MANURES. 157 



is united with 80 lbs. of sulphuric acid and 56 lbs. 

 of lime in combination with each other, forming 

 136 lbs. of sulphate of lime, or plaster-of-jDaris. 

 The whole compound contains : 



Phosphoric acid 72 lbs. 



Sulphuric acid 80 " 



Lime 84 " 



In all 236 " 



— or, 25^^ per cent, of phosphoric acid. 



The phosphoric acid, now in combination with 

 only one equivalent of lime, is readily dissolved in 

 water, and Avill be evenly distributed in the soil ; but 

 it will take the earliest opportunity to combine with 

 two more equivalents of lime in the soil, and will 

 again become insoluble. It may well be asked. 

 What is the advantage of making it soluble if it is 

 so soon again to become insoluble ? 



The answer to this question is clearly stated in 

 the following quotation from Prof. S. W. Johnson's 

 Essays on Manures : — 



" This white cloud is precipitated bone-phosphate 

 of lime, and does not essentially differ from the 

 original bone-phosphate, except that it is inconceiv- 

 ably finer than can be obtained by any mechanical 

 means. The particles of the finest bone-dust will 

 not average smaller than one-hundredth of an inch, 

 while those of the precipitated phosphate are not 

 more than one twenty-thousandth of an inch in di- 

 ameter. Since the particles of the precipitated phos- 

 phate are so very much smaller than those of the 



