cl--H, Cl- -H, 



But by far the greater number of chemical compounds 

 are more complicated in constitution, and may be looked 

 upon as formed by the replacement of one or all of the 

 elements in the above formulae by atomic groups, which 

 have the same valence as the replaced atoms. The four 

 fundamental formulae, inasmuch as they illustrate the func- 

 tions of the elements of different valence, may be conveni- 

 ently employed for the purpose of comparison with more 

 complicated formulae with the object of rendering the ex- 

 planation of the latter more simple. 



If we take any of the fundamental formulae, and divide 

 them at any part, we obtain two residues of equal valence. 

 For instance, if we divide H.C1, we obtain H and Cl, both 

 monovalent ; if we divide H.O.H, we obtain H and OH, 

 and these are both monovalent, for, as can be readily seen, 

 the group OH requires a monovalent atom or group in 



