SALTPETRE PHOSPHATES. 149 



able as a fertiliser, inasmuch as it supplies both potassium and 

 nitrogen in a directly available form, is too expensive to be 

 used as a manure except under special circumstances. Like 

 sodium nitrate, it occurs as a deposit in rainless districts in the 

 tropics, especially in India. It was also made by the so- 

 called " Nitre plantations" in France and other countries. 

 These consisted of heaps of earth, old mortar, road scrapings 

 rich in calcium carbonate, &c., mixed with decomposing animal 

 matter, protected from the rain by a shed and placed on an 

 impervious floor. The heap was watered periodically with 

 urine, liquid manure, or other liquid rich in animal nitro- 

 genous matter. Nitrification under these favourable condi- 

 tions took place rapidly and the liquid draining aw r ay contained 

 large quantities of nitrates of potassium and calcium. At long 

 intervals the heap was lixiviated with water and the solution 

 mixed with wood ashes or potassium carbonate, when calcium 

 carbonate was precipitated and removed and potassium nitrate 

 was extracted from the solution by evaporation and crystalli- 

 zation 



K a CO + Ca(N0 8 ) a = CaC0 3 + 2KN0 8 . 



It is probable that the saltpetre of India owes its origin to 

 a similar action of nitrification, the potash coming from the 

 minerals in the soil. 



Potassium nitrate is now made in large quantities from 

 sodium nitrate and potassium chloride, which when mixed in 

 solution and evaporated yield, first sodium chloride crystals, 

 and then, on cooling, potassium nitrate. 



The properties of saltpetre are well known and need not be. 

 described here. 



II. PHOSPHATIC MANURES. Several of the manures 

 already described are mainly valued for the phosphates they 

 contain ; this is the case with the non-nitrogenous guanos, 

 steamed and burnt bones, &c. There are other sources of 

 phosphoric acid of greater importance, which must now be 

 described. Before mentioning the manures themselves, it 

 may be advisable to briefly describe the various forms in 

 which phosphoric acid occurs in fertilisers. These are 



