138 AGRICULTURE 



determine the quantity of phosphoric acid 

 contained in 1000 kilograms, say a ton, of 

 the substance to be valued ; and, knowing the 

 price per ton, one arrives at the cost per 

 pound. This system may be illustrated by 

 taking the case of basic slag, which is put on 

 the market of different qualities, the phos- 

 phoric acid usually varying between 10 and 

 20 per cent., corresponding to a percentage 

 of phosphates of about 22 to 44. A common 

 type of basic slag is one that holds 37 to 42, 

 say 40, per cent, of total phosphates, which 

 corresponds to a percentage of phosphoric 

 acid of 18'32. The relationship between 

 phosphoric acid and total phosphates may 

 be explained as follows. The chemist, in 

 analysing a phosphatic manure, determines 

 the amount of phosphoric acid, and this, by 

 calculation, he converts into tribasic calcic 

 phosphate, the substance, namely, that is 

 meant when one uses the shorter term 

 *' phosphate " or " phosphates." The basis 

 of the calculation is the chemical constitution 

 of tribasic calcic phosphate, 310 Ib. of 

 which contain 168 Ib. of lime, united 

 chemically with 142 Ib. of phosphoric acid. 

 If, therefore, 142 Ib. of phosphoric acid can 

 combine with 168 Ib. of lime to form 310 Ib. 

 of tribasic calcic phosphate, 18*32 Ib. will 



