232 ESSAYS anp OBSERVATIONS 
lour from.a few drops of a ftrong cauttic 
ley. The circumftances which hinder us 
from rendering thefe falts perfectly cauttic 
by heat, are their propenfity to diffipation 
in the utmoft violence of the fire, their ex- 
treme acrimony, and the imperfection of 
our common veffels. For, before the heat 
becomes very intenfe, the alkalis either e- 
vaporate, or diffolve a part of the cruci+ 
bles in which they are contained, and oft- 
en efcape through their pores ; which hap- 
pens, efpecially as foon as they have alrea- 
dy acquired fome degree of additional a- 
crimony, by the lofs of part of their air. 
THE fufion alfo, which they fo readily 
undergo, is well known by chymifts, asa 
{trong obftacle to the feparation of the. 
volatile from the fixed parts of 1 come 
pound by - fire; accordingly, in feveral 
procefies, we are directed to. add to the, 
fufible compound fome porous fubftance 
which is incapable of fufion, and will re- 
tain the whole in a fpongy form, there-: 
by to facilitate the diflipation of the vola- 
tile parts. 
In 
