20S 



On Superphosphate of Lime. 



Bat it is possible to dissolve the coprolite in muriatic acid, 

 and the addition of any alkali to the acid liquid will cause a 

 bulky precipitate, which is phosphate of lime, precisely the same 

 in composition as it was before solution, but differing evidently 

 in the bulky and light character it has acquired. This precipi- 

 tated phosphate is readily soluble in acetic acid. I call special 

 attention to this point, because upon it hinges the whole question 

 of the manufacture of superphosphate of lime. 



You may have the same chemical substance in two different 

 mechanical states, which materially alter its relations to solvents. 



Some kinds of aggregated phosphate of lime are insoluble in 

 a weak acid like vinegar; on the contrary, when in a light, bulky, 

 highly divided form, they become readily soluble in the same 

 acid ; and what is true of one solvent is relatively true of another 

 — if by chemical action you bring the phosphate of lime into a 

 condition in which it dissolves readily in any given weak acids 

 which it did not before, it stands to reason that you in the same 

 proportion increase its solubility in other acids of a similar 

 character. And so it comes to pass, that the natural solvent in 

 the soil — water impregnated with carbonic acid — will readily dis- 

 solve the precipitated phosphate, whilst it might have little or no 

 influence on the original mineral. And it is to be observed that 

 no mechanical powdering of the coprolite practically within our 

 reach will accomplish the change of form which is so easily 

 brought about by simple chemical means. 



Bi-phosphate of Lime. — This combination of lime and phos- 

 phoric acid, which exists in well-made superphosphate of limiC, is 

 not met with naturally either in a state of purity or otherwise ; 

 it can only be prepared by artificial means. The acid or bi- 

 phosphate of lime contains in 100 parts, according to the ana- 

 lysis of Berzelius — 



Phosphoric Acid . . . 7H 

 Lime 28^* 



100 



Now, if we make a comparison of the composition of these two 

 products we shall readily see that the bi-phosphate of lime con- 

 tains much more phosphoric acid than the neutral phosphate — it 

 may, indeed, be considered as a solution of neutral phosphate in 

 phosphoric acid, just as we have spoken but now of a solution 



* Experiments have been made in my own laboratory by digesting precipitated 

 phosphate of lime in phosphoric acid until the acid liquid refused to dissolve any 

 further portion, and the result has accorded very closely with the above numbers. 

 We may safely believe therefore that Avhen phosphoric acid acts freely upon phosphate 

 of lime, in a proper state of subdivision, the liquid Avill contain a compound such as 

 we have described. 



