POTASH. 445 



boiled together for half an hour in a clean iron pan. The lime 

 abstracts carbonic acid from the potash, and becomes carbonate 

 of lime ; a reaction which may be illustrated by adding lime- 

 water to a solution of carbonate of potash, when a precipitate of 

 carbonate of lime falls. When the potash has been deprived 

 entirely of carbonic acid, a little of the clear liquid taken from 

 the pan will be found not to effervesce upon the addition of an 

 acid to it. It is remarkable that the decomposition is never 

 complete, if the carbonate of potash be dissolved in less than 

 the prescribed quantity of water. Liebig has observed that a 

 concentrated solution of potash decomposes carbonate of lime, 

 and consequently hydrate of lime could not, in the same circum- 

 stances, decompose carbonate of potash. The pan being co- 

 vered by a lid, may be allowed to cool ; as the insoluble carbo- 

 nate of lime and the excess of hydrate of lime subside, a con- 

 siderable quantity of the clear solution of potash may be drawn 

 off by a syphon, and the remainder may be obtained clear by ni- 

 tration. In the latter operation a large glass funnel may be em- 

 ployed, to support a filter of washed cotton calico, into which 

 what remains in the pan is transferred. A small portion of liquid, 

 which passes through turbid at first, should be returned to 

 the filter. As the solution of potash absorbs carbonic acid, it is 

 proper to conduct its filtration with as little exposure to air as 

 possible ; on which account the mouth of the funnel should be 

 covered by a plate, and the liquid which flows from it be im- 

 mediately received in a bottle, in the mouth of which the funnel 

 may be supported. The bottle in which potash is preserved 

 should not be of crystal, or of a material containing lead, as the 

 alkali corrodes such glass, particularly when its natural sur- 

 face has been cut. 



To obtain the solid hydrate of potash, the preceding solution 

 is rapidly evaporated in a clean iron pan or silver bason, till an 

 oily liquid remains at a high temperature, which contains no 

 more than a single equivalent of water. This liquid is poured 

 into cylindrical iron moulds, to obtain it in the form of sticks, 

 which are used by surgeons as a cautery, and are the potassa or 

 potassa fusa of the pharmacopeia ; a form in which it is also 

 convenient to have potash for some chemical purposes. The 

 sticks generally contain a portion of carbonate of potash, be- 

 sides a little oxide of iron and peroxide of potassium, the last 

 of which gives occasion to the evolution of a little oxygen gas 



