xxiv CONSTITUTION OF CHEMICAL COMPOUNDS. 



six free affinities ; if three atoms unite, C OO the result- 



* 5 



ing group must have eight free affinities, etc.; and, as this 

 chain combination may be continued indefinitely, and the 

 free affinities may be saturated by the greatest variety of 

 groups, it is evident that the number of compounds, the 

 possibility of whose existence is thus indicated, is un 

 limited. The atoms of oxygen also possess this property 

 of uniting with each other to form chains, as we see in the 

 compounds : 



H.O.O.H, Cl.O.O.H, Cl.O.O.O.H, Cl.O.O.O.O.H, 

 Br.O.O.O.H, etc. 



In these cases the oxygen-atoms, which with one of their 

 affinities are united with hydrogen, impart to this hydro 

 gen-atom. characteristic properties ; and, whenever this 

 kind of combination is found, we say, for convenience 

 sake, the compound contains hydrox}^. 



Examples of compounds formed by the chain-combina 

 tion of different elements of the same valence are the fol 

 lowing: H.O.S.O.O.H, H.O.O.S.O.O.H, in which we have 

 sulphur and oxygen, forming a continuous chain. 



Having thus seen the various methods of combination 

 of atoms, let us briefly illustrate the applications of these 

 forms to the characteristic classes of compounds. 



In the first place, chemists have long recognized the 

 existence of two classes of compounds, bases and acids, 

 the representative members of which have, in certain re 

 spects, opposite or complementary properties. The larger 

 number of acids, as well as bases, contain hydrogen and 

 oxygen ; and either all or a part of the hydrogen-atoms 

 contained in them are united with oxygen. To the pre 

 sence of these hydrogen-atoms or of the hydroxyl groups 

 (see above), of which they form a part, are due the proper 

 ties which distinguish the compounds as acids or bases. 

 Examples of acids and bases are : 



C1.0.H=C1(OH), K.O.II=K(OH). 

 H.O.O.S.O.O.H = S0 2 (OH) 2 , H.O.Ca.O.H = Ca(OH) 2 . 



0:C .O.H = IICO(OH), H-C-C-O.H = C 2 IP(OII). 



