USEFUL MINERALS. 



masses. A steam-engine is, of course, required 

 to work the winding apparatus and pumps. In 

 some of the mines, the pillars left to support the 

 roof are twenty to thirty feet square, and they are 

 generally at regular distances apart, so that when 

 a deep working is in full operation and lighted up 

 with candles, the effect is striking and brilliant 

 Far more salt is, however, obtained in Cheshire 

 and elsewhere from brine springs than from the 

 rock-salt beds. The pumps used to raise the 

 brine are placed in shafts lined with clay puddle, 

 to prevent the access of fresh water. 



As many as 1,250,000 tons of salt were pro- 

 duced in Cheshire in 1870, rather more than one- 

 tenth of which was rock-salt. In the same year, 

 the yield in Worcestershire was 220,000 tons, and 

 in the Belfast district, 9162 tons. 



In India, the excise-duty imposed upon salt is 

 a considerable source of revenue, yielding in 

 some years ^7,000,000. There, indeed, the 

 production of salt is practically a government 

 monopoly. This is the case also in Austria, where 

 the annual produce is about one-third that of 

 Great Britain. 



Under what circumstances rock-salt has been 

 deposited has not yet been satisfactorily explained. 

 Probably the theory most in favour is, that it was 

 deposited in salt lakes which had no outlet, so 

 that in the course of time, as the water flowing 

 into the lake would be more or less saline, and 

 that which evaporated pure, more salt would 

 accumulate than the water could hold in solution. 

 A deposit of salt would then take place, and 

 eventually become covered by the sediment which 

 now forms beds of marl and other strata. 



Alum. Alum is a sulphate of aluminium and 

 potassium, or the sulphate of potassium may be 

 replaced by the sulphates of ammonium, sodium, 

 or magnesium, or several other substances. Ac- 

 cording to its composition, therefore, alum is 

 distinguished as sodium-alum, ammonium-alum, 

 potassium-alum, and so on. The last two are, 

 however, the common kinds. Several of these 

 alums occur native, but only in small quantities, 

 so that, for use in the arts, it is artificially pre- 

 pared from alum-stone, alum-shale, some coal- 

 measure shales, and pure clay. The best alum 

 is made from the alum-stone of Tolfa, near Rome. 

 Immense quantities of ammonium-alum are now 

 made at Manchester, by treating the common 

 shale of the coal-measures with sulphuric acid and 

 then adding ammonia, obtained from gas liquor. 

 Potassium-alum is also largely made from a similar 

 shale near Glasgow, as well as from a Lias shale 

 at Whitby. Potassium and ammonium alum are 

 equally serviceable in the arts. 



Pure China clay, or pipeclay, treated with sul- 

 phuric acid, and ammonia or potash added, 

 yields an alum easily purified, but the high price 

 of these clays makes it unprofitable to use them. 



A sulphate of aluminium, under the name of con- 

 centrated alum, is manufactured in considerable 

 quantities by treating pure clay with sulphuric 

 acid, ferrocyanide of potassium being used to 

 purify it from iron. It is used by dyers instead of 

 common alum. 



Ordinary alum has a sweetish, astringent taste. 

 It is soluble in cold water to the extent of about 

 five parts in one hundred, but at the boiling-point 

 one hundred parts of water will dissolve about 

 four hundred and twenty parts of alum. This 



substance is extensively used in many manufac- 

 tures, but especially as a mordant or fixing agent 

 in calico-printing. It is also used to prepare the 

 colours called lakes. With it white leather is- 

 tanned or tawed, and tallow is hardened. In- 

 medicine it is used as an astringent; and after 

 wood or paper has been well soaked in it, neither 

 will easily take fire. 



Potassium Nitrate (Nitrate of Potash). This 

 I salt is known in commerce as saltpetre, and some- 

 times as nitre, although the latter term is also 

 applied to the sodium nitrate. Potassium nitrate 

 is composed of potassium oxide and nitric 

 acid. It is an important and valuable substance,, 

 from its being an indispensable ingredient in the 

 manufacture of gunpowder. The principal supply 

 of saltpetre is derived from India, whence we 

 import annually some 200,000 hundredweight. 

 Considerable quantities are also obtained in 

 Arabia, Persia, Spain, and Hungary. In these, 

 and, indeed, in most warm countries, it occurs as 

 an efflorescence on the soil, but it does not pene- 

 trate to any depth. This saltpetre earth is merely 

 lixiviated, and from the solution so obtained the 

 salt crystallises. 



Of late years, considerable quantities of salt- 

 petre have been made by treating the native 

 nitrate of soda, 'Chili saltpetre' (a more abun- 

 dant salt), with carbonate of potassium. In this 

 way carbonate of sodium is precipitated and 

 removed, and a solution of potassium nitrate 

 remains. Besides its use in making gunpowder,, 

 saltpetre is employed in fireworks, in medicine,, 

 and in curing meat It is the sal-prunella of the 

 shops. 



Sodium Nitrate (Nitrate of Soda}. There are 

 various names given to this salt, but those chiefly 

 in use are cubic nitre, cubic saltpetre, and Chili 

 saltpetre. It crystallises in rhombohedrons, while 

 potassium nitrate crystallises in six-sided prisms. 

 Sodium nitrate occurs in enormous quantity in. 

 the district of Tarapaca, Northern Chili, where it 

 is found in beds several feet thick. It is largely 

 consumed in the manufacture of nitric acid and as- 

 a manure for grass land. 



Natron Trona. Two native sodium carbon- 

 ates are found, one called natron, or the hydrated 

 carbonate, and the other termed trona, or the 

 sesquicarbonate. The second contains more soda 

 than the first, and it is also harder and less deli- 

 quescent. Natron occurs abundantly in the soda 

 lakes of Egypt and Trona, not far from Fezzan, 

 and in Barbary. In some parts of India, consider- 

 able tracts of land are rendered quite sterile by 

 an efflorescence of carbonate, but more frequently 

 of sulphate of soda, on the soil. 



Borax. Borax, or sodium borate, is a com- 

 pound of the elementary substance boron, the gas 

 oxygen, and the metal sodium. It is also called 

 tincal, and is obtained in large quantities in the 

 valley of Pugd, Ladakh district, Northern India, 

 and in Tibet In Pugd, the borax-producing 

 locality is under the influence of thermal springs, 

 and the mineral is found in deposits of two or 

 three inches in thickness, lying immediately below 

 an efflorescence of salts of soda. It is transported 

 across the Himalaya on the backs of sheep 

 and goats, and refined at Jagadri, Amritsir, and 

 other places, by simply dissolving it in water, and 

 then evaporating the solution till crystals of borax 

 form. Of this mineral, whose important uses will 



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