CALCIUM. 155 



of water of crystallization. By heating to about 115 C. (239 F.) 

 most of this water is expelled, and a nearly anhydrous sulphate 

 formed. This article readily recombines with water, becoming a 

 hard mass, for which reason it is used for making moulds and casts, 

 and in surgery. If the gypsum is heated to a considerably higher 

 temperature than the one mentioned, all water is expelled, and the 

 product thus obtained combines with water but very slowly. 



Tricalcium phosphate, Calcii phosphas praecipitatus, Ca 3 (PO 4 ) 2 

 = 310 (Precipitated calcium phosphate, Phosphate of Time, Bone- 

 phosphate). By dissolving bone-ash (bone from which all organic 

 matter has been expelled by heat) in hydrochloric acid, and precipi- 

 tating the solution with ammonia water there is obtained calcium 

 phosphate, which contains traces of calcium fluoride and magnesium 

 phosphate. 



A pure article is made by precipitating a solution of calcium 

 chloride by sodium phosphate and ammonia : 



2Xa 2 HPO, + SCaCL, + 2NH 4 OH = Ca^POJ., + 4NaCl + 2NH 4 C1 + 2H 2 O. 



It is a white, tasteless, amorphous powder, insoluble in cold water, 

 soluble in hydrochloric or nitric acids. 



Superphosphate, or acid phosphate of lime. Among the inorganic sub- 

 stances which serve as plant-food, calcium phosphate is a highly important 

 one. As this compound is found usually in very small quantities as a con- 

 stituent of the soil, and as this small quantity is soon removed by the various 

 crops taken from a cultivated soil, it becomes necessary to replace it in order 

 to enable the plant to grow and to form seeds. 



For this purpose the various phosphatic rocks (chiefly calcium phosphate) 

 are converted into commercial fertilizers, which is accomplished by the addi- 

 tion of sulphuric acid to the ground rock. The sulphuric acid removes from 

 the tricalcium phosphate one or two atoms of calcium, forming mono- or 

 dicalcium phosphate and calcium sulphate. The mixture of these substances, 

 containing also the impurities originally present in the phosphatic rocks, is 

 sold as acid phosphate or superphosphate. 



Bone-black and bone-ash. Phosphates enter the animal system 

 in the various kinds of food, and are to be found in every tissue and 

 fluid, but most abundantly in the bones and teeth. Bones contain 

 about 30 per cent, of organic and 70 per cent, of inorganic matter, 

 most of which is tricalcium phosphate. When bones are burned until 

 all the organic matter has been destroyed and volatilized, the result- 

 ing product is known as bone-ash. If, however, the bones are sub- 

 jected to the process of destructive distillation (heating with exclusion 



