242 Mr. Cavendish’s Account of forne 
and vitriolic acids, and the phenomena of freezing mixtures 5 
but has alfo produced degrees of cold greatly fuperior to any 
before known. 
1 . In the above-mentioned Paper I faid, that the cold pro- 
duced by mixing fpirit of nitre with fnoW, is owing to the 
melting of the fnow ; and that in all probability there is a 
certain degree of cold, in which fpirit of nitre is fo far from, 
diffolving fnow, that it will yield out part of its own water* 
and fuffer that to freeze, as is the cafe with folutions of com- 
mon fait ; fo that if the cold of the materials, before mixing* 
is equal to this, no additional cold can be produced. A cir- 
cumftance, however, which at firft fight feems repugnant to 
this opinion, occurred in an experiment of Fahrenheit’s for 
producing cold by a mixture of fpirit of nitre and ice ; namely,, 
that the acid, which had been repeatedly cooled by different fri- 
jrorific mixtures, was found frozen before it was mixed with 
the ice ; notwithftanding which, cold was produced by the mix- 
ture. Profeflor Braun alfo found, that cold was produced hp 
mixing frozen fpirit of nitre with fnow. On confideration, 
however, this appeared by no means inconfiftent with the opi- 
nion there laid down, as there was great reafon to think* that 
the freezing of the acid was of a different kind from that con- 
fidered in the above-mentioned Paper, and that it did not pro- 
ceed from the watery part feparating from the reft and freez- 
ing ; but that the whole acid, or perhaps the more concentrated 
part, froze; in which cafe it would not be extraordinary that 
the acid fhould diffolve more fnow, and produce cold. 
2. To clear up this point, I fent to Hudfon’s Bay a bottle of 
fpirit of nitre, of nearly the fame ftrengtli as Fahrenheit’s ; 
and defired Mr. M c Nab to expofe it to the cold, and, if it 
froze, to afcertain the temperature, and decant the fluid part into 
. 3 another 
