Experiments on freezing Mixtures \ 265 
contained Tome undiffolved fnow, and fome more was added to 
it after that time, which before the firft of January diffolved 
and mixed with the acid ; fo that the acid in the mixture, at the 
time it funk to ~ 55 0 §, was not quite fo much diluted as that 
which froze on January 1. This is the reverfe of what hap- 
pened in the trial of the nitrous acid in Art. 15. as in that 
experiment the fluid part, at the time of the greateft cold, was 
weaker than the whole mixture together; but it muft be con- 
fidered, that that mixture contained much congealed acid, as 
well as undiffolved fnow, whereas this contained only the latter. 
25. On January 1, fnow was added to the other half of the 
acid diluted on December 21. The cold produced was much 
greater than before, namely - 68°| ; this feems to have pro- 
ceeded, partly from the air and materials having been 1 2 degrees 
colder in this than in the former experiment, and partly from 
the fnow having been added fafter, fo that the mixture arrived 
at its greateft degree of cold in 2o\ whereas it before took up 
46'. Another reafon is, that the former mixture was made in 
too fmall ajar, in confequence of which it was poured into a 
larger before the experiment was completed, whereby fome 
cold was loft. The quantity of fnow ufed in this experiment 
was lefs than in the former, fo that the ftrength of the acid 
after the experiment was about ,343. The mixture alfo grew 
much thicker, and had a degree of elafticity refembling jelly ; 
but whether this was owing only to more fnow remaining 
undiffolved, or to any other caule, I cannot telL 
26. Great as the foregoing degree of cold is, Mr. M c Nab, 
on February 2, produced one much greater. In hopes of ob- 
taining a greater degree of cold by previoufly cooling the ma- 
terials, he cooled about feven ounces of oil of vitriol, whofe 
ftrength was ,629, that is, rather ftronger than the foregoing, 
Vo l. LXXVI. M m by 
