PHYSICAL AND.LITERARY. sir 



will fobn take a very deep colour from a few 

 drops of a ftrong cauftic ley. The circum- 

 ftances which hindac us from rendering thefe 

 falts perfeftly cauftic by heat, are their pro- 

 peniity to diflipation in the utmoft violence 

 of the fire, their extreme acrimony^i and the 

 imperfedion of our common veffeis. For 

 before the heat becomes very intenfe, the at-^ 

 kalis either evaporate, or diiTolve a part of the 

 crucibles in which they are contained, and 

 often efcape thro' their pores ; which hap^ 

 pens, efpecially.as ibon as they have already 

 acquired fome degree of additional acrimony, 

 by the lofs of part of their air. 



The fufion alfo, which they fo readily.un- 

 dergo, is well known by Chemifts;, as a ftrong 

 obftacle to the feparation of the volatile from the 

 fixed parts of a compound by fire ; according- 

 ly, in feveral procefles, we are directed to add 

 to the fufible compound fbme porous fubftance 

 which is incapable of fufion, and- will retain 

 the whole in a fpongy form, thereby to faciii-f 

 tate the diffipation of the volatile parts. 



In order to know whether an alkali would 

 lofe a part of its air, and acquire a degree of 

 caufticity, when expofed, with this precauti- 

 on, to the adtion of a ftrong fire, 1 mixed an 



ounce 



