Of WHJNSTONE and LAVA. 97 



of the alkali. This mixture is frequently flirred during two or 

 three days ; for when no heat is applied, the lime requires a cer- 

 tain time to attrad; the whole of the carbonic acid. In the next 

 place, the mixture is filtered through a piece of linen placed in a 

 funnel, and water poured on it till the whole of the alkali is wafli- 

 ed out. As it pafles through, it is poured, at intervals, from the 

 bottle which receives it firft, into a fecond that is clofely flopped. 



In making the fixed alkalis cauftic, it is ufuai to boil them 

 with the lime ; but as mofl kinds of limeftone contain a fmall 

 quantity of filex or argil, and as thefe earths, when in a ftate of 

 divifion, are fohible in boiling cauftic alkali, there is a probabi- 

 lity, when heat is applied, of its being thus rendered impure. 

 This is my reafon for carefully avoiding heat in the firft part of 

 the procefs ; and I have not found that lime made from chalk, 

 or from the purer kinds of limeftone *, give any impurity what- 

 ever to the alkali, when mixed with it cold. 



The folution, in its weak ftate, is firft evaporated in a bafon 

 of hammered iron, polifhed ; but when it is fomewJjat con- 

 centrated, I carry on the evaporation in a difli made of the pUr 

 reft filver, reduced from luna cornea. After being boiled to a 

 fmall quantity, it is allowed 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 LovviTZ has pointed out. Afterwards, the Iblution is care- 

 fully decanted from thefe falts, and again evaporated in the fiL- 

 ver difh, till it acquires the confiftence of thin oil f. In this 

 ftate, fo little water is prefent,, that any part of the alkali, which 



Part I. N may 



* Mr Klaproth ufes lime made of Carrara marble, which he boils with the al- 

 kali. ( Deitreige, vol. i. preface). 



t Mr LowiTZ,in defcribing his procefs for cryflallizing cauftic potaQi, direfls the 

 laft evaporation to be performed in a glafs retort. This method is very erroneous ; as 

 rte alkali, when heated and concentrated, will diflblve large quantities of the glafs^ 



