Ptirijicaiten of Pot^AJh in the Large Wwj. 331 



If the lapis caufLIcus be wanted, the concentrated liquor is to' be poured into a fraaller pot, 

 and the evaporation carried fo far that the fait may congeal when poured out on a plate of. 

 iron or marble *. 



If a purer alkali be required for the delicate experiments of chemiftry, inftead of pot-afh, 

 the tartareous acidule or cream of tartar may be calcined and made ufe of; or otherwife 

 the fufed pot-afh before mentioned may be purified by alcohol after the manner of Ber- 

 thollet. Experiment has proved to us, that very pure alkali may by this means be obtained f . 



In this cafe, the lixivium is to be evaporated to the confiflence of a thick fyrup in a filver 

 bafon I, or preferably in clofe veflels. This matter is then to be diflblved in alcohol. The 

 pot-afh alone combines with the fluid : the fulphate and muriate of pot-afti, the portions of 

 earth and even of carbonic acid which it obftinately retains, or which it has refumed from 

 the air during the evaporation, remain at the bottom of the folution. If the alcohol has 

 been poured on the matter whilft (till hot, and a lefs quantity of this folvent has been em- 

 ployed than was requifite for diffblving the pot-afli, it cryftallizes by cooling in white blades, 

 which are fometimes feveral inches long. If it be required to feparate the pot-adi from the 

 alcohol, and to obtain it in the dry flate, the folution muft be evaporated in a filver bafon, 

 and not in a glafs veflel ; for the pot-afh often diffolvcs a portion of filex, which alters its 

 purity. 



We fee by this operation, of which the details deferve to be confulted, in the memoir of 

 Berthollct, that the cauflic pot-afh is deprived of filex, of carbonic acid, of all the foreign 

 falts, and of the fmall portion of iron it may have taken up from the velTel in which the 

 evaporation was performed. 



* It is proper to remark, that the folid caullic alkali thus prepared is much more aftive than that ufually 

 jnailc by chtniifts for fale. For this reafon it is neceffary to be very prudent in its application. G. 

 + See, however, the remark of Lowitz on this fubjeft. " Philof. Joum. I. 163. N." 

 % What follows is extraflcd from the memoir of the celebrated BerthoUet, for tlie advantage of thofe who 

 ir.ay not poffcfs the means of confulting the original. G. 



Uua X. A TABLE 



