MILLS FOR GRINDING . 523 



ing feldspar for the pottery would also be suitable for preparing 

 it for agricultural purposes. Cushman says: 



"For fertilizer purposes the fine grinding of feldspar could be 

 done in iron mills similar to those which are used for grinding 

 limestone in the cement industry. The only important points to 

 consider would be the percentage of total potash present and the 

 fineness of grinding. At the present time there are few data 

 available on the cost of grinding feldspar to a 2OO-mesh powder, 

 but with modern machinery there is little doubt that it can be 

 done much more economically than would have been considered 

 possible only a few years ago. Under the stimulus of the cement 

 industry a great development has been made in recent years in 

 the methods and art of fine grinding. The following table is 

 of interest, as it shows at a glance what the potash in ground 

 feldspar would cost if the percentage is compared with a cost 

 of grinding varying from $i to $10 per long ton. 



PRICE PER POUND OF POTASH UNIT IN FELDSPAR. 



The prices are given in cents per pound, so that if, for in- 

 stance, rock carrying eight per cent, of potash could be de- 

 livered for $9 per ton, the potash contained in it would be added 

 to the land at a cost of 5 cents per pound. At $5 per ton the 

 cost per pound would fall to 28 mills. The figures are of course 

 only applicable provided the potash in the ground material can 

 be proved available as a plant food." 



On this subject Cushman makes the following observation: 

 "It must be remembered that the only real measure of avail- 

 able potash is that which is made use of by the crop. It is not 



