ALKALIES. 239 



and the acids being destroyed by their action on each other ; also by 

 igniting nitre in a crucible of gold. 



3. PREPARATION OF POTASSA, OR PURE POTASH. 



Take 1 part potashes, or pearl ashes, and good quick lime 2, with 

 abundance of water ; boil for an hour, in an iron or copper kettle, till 

 the fluid neither effervesces with acids, nor precipitates lime water.* 

 Strain it through a coarse brown towel, stretched on a frame with ten- 

 ter hooks, and hot water should be repeatedly passed through, until we 

 have used ten times as much as the weight of the carbonate of pot- 

 ash employed. The caustic fluid may be put up in black bot- 

 tles, and allowed to settle over night ; the next morning it may be 

 drawn off by a glass syphon. To avoid burning the rnouth, the sy- 

 phon tube may be filled with water, and the finger being pressed up- 

 on the mouth of the longer leg, the shorter may be dexterously turn- 

 ed into the bottle's mouth, without breaking the column in the sy- 

 phon, the water in which maybe allowed to run off, 

 and the fluid is then saved for evaporation. 



In general, filtering succeeds badly with caustic 

 alkalies, unless very weak, as they are apt to corrode 

 the filters, and paper can scarcely be used, unless 

 for small assays. If the filtering is slow, the car-- 

 bonic acid of the air is apt to combine with the alkali, 

 and to prevent this, Mr. Donovan contrived the an- 

 nexed apparatus, in which A is the filtering funnel, 

 whose mouth is obstructed by folds of linen ;f D is 

 the receiving vessel, and c is a connecting tube, to 

 prevent, at once, any communication with the exter-D| 

 nal air, and any accumulation of pressure in the 

 lower vessel. { 



(b.) Boil the solution^ down to dryness in a clean iron kettle ; fuse 

 the mass in a silver crucible ; pour it out on a marble slab ; break it up, 

 without delay, and cork it tight from the air in a glass bottle. Cream 

 of tartar, ignited in a crucible, dissolved in water, filtered, boiled 

 with sufficient lime, obtained clear by subsidence and decantation, 

 and solid by evaporation in a silver vessel, to the consistence of oil, 

 gives a cake of the pure hydrate of potassa, without the trouble of 

 using alcohol. It must be put up immediately, in close bottles. Ure. 



* Taking care, provided the solution of alkali is strong, to dilute it with pure 

 water; otherwise it may precipitate the lime, by seizing the water, and thus give a 

 delusive indication. 



t Better by fragments of glass, coarser below and finer and finer above ; water is 

 passed through, both before and after an experiment, to remove impurities, and thus 

 a permanent filter is obtained for acids and other corrosive fluids. D. O. 



t Ann. Phil. 26, 115, and Turner, 2d Ed. p. 405. 



For a table shewing the real quantities of alkali in aqueous solutions, see Henry. 

 Vol. I, p. 528, 10th London Ed. 



