CALCIUM. 279 



Experiment 28. Add to about 10 grammes of marble (calcium carbonate), in 

 small pieces, hydrochloric acid as long as effervescence takes place ; filter the 

 solution of calcium chloride thus obtained and add to it solution of sodium 

 carbonate as long as a precipitate is formed, collect the precipitate on a filter, 

 wash and dry it. 



Dried calcium sulphate, Calcii sulphas exsiccatus, CaSO 4 =; 

 135.15 (Dried gypsum, Plaster-of-Paris, Calcined plaster). It has 

 been mentioned above that the mineral gypsum is native calcium 

 sulphate in combination with 2 molecules of water of crystallization, 

 By heating to about 115 C. (239 F.) about three-fourths 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 higher temperature than the one men- 

 tioned, all water is expelled, and the product thus obtained combines 

 with water but very slowly. 



Precipitated calcium phosphate, Calcii phosphas preecipitatus, 

 Ca s( p 04)2 = 307.98 (Phosphate of lime). By dissolving bone-ash 

 (bone from which all organic matter has been expelled hy heat) in 

 hydrochloric acid, and precipitating 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 : 



2Na 2 HPO 4 + 3CaCl 2 + 2NH 4 OH = Ca 3 (PO 4 ) 2 + 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 



