210 



Oti Superphosphate of L ime. 



parts of absolute phosphate of lime, we must employ 45 parts of 

 absolute sulphuric acid, and we shall obtain 68 parts of bi-phos- 

 phale of lime and 77 parts of sulphate of lime. But though we 

 have spoken of abstracting or separating a certain quantity of 

 lime from the phosphate, it is plain that this is only to be under- 

 stood in a chemical sense ; the bi-phosphate and the sulphate of 

 lime reinain together, and the product is a mixture of these two 

 substances. 



It is also plain^ from what has already been said, that there is a 

 definite quantity of acid required to convert a definite quantity of 

 phosphate of lime into bi-phosphate — if less than this quantity is 

 employed, some of the phosphate remains unacted on and un- 

 changed — limore than enough is added, then the more powerful acid 

 (the sulphuric) proceeds to take the lime from the bi-phosphate ; 

 and if the acid be used in great excess the products would eventu- 

 ally be a mixture of sulphate of lime with uncombined phosphoric 

 acid, and a certain excess of sulphuric acid also uncombined. 



I have before said that bi-phosphate of lime can be prepared by 

 dissolving phosphate of lime in phosphoric acid. The substance 

 so prepared is exceedingly soluble in water, but capable of being 

 crystallized by evaporation : the solution of bi-phosphate of lime is 

 acid ; when brought in contact with lime, potash, or soda in the 

 caustic state, or with the carbonates of these bases, a portion of the 

 phosphoric acid is neutralised, and the phosphate of lime is pre- 

 cipitated and rendered insoluble. This point deserves great atten- 

 tion, for it explains what happens to the soluble phosphate of lime 

 in the soil. We have before seen that we can convert the neutral 

 and insoluble phosphates of lime into the bi-phosphate or soluble 

 compound, by removing some of its lime, and conversely we can 

 retransform this soluble phosphate into the insoluble condition by 

 the addition of lime or its carbonate. But by so doing we do not 

 recover the phosphate of lime in the same form ; it is, indeed, 

 still comparatively insoluble in water, but it is in the bulky sub- 

 divided state in which it is easily attacked by the weakest acids. 

 These facts serve to explain the nature of the advantage which 

 superphosphate of lime possesses over phosphate of lime that has 

 not been treated with acids. It has been frequently supposed 

 that the superphosphate of lime from its solubility passed as such 

 directly into the roots of plants, but there is every reason to be- 

 lieve that this opinion is entirely erroneous. In the first place it 

 is very unlikely that a substance, possessed of powerful acid pro- 

 perties, could enter, without injuring them, into the delicate roots 

 of plants; and, in the second, it is clear that the superphosphate 

 of lime, when it once becomes incorporated with and dissolved by 

 the moisture of the soil, must meet with lime or other bases, and 

 be speedily neutralized. If, indeed, the value of this substance 



