140 AGRICULTURE 



of a unit is got by dividing the price per 

 ton by the total percentage of phosphates, 

 namely 40, when the price per unit will be 

 found to work out at Is. 3d. Using this 

 unit value to assess the corresponding price 

 of other samples, we should find, for in- 

 stance, that a slag guaranteed to contain 

 35 per cent, of phosphates would be worth 

 2, 3s. 9d. per ton ; while one holding only 

 28 per cent, of phosphates would, at the 

 same rate of unit valuation, be worth l, 15s. 

 per ton. It is probably hardly necessary 

 to point out that the more concentrated 

 slags have some value beyond their " equi- 

 valent " price, where railway charges and 

 cartage are heavy. On the other hand, low- 

 grade slags contain more free lime, and 

 buyers may be inclined to allow something 

 for this. 



During the past few years the opinion of 

 chemists has gone in the direction of valuing 

 slag, not upon its total contents of phos- 

 phoric acid or tribasic calcic phosphate, 

 but only upon the quantity soluble in a 

 2 per cent, solution of citric acid. Basic 

 slag is not soluble in water, and although 

 its phosphate becomes available with con- 

 siderable rapidity, it cannot be made use 

 of by plants until it has been dissolved by 



