274 .Mr. Ketr’s Experiments on 
feems little reafon to doubt, that, by greater augmentations of 
cold, thefe limits may be further extended ; but in what ratio 
thefe augmentations and extenfions proceed cannot be determined 
without many obfervations made in different temperatures. 
Although it is probable, that the moft concentrated acids 
may be frozen, provided the cold be fufficiently intenfe, yet 
there feems to me reafon to believe, that fome of the congela- 
tions which have been obferved in highly concentrated acids 
have been effected in confequence of the denfity of thefe acids 
having been reduced nearly to the point of eafy freezing by 
their having abforbed moifture from the air : for the Duke 
d’Ayen and M. deMorveau expofed their acids to the air, in 
cups or opeai veffels ; and the latter author even acquaints us, 
that, on examining the fpecific gravity of the acid which had 
frozen, he found it to be to that of water as 129 to 74 ; which 
denfity being lefs than the point of eafieft freezing, proves that 
the acid which he employed, and which he had previouflv 
concentrated, had actually been weakened during the experi- 
ment. I have feveral times expofed concentrated oil of vitriol in 
open veffels in frofty weather; and I have fometimes, but not 
always, obferved a congelation take place. Upon feparating the 
fluid from the congealed part, and upon examining the fpecific 
gravity of the latter, after it had thawed, I found that it had 
been reduced to the ftandard of eafieft: freezing. When the 
congealed acid was kept longer expofed, it gradually thawed, 
even when the cold of the air increafed ; the reafon of which 
is not to be imputed to the heat produced by the moifture of 
the air mixing with the acid, for this caufe operated during 
the congelation, but principally to the diminution of denfity 
below the point of eafy freezing, which was occafioned by the 
continued abforption of moifture from the air, and which 
rendered 
