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 states of combination as basic and saline water. Thus the hydrate of sulphuric acid, already mentioned, contains one equivalent of basic, and one equivalent of saline water. It is, in his nomencla- ture, a sulphate of water with saline water, as the hydrous sulphate of zinc is a sulphate of zinc with saline water. The bi-sulphate of potassa is also a sulphate of water with sulphate of potassa, and cor- responds with the double salt of sulphate of zinc with sulphate of potassa.
The results which Professor Graham has thus obtained, and which he has communicated, partly to the Royal Society, and partly to the Royal Society of Edinburgh, suggested to him the probability that the law with respect to water in the constitution of the sul- phates would extend to any hydrated acid, and the magnesian salt of that acid ^ and his researches on this extension of the subject constitute the substance of his last communication to the Royal So- ciety. As he had already found that the sulphate of water is con- stituted like the sulphate of magnesia, so he now finds oxalate of water to resemble the oxalate of magnesia, and the nitrate of water to resemble the nitrate of magnesia. He is moreover of opinion, that this correspondence between water and the magnesian class of ox- ides extends beyond their character as bases, and that, in certain subsalts of this class, the metallic oxide replaces the water of cry- stallization of the neutral salt, and discharges a function which wa& thought peculiar to water.
The same kind of displacement, which takes place in the formation of a double sulphate by the substitution of a salt of the same class for an equivalent of water, appears to occur likewise in the constitution of double oxalates ; and the application of this principle elucidates the constitution of that class of salts, as well as of the super-oxalates, and to explain the mode in which they are derived.
