298 Mr Graham on Water as a Constituent of Salts. 



the circumstance that many salts usually hydrated, are likewise 

 capable of existing in a crystalline state without water. 



In the hydrates of the caustic alkalies and of the earths, 

 water is retained by a strong affinity, and is generally supposed 

 to be united, like an acid, to the alkali or earth. In such hy- 

 drates, water discharges an acid function. 



In the case of hydrates of the acids, the portion of water 

 which is found to be inseparable by heat, or to be very strongly 

 retained, has generally been presumed to be in the place of a base 

 to the acid, although little attention has been paid to the sub- 

 ject. The most highly concentrated sulphuric acid retains one 

 atom of water, and is supposed to be a sulphate of water. In 

 the case, too, of such a supersalt as bisulphate of potash or bitar- 

 trate of potash, the single atom of water which is known to be 

 persistently attached to the salt, has been viewed of late, by our 

 most enlightened chemical theorists, as essential to its constitu- 

 tion, and the possibility admitted that such salts may really be 

 double salts ; the bisulphate of potash, a sulphate of potash com- 

 bined with sulphate of water, and the bitartrate of potash, a 

 tartrate of potash combined with tartrate of water. 



In a late publication, I have developed this view of water 

 acting as a base in the case of phosphoric acid. That acid is 

 capable of combining with water in three different proportions ; 

 and the number of atoms of water with which the acid combines 

 at any time, depends upon circumstances which are understood. 

 That the water is basic in these different hydrates, follows from 

 the fact, that, on treating them with an alkali, the water is con- 

 stantly replaced by a quantity of alkali chemically equivalent to 

 the water. By nitrate of silver, the same precipitate is thrown 

 down from any phosphate of soda and from the corresponding 

 phosphate of water ; the composition of the precipitate being de- 

 termined in both cases by the same double decomposition. The 

 peculiarity of phosphoric acid is, that it is capable of uniting 

 with water as a base, in several proportions, while all other acids 



