Chap. 7.] Natural Hift cry of common Salt. 35 



obtain its acid by heat alone, but this is troublefome, 

 and requires veffels capable of enduring great heat. 

 It is moft readily decompofed by the vitriolic acid, 

 which has the ftrongeft attraction for the alkali. The 

 nitrous acid will alfo decompofe it, but is more liable 

 than the vitriolic acid to come over with the muriatic. 

 Some water muft be put into the retort with the vi- 

 triolic acid, otherwife the muriatic acid will come over 

 in fumes fo copioufly as to burft the vefTels. The 

 quantity of the water mould be about half that of the 

 vitriolic acid ; one half of which fhould be put into 

 the receiver to condenfe the fumes of the muriatic 

 acid, and the other half mixed with the vitriolic to 

 prevent its too fudden action. Glauber's fait remains 

 after this operation, and this is the ufual way of prepar- 

 ing it. The way to decompofe marine fait, fo as to 

 obtain its alkali, is, firft to expel the muriatic acid by 

 means of the nitrous; we thus obtain cubic nitre, 

 which muft be deflagrated to obtain the alkali. A 

 better method, however, is to mix a folution of ma- 

 rine fait with cauftic vegetable alkali j by evaporation 

 muriat of pot-am is obtained, and the remaining 

 water contains the mineral alkali, pure and difen- 

 gaged. 



. Common fait is the moft ufeful of faline bodies ; 

 for though there are fome which refift putrefaction 

 equally well, there is none which is fo friendly and 

 agreeable to the human ftomach. Its agreeable qua- 

 lities are not indeed confined to man ; moft other ani- 

 mals indicate a great fondnefs for it. The rock fait, 

 or fal gem, though only one form of common fait, 

 does not melt fo eafily as the common cryftals. The 

 moft remarkable mine of this fait is at Cracow, in 

 Poland, where there is thought to be fufficient to fup- 

 ply the whole world n\any thoufand years. In this 

 D 2 mine 



