134) Boyal Society. 



this very complicated subject. According to this theory, there are 

 three sets of phosphates, in which the oxygen of the acid being 5, 

 the oxygen in the base is respectively 3, 2, or 1 ; the remaining 

 equivalents of oxygen, in the two first cases, being supplied by that 

 portion which exists in the 2 or 3 equivalents, respectively, of the 

 basic water, which water is wholly absent in the third case. These 

 three classes of salts Professor Graham proposes to term, respect- 

 ively, monobasic, bibasic, and tribasic salts. Professor Graham has 

 extended these views of the basic formation of water in salts to the 

 case of the sulphates, in a paper communicated to the Royal Soci- 

 ety of Edinburgh, and published in the 13th volume of their Trans- 

 actions, on " Water as a constituent of Salts*." The principal ob- 

 iect of this paper, however, Avas to show that water exists in a dif- 

 ferent state in certain salts, and does not exercise a true basic func- 

 tion, being capable of being replaced by a salt, and not by an alka- 

 line base, and giving rise to a class of double salts. This inquiry 

 was suggested by the tendency of phosphate of soda to unite with 

 an additional dose of soda, and form a subsalt,vf\\ic\\ had been traced 

 to the existence of basic water in the former. The result was, that 

 in the well-known class of sulphates, consisting of sulphates of mag- 

 nesia, zinc, iron, manganese, copper, nickel and cobalt, all of which 

 crystallize with either five or seven equivalents of water, one equi- 

 valent proved to be much more strongly united to the salt than the 

 other four or six. The latter, to which the name of water of cry- 

 stallization should be restricted, may generally be expelled by a heat 

 tinder the boiling point of water ; while the remaining equivalent 

 uniformly requires a heat above 400° of Fahrenheit for its expul- 

 sion, and seems to be, in a manner, essential to the salt. Thus in 

 the double sulphate of zinc and potassa, the single equivalent of 

 water, existing in the sulphate of zinc, is replaced by an equivalent 

 of sulphate of potassa, while the six equivalents of water of crystal- 

 lization remain ; and all the other salts of this class combine with 

 one another in a similar manner. 



The super-sulphates must also be regarded as analogous to dou- 

 ble salts ; the bisulphate of potassa, for example, being a sulphate 

 of water and potassa. 



There is likewise a provision in the constitution of hydriated sul- 

 phuric acid for the production of a double salt analogous in its con- 

 stitution to sulphate of zinc. Sulphuric acid, of the specific gravity 

 1*78, contains two equivalents of water, and is capable of crystal- 

 lizing at a temperature of 40" of Fahrenheit, being, in fact, the only 

 known crystallizable hydrate of sulphuric acid. The second equi- 

 valent of water, contained in the hydrated acid, is capable of being 

 replaced by an equivalent of sulphate of potassa, which is itself a 

 salt, and a bisulphate of potassa is the result of this substitution. 

 But the first equivalent of water can be replaced only by an alkali 

 or true base. Professor Graham distinguishes water in these two 



* The paper here referred to will be found also in Lond. and Edinb. 

 Phil. Mag., vol. vi. p. 327 et seq. — Edit. 



