as ObjeSfsof the Art of Dpngy &c, 373 



Alum, being a cheap fubftance, is moft com- 

 monly ufed. It confifts of vitriolic acid, pure 

 clay*, or argillaceous earth, and water. Accord- 

 ing to Bergman, one hundred parts of cryftalifed 

 alum contain thirty-eight of vitriolic acid, eigh- 

 teen of clay, and forty-four of water. The clay 

 is generally fuperfaturaced with acid, which is 

 proved by the phenomena produced pn the ad- 

 dition of mild, vegetable, fixed, alkali. On 

 the fira portions of alkali being added a fmall 

 portion of the earth precipitates from thofe parts 

 of the alum with which the alkali cornes in con- 

 tad s and as pure clay has an attraftion for aerial 

 acid, the effervefcence produced is at firft fmall ; 

 but prefently the remaining free acid, attacking 

 this precipitate, rediflblves it, an effervefcence 

 appears, occafioned by the difcharge of the aerial 

 acid from the clay. This precipitation, follow- 

 ed by rediffolution, and a difcharge of gas, conti- 

 nues, till the acid be perfeAly faturatedj the 

 precipitation then goes on regularly and the 

 earth is no longer diflblved a new, except the 

 alkali be continued to be added, after the pre- 

 cipitation is fully accomplilhed ; nor does apy ' 

 effervefcence follow, when frefh portions of al- 

 kali are added. 



Thefe are the appearances when mild, or aerated, 

 vegetable alkali is ufed i but if the pure or cauftic 



♦ The conftituent parts of common clay are argillaceous, 

 mixed with filiceous earth, in various proportions. Pure 

 argillaceous earth is only obtainable from alum. 



B b 3 alkali 



