PHOSPHATIC MANURES 139 



combine with the proportionate amount of 

 lime to form^40jUb. of tribasic calcic phos- 



"!. 310x18-32 



phate, the equation being - = 40. 



If 310 be divided by 142 we get 2*2 (nearly), 

 and this figure can be used as a multiplying 

 factor to convert phosphoric acid into terms 

 of phosphate ; or as a dividing factor, in 

 making the change from phosphate to phos- 

 phoric acid. In the basic slag, therefore, 

 that we have assumed, there is 18' 32 per 

 cent, of phosphoric acid, so that a ton holds 

 410 Ib. of this substance. Assuming that 

 the basic slag costs 2, 10s. per ton, free on 

 rail, this will mean that the phosphoric acid 

 is being purchased at the rate of just under 

 l|d. (really T46d.) per pound. It is easy, 

 therefore, to calculate what will be the price 

 of a ton of any other quality that may be 

 quoted at any particular rate per pound of 

 phosphoric acid. 



In this country, however, phosphatic 

 manures are not usually valued in this way, 

 but by the system of units, and we have 

 already seen the working of this method 

 in connection with the valuation of nitro- 

 genous manures. In the case of basic slag 

 holding 40 per cent, of total phosphates, 

 and costing f.o.r. 2, 10s. per ton, the value 



