5'9 



the liquor. The diminishing liquid is from time to time reple- 

 nished by fresh supplies. The impurities that rise are care- 

 fully skimmed off. On attaining a certain degree of concen- 

 tration the impure saltpetre is copiously precipitated. This 

 impure precipitate is scooped out from the bottom of the pan 

 at intervals and heaped by the side of the boiler. After 30 

 to 36 hours, of continuous labour 8 to 16 Ibs. of crude saltpetre 

 are made ; the larger pans yielding up to 30 Ibs. Solar heat is 

 also used for evaporating the liquor. This crude saltpetre 

 is exported to Europe and refined there. The principal 

 impurities are Chlorides of Potassium and Sodium. Weight for 

 weight, sodium nitrate is a richer manure than potassium 

 nitrate in as much as it contains 7 per cent, more nitric acid. In 

 the manufacture of gunpowder, however, potassium nitrate is 

 in use, but for the manufacture of nitric acid, sodium nitrate. In 

 India, potassium nitrate being much cheaper, it is the best 

 nitrogenous manure to use, the potassium also being a valuable 

 plant- food. 



827. Nitrogen is absorbed by plants more readily in the 

 form of nitrate than in any other form. In water-culture ex- 

 periments nitrates are relied upon as the best source of nitro- 

 gen. Ammonia salts are less certain. Nitrates chiefly promote 

 the growth of leaves and impart to them a rich green colour. 

 In ,Peru, crude nitrate of soda is found incrusting the soil 

 of a desert. Hence this article rather than saltpetre is 

 chiefly used for manurial purpose in America and Europe. 

 Scrapings from pucca walls or damp and dirty limestone 

 buildings are rich in nitrate of lime which is also a good 

 manure. 



828. Saltpetre, Chili-saltpetre or sodium nitrate and 

 calcium nitrate should be applied as manure mixed and 

 diluted with some other substance such as water, loam or dung, 

 at the rate of 100 to 150 Ibs. per acre, mixed with 3 or 4 

 tim.es. as much dung or 10 times as much loam or water. The 

 application should be only as top dressing when the plants 



