'Experiments on freezing Mixtures * 259 
expe&ed, and confequently ferves in fome meafure to confirm 
the reafoning in this and the 12th Article. 
16. After Mr. M c Nab had diluted thefe acids as above- 
mentioned, he divided each of them into two parts, and tried 
what degree of cold could be produced by mixing them with 
fnow. On January 15th, one of thefe parts of the common 
fpirit of nitre was tried. It was fluid when the experiment 
began, though its temperature, as well as that of the fnow, 
was - 2i°i ; but on adding fnow it immediately began to 
freeze, and grew thick, and its heat increafed to — 2°f ; but by 
the addition of more fnow it quickly funk again, and at lad 
got to — 43°5* During the addition of the fnow, the mixture 
grew thinner, and by the time it arrived at nearly the greated 
degree of cold, confided vifibly of three parts : the, lowed 
party which confided of frozen acid, was white and felt 
gritty ; the upper part, which occupied about an equal fpace, 
was alfo white, but felt foft, and mud have confided of un- 
melted fnow ; the other part, which occupied by much the 
fmalled fpace, was clear and fluid. The quantity of fnow 
added was about T 9 T of the weight of the acid, and confe- 
quently its drength was reduced to ,243. 
Though fnow was added to the acid in this experiment as 
long as, and even longer than, it produced any increafe of 
cold, yet fome days after, on adding more fnow to the mix- 
ture, while it was fluid, and of the temperature of —40°!, 
the cold was increafed to - 44 0 !, or 1 degree lower than before. 
Mr. M c Nab did not perceive the fnow to melt, though in all 
probability fome mud have done fo, or no cold would have 
been produced. 
The caufe of this feems to be, that in the preceding expe- 
riment the congealed part of the acid was dronger than the 
L 1 2 fluid 
