296 CHEMICAL MANURES. 



per cent of carbonate of potash and 6 per cent of carbonate of soda, 

 and sodium chloride. To appreciate the economic bearing of this 

 idea, it will suftice to recall that plants can only draw potash from 

 the soil, and that by growing potash-loving plants too often on the 

 same ground it would soon become exhausted and sterile, unless the 

 amount of potash extracted by the plants were restored. But then 

 that would be going round in a bad circle : furnishing potash to the 

 soil, as say potassium chloride, only to convert it into carbonate of 

 potash in the plant which must be extracted by burning. It is un- 

 necessary to dwell further on the subject. 



Until comparatively recently plants were the sole source of 

 potash, and as the industrial consumption of this plant was formerly 

 much greater than to-day (they did not then know that soda could 

 replace potash in most of its applications), the product was always 

 insufficient to meet the wants of industry. 



About the end of the eighteenth century the invention of Le- 

 blanc, which consisted in extracting soda from common salt, chloride 

 of sodium came to deliver the farmer from the incalculable tribute 

 which he was obliged to pay annually to industry under the form of 

 potash. Some industries, however, especiall}^ glass manufacture, 

 continue to use potash owing to the impossibility of their replacing 

 it by soda. Attempts were then made to reduce the potash which 

 occurs in abundance in a great number of insoluble and difficultly 

 soluble minerals such as granite, porphyry, potash-felspar with 

 16"6 per cent of potash. But soon the discovery and the exploitation 

 of the enormous deposits of potash salts of Stassfurt rendered this 

 useless. Now, and for some time back, potash, or better carbonate 

 of potash (for the word potash is no longer used except to denote 

 the impure product), has been made in considerable quantities by 

 the Leblanc process. The raw material used, the kainit, hard salt 

 (p. 299), is supplied by the Stassfurt mines. 



Stassfurt Salts. — In the Stassfurt mines very soluble potash 

 salts occur in sufficient quantity to meet the wants of industry for an 

 unlimited period. Far from having recourse, henceforth, to the 

 farmer to borrow potash, industry is enabled to restore to him the 

 enormous quantities which it had taken from him in the course of 

 centuries. Encouraged by the geognostic conditions of the Stassfurt 

 region, the first boring was begun on 3 April, 1839, in the hope 

 of finding new deposits of common salt, the production of which was 

 insufficient in the district. In the month of June, 18i3, the first 

 portions of salt were brought to the surface, and in 1851 a bed of 

 salt 325 metres thick had been passed without reaching the bottom. 

 The borehole was then 581 metres deep. The salt extracted from 

 the borehole in 1843 had a density of 1*205, and the deeper the bed 

 was penetrated the more the density increased. The analysis of 

 the product gave the following results : — 



