NITRATE OF POTASH.

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to the process of solution and filtration. This is effected by a large mudfilter, lined on the inside with stiff' clay.” It has a false bottom ofbamboo, covered with close wrought grass mats, on which are placedvegetable ashes. Upon these the nitrous earth is laid. Water is thenadded to dissolve the saline matters of the earth, and the solution thusobtained, filtering through the mats, drops into the empty space betweenthe real and false bottom, and is conveyed away into an earthen receiver.In its passage through the wood-ashes the carbonate of potash containedin the latter re-acts on the nitrate of lime of the solution, and producesnitrate of potash and carbonate of lime. The solution is afterwardsevaporated in earthen pots, filtered, and put aside to crystallize. Theimpure nitre thus procured is termed dhouah: it contains from 45 to 70per cent, of pure nitrate of potash. It is re-dissolved and crystallized bythe native merchants, who supply the Calcutta bazaars, and when thuspurified is called by the natives kalmee. (See Stevenson, op. cit. ; alsoIndia Jour, of Med. and Phys. Science, new series, vol. i. p. 10, 1836.)

Saltpetre is imported into this country principally from Calcutta , butsome comes from Madras. It is brought over in cloth bags which containfrom 150 to 175 lbs. each. Its quality varies considerably. It is alwaysmore or less impure: but the common varieties, which have a dirtyyellowish appearance, are termed rough or crude saltpetre, while the purerand cleaner looking kinds are called East India refined. The loss whichit suffers in refining, or in other words the impurities which it contains,are technically designated refraction. This varies greatly in differentsamples, but is usually between 5 and 15 per cent. (For the methods ofdetermining it, consult Dumas, Trait'c de Chimie, t. 2 me . p. 762 ; andBrande’s Manual of Chemistry , 4th ed. p. 549.)

Nitre is purified by repeated crystallization. When it has been dis-solved and crystallized once only it is called singly refined nitre: whentwice, doubly refined. Its purity may be ascertained by testing it withnitrate of silver, chloride of barium, and oxalate of ammonia. Thefirst detects the chlorides, the second the sulphates, and the third thecalcareous salts.

2. From artificial sources. —The artificial preparation of nitre is prac-tised in several parts of Europe . The establishments in which it iscarried on are called artificial Nitrieres. The mode adopted varies,however, in different places.

At Appenzel, a canton of Switzerland , nitre is formed from the urineof animals. A hole is dug near to stables, and in this • is put a sandykind of earth, which is kept moistened with the water running from thestables. In two or three years this earth yields nitre.

In Sweden , where each landed proprietor is compelled to furnish acertain quantity of nitre, it is prepared as follows:—Decomposing animaland vegetable matters, mixed with cinders, lime, or marl, are placed inheaps (called nitre beds) under cover, the mass being occasionally moved,or holes made in it, so that they are exposed to the air. From time totime they are watered with urine (a liquid containing more nitrogen thanany other animal substance.) At the end of two or three years the nitro-gen has combined with oxygen, and this with bases to form nitrates. Bylixiviation the salts may be separated, and any nitrate of lime presentmay be converted into nitrate of potash by adding wood-ashes, whichcontain carbonate of potash.