142 Fertilisers [pt. hi 



it has not proved as useful as basic slag or superphos- 

 phate and has not justified its popularity. 



Steamed bone flour contains less nitrogen, but so far 

 as the phosphate is concerned it has the advantage that 

 it is very finely divided and can readily be distributed. 

 It gives good results on light alluvial loams. 



Dissolved bones resemble superphosphate in their 

 action but are on the whole more expensive and less 

 satisfactory. 



Superphosphate 



On May 23rd, 1842, Lawes patented his process for 

 manufacturing superphosphate and thus founded the 

 artificial fertiliser industry which has since attained 

 enormous dimensions. The principle of the process is 

 very simple: rock phosphates (themselves of no great 

 fertilising value in this country) are treated with sul- 

 phuric acid so as to convert the tricalcic phosphate 

 Ca3(P04)2 into the more soluble compound to which the 

 formula Ca(H2P04)2 is assigned: in addition calcium 

 sulphate is formed. The following is the usual expres- 

 sion of the reaction ; it is not, however, strict^ correct : 



Ca3(P04)2 + 2H0SO4 = 2CaS04 + Ca(H2P04)2. 



The mixture of calcium sulphate, monocalcic phos- 

 phate and some free phosphoric acid^ constitutes the 

 superphosphate. No separation is attempted, and the 

 calcium sulphate or gypsum is left in : it not only docs 

 no harm but has itself some fertilising value and indeed 

 was much used in the past: it also serves to get the 

 superphosphate into a dry condition because it absorbs 

 water very completely. The process has attained a con- 

 siderable degree of perfection, and allows of the pro- 

 ^ J. H. Coste, Journ. Soc. Chem. Ind. 1897, xvi. 195. 



