134 METALS AND THEIR COMBINATIONS. 



is the native calcium sulphate in combination with 2 molecules 

 of water of crystallization. By heating to about 115 this water 

 is expelled, and the anhydrous sulphate formed. It readily re- 

 combines with water, becoming a hard mass, for which reason 

 it is used for making moulds and casts, and in surgery. 

 For the latter purpose plaster is often mixed with some alum 

 and gelatine before adding the water, this mixture being pre- 

 ferred on account of forming a harder, less porous mass, with 

 a smooth surface that can be washed with water containing 

 disinfecting agents. 



Tricalcium phosphate, Calcii phosphas praecipitatus, Ca 3 2P0 4 = 310 



(Precipitated phosphate of calcium, Phosphate of lime, Bone-phos- 

 phate}. By dissolving bone-ash (bones from which all organic 

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

 cipitating the solution with ammonia water a calcium phosphate 

 is obtained, which contains traces of calcium fluoride and mag- 

 nesium phosphate. 



A pure article is made by precipitating a solution of calcium 

 chloride by sodium phosphate and ammonia : 



2Na 2 HPO 4 -f 3CaCl 2 + 2NH 4 HO = Ca 3 2PO 4 + 4NaCl + 2NH 4 C1 + 2H 2 O. 



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

 soluble in all acids. 



Superphosphate, or acid phosphate of lime. Among the inor- 

 ganic substances which serve as plant-food, calcium phosphate 

 is a highly important one. As this compound is usually found 

 in very small quantities as a constituent 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 addition 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 phos- 

 phate and calcium sulphate. The mixture of these substances, 

 containing also more or less of the impurities originally present 



