2 jo Mr. Ketr’s Experiments on 
(fuch as he had difcovered to be pofleffed by fpirit of nitre), 
or whether the cold requifite for congelation does not con- 
tinually diminifh, as the ftrength of the acid increafes, without 
limitation. This latter opinion he thinks the mod: probable, 
from the circumftance of the Duke d’A yen’s and M. de Mor- 
veau’s acids having frozen with a confiderably lefs intenfe* 
cold than thole of Mr. M c Nab, which, he fuppofes, were 
weaker, as the former acids had been concentrated purpofely. 
The obfervations which I have made, and am going to relate, 
apply lolely to the latter kind of congelation of the vitriolic 
acid, as the acid which I employed was of the kind that is 
prepared by burning fulphur, and is commonly fold in Eng- 
land under the name of oil of vitriol, and was perfeflly free 
from colour, fmell, or fmoking quality. After a fevere frofl 
at the end of the year 1784, and beginning of 1785, I ob- 
ferved that fome vitriolic acid, contained in a corked phial, had 
congealed ; while other parcels of the fame acid, fome Wronger 
and fome weaker, equally expofed to the cold, had remained 
fluid. As I imputed the congelation to the great intenflty of 
the cold, I was afterwards much furprifed, when the frofl 
ceafed, to find that the acid remained frozen during many days, 
when the temperature of the air was fometimes above 40° of 
Fahrenheit’s fcale; and when the congealed acid was 
brought into a warm room, purpofely to thaw it, a thermo- 
meter, placed in contact with it during its thawing, continued 
ftationary at 45 0 . From thefe circumftances I concluded, that 
the freezing and thawing point of this acid was very near the 
laft mentioned degree ; and, accordingly, upon expofing the 
liquor which had been thawed to the air, at the temperature of 
30°, the congelation again took place in a few hours. From 
the circumflance of other parcels of the fame acid, but of 
4 different 
