MINERAL DEPOSITS OF FERTILIZERS 25 



are just rebuilt on top of the old debris, whilst in other 

 places the buildings are erected a little apart from the old 

 site. There is, therefore, a tendency for these villages to be 

 somewhat raised above the general level of the plain, owing to 

 the accumulations of mud and refuse for, possibly, thousands 

 of years. The decaying organic refuse in these sites under- 

 goes the process of nitrification, and drains into some of the 

 lower levels. The e.arths from which the crude saltpetre is 

 obtained contain very varying amounts of nitrate, as little 

 as i % and as much as 29 % having been observed, but few 

 beds of nitre earth contain more than 5 %. In addition to 

 nitrate, the earth always contains sodium chloride and sodium 

 sulphate in considerable amounts. 



The general method of extraction consists in building 

 a small earthen chamber, with a hole in front to allow the 

 nitrate liquors to drain away. A false bottom of bamboos 

 and matting is placed in this earthen chamber. The earth 

 is laid on this false bottom, and trodden into position with 

 much care. Small quantities of wood ashes are very fre- 

 quently mixed with the earth during the process of filling. 

 Water is now poured on to the surface, in such a way as 

 to avoid disturbing the earth. As the water soaks into 

 the earth, the clay begins to swell, and the rate of drainage 

 becomes very slow. The water soaks down slowly, and after 

 the lapse of a few hours, reaches the bamboo floor and drains 

 away out of the hole in the front. As the water slowly 

 trickles downwards, the most soluble salts are in excess and 

 produce a saturated solution, whilst the less soluble salts are 

 to a large extent left behind. At the same time, double 

 decomposition takes place between any calcium nitrate 

 in the earth and any potassium carbonate in the ashes with 

 which the earth has been mixed. As the.se earths always 

 contain much organic matter, the drainage is usually coloured 

 brown. The composition of the drainage liquors varies 

 considerably, but is given approximately in Table 4. 



