TREATMENT OF BONES. 105 



with sulphuric acid. This treatment, as already ex- 

 plained in the first part of this volume, gives us the 

 double, or bi-phosphate of lime, and if continued by- 

 further additions of sulphuric acid, the substance 

 called by fertilizer men "superphosphate." In this 

 we have most of the phosphoric acid in a soluble 

 form, or immediately " available," and just in this 

 form it exists in our high-grade fertilizers. We 

 have no means of counteracting the natural ten- 

 dency of the free phosphoric acid to "revert" 

 whtm applied to the soil. But this is not usually a 

 serious matter. The " reverted " phosphoric acid is 

 again subject to a chemical action and decomposi- 

 tion in the soil, and therefore may well be con- 

 sidered available, even if not soluble in distilled 

 water. Under average circumstances the reverting 

 process is but slow, and the crops have a good op- 

 portunity to help themselves to the free article. 

 The presence of free lime in the soil, of course, 

 accelerates the process of reversion, and where su- 

 perphosphate is used, or to be used, lime should 

 not be applied. 



We have seen that the sulphuric acid treat- 

 ment, by which the phosphoric acid is made immedi- 

 ately soluble, also results in the formation of 

 sulphate of lime. Consequently, the more soluble, 

 and therefore more valuable, the phosphoric acid in 

 bone phosphate, the greater is the quantity of sul- 

 phate of lime contained in it. 



The phosphoric acid in bones can also be made 

 partially available by burning, either in open fire 

 or in closed vessels. By the latter 

 procedure we obtain what is called 

 " bone-black." The result of burning bones is chief- 

 ly phosphate of lime, whithout nitrogen. To make 



