344 Preparations and Compositions. Part III.

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the mixture shall entirely cease to effervesce ; then evapo-rate to dryness •, and having increased the fire a little, bringthe saline mass cautiously into a state of fusion. Dissolvethe salt, after it has cooled, in water: filter the solution,and evaporate, until, on removing it from the fire, it shallconcrete into a crystalline mass, which should be very white.Put this, as quickly as possible, into vessels accurately clo-sed.

This is both a troublesome and expensive preparation j for,when attempted to be made by simply evaporating to dryness,the salt has always a dark unpleasant colour, which can nei-ther be removed by repeated solution and crystallization, noieven by solution in alcohol. It is doubtful to what the colouris owing. It has been ascribed by some to part of the aceticacid being decomposed by heat during the exsiccation of thesalt: they accordingly recommend the evaporation to be con-ducted very gently, and the pellicles to be skimmed fromthe surface of the liquor as fast as they are formed ; and inthis way, they say, they have procured, at once, a very whitesalt. Others again ascribe it to accidental impurities, con-tracted during the operation, and recommend the utmost at-tention to cleanliness, and the use of earthen vessels; whileothers ascribe it to some foreign matter, which rises in distil-lation with the last portions of the acetous acid, and thereforedirect, that only the first portions which come over should beused, or that the acetous acid should be distilled with charcoal.The last opinion appears to be the most probable, since, whenacetic acid procured from the distillation of an acetate is em-ployed, a colourless solution is obtained, and solutions whichbecome coloured do not at the same time become alkaline.But to whatever cause it be owing, the colour is most effec-tually destroyed by fusing the salt. The heat necessary to dothis decomposes the colouring matter ; and on dissolving thefused mass in water, and filtering the solution, we find a finelight charcoal on the filter. But this fusion is attended withconsiderable loss ; for part of the acetic acid itself is decom-posed.

To ascertain the exact saturation, litmus and turmericpaper should be alternately employed. Mr Phillips says,that rather more than 21 pints o fdistiiled vinegar, of 1.007,are required to saturate 18 ounces of subcarbonate of po-tass.

The operator must be particularly careful, in melting i),not to use a greater heat, nor keep it longer liquefied, than