IV. J THE PHOSPHATES OF CALCIUM 105 



solution in acids it is little used in manure-making. 

 The typical phosphate of lime, which is regarded as 

 the starting-point for the manures, is the tricalcium 

 phosphate CagPgOg, which is supposed to exist in bone 

 ash and in the natural uncrystalline phosphate rock, 

 such as is mined in Algeria or Florida. It is, however, 

 doubtful if such a phosphate really exists in any stable 

 condition ; it has been shown that bone ash and such 

 phosphates, when treated with water, continue to yield a 

 little phosphoric acid to solution and become more and 

 more basic ; crystals of the composition CagPgOg do not 

 exist, nor can a substance corresponding to this formula 

 be precipitated. This, however, is an academic ques- 

 tion ; in all dealings with manures tri-calcium phosphate 

 is supposed to exist, and whatever the actual compound 

 in the manure may be, the quantity of phosphoric acid 

 is always expressed as if it were combined as tri-calcium 

 phosphate. Thus 310 parts of calcium phosphate are 

 equivalent to 142 parts of phosphoric acid, hence what- 

 ever the percentage of phosphoric acid found by analysis 



it is multiplied by 2- 18 {=' — ) and expressed as 



percentage of tri-calcium phosphate. This puts all 

 phosphatic manures on an equal basis and enables a 

 comparison to be made of one against the other, just as 

 would the percentages of phosphoric acid which are really 

 obtained by analysis, but which are not in favour with 

 manufacturers because, being so much smaller numbers, 

 they seem to give the manure too poor a showing. 



When a solution of phosphoric acid, such as is 

 obtained by treating any of the natural phosphates with 

 sulphuric or hydrochloric acid, is precipitated with lime 

 water, a salt of the composition CaHPO^, di-calcium 

 hydrogen phosphate, is obtained, and this is a perfectly 

 stable and definite compound. It sometimes comes on 



