58 Existence of Compound Radicals in Amphide Salts. 



acid and base really exist in the salt formed by the action of hy- 

 drated acids on a base, is purely theoretical. 



" When we compare the constitution of a neutral salt with that 

 of the hydrated acid by which it is formed, we find the positive 

 result to be the substitution of a metal for the hydrogen of the 

 latter, thus, S0 3 -f HO gives with zinc S0 3 -fZnO; and where 

 a metal is acted on by an hydrated acid, the hydrogen is thus 

 evolved either directly as gas, or it reacts on the elements of the 

 acid and gives rise to secondary products which are evolved, such 

 as sulphurous acid, nitric oxide, &c. In all cases we may con- 

 sider the action of a metal on a hydrated acid, to be primarily 

 the elimination of hydrogen and the formation of a neutral salt. 

 But in this respect the action becomes completely analogous to 

 that of the metal on a hydracid, except that in the latter case a 

 haloid salt is formed, and hence we assimilate the two classes in 

 constitution by a very simple arrangement of their formulae. 



" There are, however, a number of acids which may be ob- 

 tained in a dry and isolated form, as the sulphuric, the silicic, the 

 telluric, the stannic, the arsenic, the phosphoric, &c., and when 

 they combine with bases, it is most natural to consider the union 

 as being direct, and that the salt contains acid and base really as 

 such. This is accordingly the strongest point of the ordinary 

 theory. But other and important circumstances intervene. These 

 acids, although they may be obtained free from water, yet in that 

 state they combine with bases but very feebly, and require a high 

 temperature in order to bring their affinities into play. On the 

 other hand, in all cases where these bodies manifest their acid 

 characters in the highest degree, they are combined with water, 

 as in oil of vitriol and phosphoric acid, and when expelled from 

 combination with a base, they immediately enter into combina- 

 tion with water in an equivalent proportion. Thus where phos- 

 phate of lime is decomposed by oil of vitriol, it is not phospho- 

 ric acid (PO 5 ) which is found in the liquor, but its terhydrate 

 (P0 5 4-3HO), as is shown by its forming with oxide of silver 

 the yellow phosphate PO 5 -f-3AgO. In the case of telluric acid, 

 its hydrate (Te0 3 + 3HO) is very soluble in water, it crystallizes 

 in large prisms ; by 212° two atoms of water are given off, but 

 its nature is not changed, the body which remains (Te0 3 -fHO) 

 is still acid and soluble in water, perfectly neutralizing the alka- 

 lies ; but by a red heat this last atom of water is driven off, and 



