280 Mr, Keir’s Experiments on 
Like water, this acid poflefles the property of retaining its 
fluidity when cooled feveral degrees below its freezing point ; 
and of riling fuddenly to this point, when its congelation is 
promoted by agitation, or by contact with even a warmer 
thermometer. 
4thly, That, like water and other congelabte fluids, the 
vitriolic acid generates cold during its liquefaction, and heat 
during its congelation ; the quantity of which heat and cold, 
fo generated, remains to be determined by future experiments. 
5 thly , That the acid, by congelation, when the circum- 
ftances for didinCl crydallization are favourable, affumes a 
regular crydalline form, a confiderable folidity and bardnefs, 
and a denfity much greater than it poflefled in a fluid ftate. 
With refpeCt to the firft mentioned fpecies of congelation, 
which is peculiar to the fmoking vitriolic acid that is procured 
from martial vitriol, although I have had no opportunity of 
feeing it, as all the vitriolic acid, that is ufed in this country, 
is obtained by burning lulphur, yet I will beg leave to fug- 
ged, that it may be worth the attention of thofe chemids to 
whom it occurs, to obferve more accurately than has been 
done, the freezing temperature and the denfity of the con^ 
gelable acids ; and to examine whether the denfity of this 
fmoking acid alfo is connected with the glacial property. It 
feems further to be deferving of invedigation, whether there 
is not fome analogy between the congelation of the fmoking 
oil of vitriol, and the very curious crydallization which Dr. 
Priestley obferved in a concentrated vitriolic acid, faturated 
with nitrous acid vapours and whether this fmoking quality 
does 
* Experiments and Obfervations relating to various Branches of Natural 
Philofophy, 
