136 



AGRICULTURAL CHEMISTRY 



of testing for carbon dioxid. If a small amount of 

 any material supposed to contain carbonates is placed 

 in a test-tube along with a little water, and then a small 

 glass tube or loop-tube containing a few drops of lime 

 water is inserted in the test-tube, after the gas is liberated 

 by hydrochloric acid, the drop of lime water becomes 

 turbid, due to the formation of CaCO 3 (see Fig. 57). 



170. Calcium Sulfate. Deposits of this salt known as 

 gypsum, CaSO 4 .2H 2 O, are found abundantly in some lo- 

 calities. Gypsum or land plaster is used as a fertilizer 

 and also for the preparation of plaster of Paris. The 

 ' 'setting" of plaster of Paris is due to the fact that when 

 the water of crystallization is expelled, the substance is 

 again capable of taking up water, expanding, and form- 

 ing a hard mass. 



171. Calcium Chlorid. This salt is not found in nature 

 to any appreciable extent. It is employed in the labora- 

 tory in desiccators and for the drying of gases. 



172. Bleaching=Powder. This mate- 

 rial is made by passing chlorin into a 

 solution of lime water. The chlorin is 

 held in chemical combination, forming 

 calcium hypochlorite, Ca(ClO) 2 , which 

 readily gives up its chlorin and is exten- 

 sively used for bleaching and disinfect- 

 ing purposes as explained in Section 89. 



173. Calcium Phosphate. Deposits 

 Fig. 58. Apatite rock. o f this material are found in nature in 

 various physical forms as soft phosphate, and in crystalline 



