^30 Purification of P'J-AJh in the Lnfgt Wa'j, 



hole 16 pieiced, to Infcrt a tube of glafs, w'.iich comes out under an angle of 45 degrees with 

 the horizon. 



The channels are covered with tubes of glafs ranged acrofs, upon which is placed a 

 cloth in fuch a manner as to cover them completely. Upon this a thin layer of wood- 

 aflu-s is fprinkkd, and afterwards the mixture which we (hall proceed to mention is put 

 into th.e vclTcls. 



For want of veflels of Hone, we may operate by ufing fmall tubs of white wood, as irj 

 Figure 5 ; and inllcad of the grooves, river fand mud be fpread on the bottom, after being 

 firft carefully waflied; over this a finer flratum muft be fpread, and the whole then covered 

 with a cloth, and wood-aflies (Irewed thereon. Thefe veflels, like the others, muft each 

 have a tube to convey the fluid whicli filters through. 



It is not neceflary to remark, thai the firft method is to be preferred. For the cauftic alkali 

 always feizes a portion of the colouring matter of the wood, and may even, according to it* 

 degree of concentration, take up a fmall quantity of filex by folution. But tliefe incon- 

 veniencies are of little confcquence, when merely the cauftic alkali or lapis caufticus is 

 wanted. 



For the arts, and for the delicate experiments of chemiftry, the cifterns of calcareous ftone 

 are to be preferred: with thefe the liquor is obtained perfedlly limpid. 



Things being thus Hlfpofed, equal parts are to be taken of quicklime and of pot afh, more 

 efpecially v/hen the lime is very cauftic. In the contrary cafe, twenty parts of lime may he 

 taken to fifteen of pot-afli. Water is then to be heated in an iron pot till it nearly boils ; 

 in this ftate the lime is to be added, which by its extiniflion brings it to the boiling heat. 

 When the extinction is complete, the pot-afb is mixed, and the whole formed into a thick 

 mafs, which is left to cool a little. 



The mixture is afterwards poured into the cifterns, and Immediately covered with wafer. 

 In order to avoid making perforations in the mixture by pouring the water, a fmall board is 

 laid to receive the ftream, which rifes with the fluid. 



Care muft be taken to place pitchers or other veflels to receive the liquor which flows 

 through the tube ; and in order to prevent the lixivium from abforbing carbonic acid from 

 the atmofphere, the veflels muft be flightly clofed to prevent the circulation of the exter- 

 nal air. 



It is likewife neceflfary that a fupply of water fliould be kept up over the mixture. When 

 it comes out of the tube in an infipid ftate, it is unneceflary to draw ofl^any more. 



The folutions obtained arc all to the laft very nearly of the fame ftrength ; for they become 

 weak all at once : from this circumftance the very aqueous folutions are avoided. 



Iron pots may be ufed to evaporate the waters. The laft run muft be taken firft, becaufc 

 it is rather weakeft ; and by this means the ftrongcr folutions are not fo long expofed to 

 the contact of the air. The evaporation muft be performed by a ftrong ebullition. 



When the concentration has arrived to a certain point, the fulphate of pot-afli cryftallizes 

 and falls down. It may eafily be taken out by placing at the bottom of the pot a perforated 

 iron ladle, in which the fait is colleded. Strong ebullition is neceffary to keep the atmo- 

 fpheric air at a diftance from the furface, and towards the end the motion of boiling ferves 

 to carry the fulphate of pot-aih into the ladle. 



