124 PHOSPHATIC MANURES [chap. 



Lawes and sold at about £'j a ton, was a mixture of 

 soluble and insoluble phosphate derived from coprolites 

 or from guanos, mixed with animal substances and am- 

 moniacal salts, resembling, in fact, dissolved bones and 

 very much the kind of thing nowadays sold as soluble 

 bone compound. Way in 1851 gives analyses ranging 

 from 3-24 to 0-I2 per cent, of nitrogen, soluble phosphate 

 of lime from 18-5 down to i-6 per cent., insoluble from 

 28-3 down to o-o6 per cent. By 1862 the manufacture 

 had risen to 150,000 to 200,000 tons per annum ; in 1907 

 about 700,000 to 800,000 tons were made in the United 

 Kingdom, of which about 120,000 tons were exported. 



In the manufacture of superphosphate, the finely- 

 ground materials, graded and mixed after analysis so as 

 to produce superphosphate of the desired quality, are 

 mixed with sufificient dilute oil of vitriol, containing 

 about 60 per cent of pure acid, to bring about the 

 following reaction :-— 



Ca3P208 + 2H2S04 + 4H20 = CaH^PgOg + sCaSO^, 2H2O. 



At the same time, an excess of acid must also be em- 

 ployed to convert into sulphates the carbonate, fluoride, 

 and chloride of calcium present. The mixing is per- 

 formed mechanically, a considerable rise of temperature 

 ensues, and there is an evolution of water vapour, car- 

 bonic, hydrochloric, and hydrofluoric acids ; the two latter, 

 besides causing a waste of sulphuric acid, are trouble- 

 some to the manufacturer, because they must be con- 

 densed, and not allowed to escape into the atmosphere. 

 After mixing, the hot damp mass is dropped into a 

 lower chamber where the reaction completes itself, and 

 the whole solidifies as the gypsum combines with the 

 remaining water. The mass is easily friable, and is 

 dug out, crushed, and put into store. At one time 

 artificial drying had to be employed, in order to obtain 



