On Superphospliate of Lime, 



209 



of the phosphate in acetic acid. As the only difference between 

 the neutral and bi-phosphate is in the relative proportion of its 

 in2:redients, there will manifestly be two methods by which we 

 might set about converting the one into the other. Either we 

 may add phosphoric acid, or we may take away lime. 



The latter method is the only one practicable in a manufac- 

 turing sense, and it is accomplished in this way : — Phosphoric 

 acid is what chemists call a weak acid, that is to say, it does not 

 unite with alkalies and earths in so powerful a manner as some 

 other acids. Sulphuric acid, on the contrary, is a very powerful 

 acid, and is capable of depriving a weaker acid than itself of any 

 base with which it may be united. If sulphuric acid be added 

 to phosphate of lime it immediately lays hold of the whole or of 

 a portion of the lime according to the quantity employed, and 

 the phosphoric acid is either liberated or remains united with a 

 smaller quantity of lime. 



In order to make this part of the subject understood, it will be 

 necessary to put a case. 



Let us suppose that 100 parts or pounds of chemically pure 

 phosphate of lime are taken. This quantity contains 48^ parts 

 of phosphoric acid. The first point is to ascertain how much 

 lime must be taken away to convert this neutral phosphate into 

 superphosphate. It was seen before that 100 parts of pure 

 bi-phosphate contain 71^ parts of phosphoric acid. 



If, then, 7H parts of phosphoric acid produce 100 of bi-phos- 

 phate, 48^ parts (or the quantity contained in 100 of the neutral 

 phosphate) will be equal to 68 parts of bi-phosphate. In other 

 words, 100 parts of neutral phosphate can be converted into 68 

 parts of the bi-phosphate : and this change is to be effected by 

 the abstraction of 32 parts of lime. 



If we have succeeded in making this plain, it will be seen 

 that a given quantity of neutral phosphate is capable of furnish- 

 ing a somewhat less quantity of superphosphate, and the differ- 

 ence between the first and the second is the quantity of lime to 

 be removed. The next point to decide is the quantity of sul- 

 phuric acid to be employed for this purpose ; and in speaking 

 of this, I shall treat it quite irrespectively of the state in which 

 the acid occurs in practice, and as if we were dealing with an 

 absolutely pure and undiluted substance. The practical bearing- 

 of these laws will be discussed subsequently. 4:1^ parts of lime 

 combine with 58 J parts of sulphuric acid to produce 100 parts of 

 sulphate of lime. Now, to convert 100 parts of phosphate into 

 superphosphate we have found that it is required to abstract or 

 separate 32 parts of lime ; and it will be found that these 32 

 parts to be separated will require 45 parts of acid. 



So that the result of our calculations is, that for every 100 



VOL. XII, P 



