554 PRINCIPLES OF CHEMISTRY 



temperate climates during the summer months, fertile soils produce a 

 small quantity of nitre. In this respect India is especially known as 

 affording a considerable supply of nitre extracted from the soil. The" 

 nitre-bearing soil after the rainy season sometimes becomes covered 

 during the summer with crystals of nitre, formed by the evaporation 

 of the water in which it was previously dissolved. This soil is col*' 

 lected, subjected to repeated lixiviations, and treated for nitre as will 

 be presently described In temperate climates nitrates are obtained 

 from the lime rubbish of demolished buildings which have stood for 

 many years, and especially from those portions which have been in con- 

 tact with the ground. The conditions there are very favourable for 

 the formation of nitre, because the lime used as a cement in buildings 

 contains the base necessary for the formation of nitrates, while the 

 excrement, urine, and animal refuse are sources of nitrogen. By the 

 methodical lixiviation of this kind of rubbish a solution of nitrogenous 

 salts is formed similar to that obtained by the lixiviation of fertile 

 soil. A similar solution is also obtained by the lixiviation of the so 

 c"alled nitre plantations. They are composed of manure interlaid with 

 brush-wood, and strewn over with ashes, lime, and other alkaline 

 rubbish. These nitre plantations are set up in those localities where 

 the manure is not required for the fertilisation of the soil, as, for 

 example, in the south-eastern 'black earth f Governments of Russia. 

 The same process of oxidation of nitrogenous matter freely exposed to 

 air and moisture during the warm season in the presence of alkalis 

 takes place in nitre plantations as in fertile soil and in the walls of 

 buildings. From all these sources there is obtained a solution con- 

 taining various salts of nitric acid mixed with soluble organic matter. 

 The simplest method of treating this impure solution of nitre is to 

 add a solution of potassium carbonate, or to simply treat it with 

 ashes containing this substance. The potassium carbonate entejs into 

 double decomposition with the calcium and magnesium salts, forming 

 insoluble carbonates of these bases and leaving the nitre in solution. 

 Thus, for instance, K 2 C0 3 + Ca(NO 3 ) 2 == 2KNO 3 -I- Ca00 3 . Both 

 calcium and magnesium -carbc-nates are insoluble, and therefore after 

 treatment' with potassium .carbonate the solution no longer contains 

 salts of these metals but only the salts of sodium and potassium 

 together with organic matter. The latter partially separates on 

 heating in an insoluble form, and is entirely destroyed by heating the 

 nitre to a low red heat, The nitre thus obtained is easily purified by 



accomplished by the. aid of peculiar micro-organisms (ferments), without which the 

 simultaneous action of the other necessary conditions (alkalis, moisture, a temperature 

 of 87, airland nitrogenous substances) cannot give nitre. 



