_Superphosphate; Dissolved Bones._--These names were at first applied to bones which had been treated with sulphuric acid; but superphosphates are now rarely made from bones alone, but bone ash and some of the mineral phosphates just described are employed, either along with them, or very frequently alone. The manufacture of superphosphates depends on the existence of two different compounds of phosphoric acid and lime, one of which contains three times as much lime as the other. That which contains the larger quantity of lime is found in the bones and all other natural phosphates, and is quite insoluble in water; but when two-thirds of its lime are removed, it is converted into the other compound, which is exceedingly soluble. This change is effected by the use of sulphuric acid, which combines with two-thirds of the lime of the ordinary insoluble phosphate of lime, and converts it into the _biphosphate of lime_, which is soluble. When, therefore, we add to 100 lbs. of common phosphate of lime the necessary quantity of sulphuric acid, it yields 64 lbs. of biphosphate, containing the whole of the phosphoric acid, which is the valuable constituent, the diminution in weight being due to the removal of the valueless lime. Hence it follows, also, that as the lime so removed is converted into sulphate, there must, for every 100 lbs. of phosphate of lime converted into biphosphate, be produced 87 lbs. of dry sulphate of lime, or 110 of the ordinary sulphate called gypsum. This is the minimum quantity which can be present, but in actual practice it is liable to be greatly exceeded, more especially where coprolites are used, owing to the large amount of carbonate of lime they contain, which is also converted into sulphate by the action of the acid, so that it is far from uncommon to find the gypsum twice as great as it would be if materials free from carbonates could be obtained. By employing a sufficiency of sulphuric acid, the whole quantity of phosphoric acid in the bones may be thus brought into a soluble state, but in actual practice it is found preferable to leave part of it in the insoluble condition; as where it is entirely soluble, its effect is too great during the early part of the season, and deficient at its end. In order to dissolve bones, bone ash, or mineral phosphates, they are mixed with from a third to half their weight of sulphuric acid, of specific gravity 1.70 or 140 deg. Twaddell. When mineral phosphates, and particularly coprolites, are used, the quantity of sulphuric acid must be increased so as to compensate for the loss of that which is consumed in decomposing the carbonate of lime they contain. When operating on the small scale, the materials are put into a vessel of wood, stone, or lead (iron is to be avoided, as it is rapidly corroded by the acid), and mixed with from a sixth to a fourth of their weight of water, which may with advantage be used hot. The sulphuric acid is then added, and mixed