442 A Chemical Andlyjis of Three Species ofWhinJlone^ and Two of Lam. 



In making the fixed alkalis cauftic, it is ufual to boil them with the lime ; but as moft 

 kinds of limeftone contain a fmall quantity of filex or argil, and as thefe earths, when in 

 a ftate of divifion, are foluble in boiling cauftic alkali, there is a probability, when heat is 

 applied, of its being thus rendered impure. This is my reafon fo/ carefully avoiding heat 

 in the firft part of tlie procefs ; and I have not found that lime made from chalk, or from 

 the purer kinds of limeftone *, give any impurity whatever to the alkali, when mixed witli 

 it cold. I 



The folution, in its weak ftate, is firft evaporated in a bafon of hammered iron, polifhed ; 

 but when it is fomewhat concentrated, I carry on the evaporation in a difii made of the 

 pureft filver, reduced from luna cornea. After being boiled to a fmall quantity, it is al- 

 lowed to cool, and then put into, a well-ftopped bottle for fome days ; during which, if 

 the evaporation has been continued long enough, the neutral falts cryftallize, as Mr. 

 Lowifz has pointed out. Afterwards, the folution is carefully decanted from thefe falts, 

 and again evaporated in the filver difti, till it acquires the confiftence of thin oil f. In this 

 ftate, fo little water is prefent, that any part of the alkali, which may be united to carbonic 

 acid, cryftallizes, and alfo any neutral fait that may remain ; and the folution being de- 

 canted a fecond time, is obtained perfe£lly pure, colourlefs, and tranfparent. 



In the laft boiling it is fomewhat difficult to obferve the exafl: degree of concentration 

 at which all the alkaline carbonate will cryftallize ; and if the evaporation is carried too 

 . far, the cauftic alkali, if pot-afli, cryftallizes itfelf : fo that feveral evaporations are fome- 

 times requifite. 



When no more water remains in the folution than is juft fufficient to hold the cauftic 

 alkali diflblved, it contains nearly half its weight of alkali ; but the exaft quantity is eafily 

 known, by evaporating a portion to drynefs in a filver crucible. Before ufing fuch a folu- 

 tion for analyfes-, I afcertain its purity in the following manner : fome of it being fuper- 

 faturated with perfectly pure nitric acid, is examined with nitrate of barytes and of filver ; 

 with neither of which, if properly made, it will give the fmalleft cloud ; confequently it 

 can contain no fulphuric or muriatic acid. 



Another portion being faturated exaftly with a pure acid, the whole is evaporated to 

 drynefs, and the fait left is rediffolved in a little water. If any earth were contained in the 

 cauftic alkali, it would remain thus undiflblved ; but when made as above defcribed, I have 

 never, in this examination, obferved the fmalleft fediment. 



After the alkali is purified from neutral falts, and from the part united to carbonic acid, 

 it may itfelf be cryftallized by farther evaporation, as Mr. Lowitz has fhown. But this 

 piocefs feems of no ufe in chemical analyfis, as the alkali is previoufly obtained altogether 

 pure. 



• Mr. Klaproth ufes lime made of Carrara marble, which he boils with the alkali. {Beitrage, vol. i. 

 preface.) 



f Mr. Lowitz, in defcribing his procefs for cr) ftalliziug cauftic pot-afli, direfts the laft evaporation to be 

 peiformed in a glafs retort. This method is very erroneous j as the alkali, when heated and concentrated, 

 will diiiblve large quantities of the glafs, 



4 ' Defcription 



