Experiments on the freezing of Acids . i yy 
On the Vitriolic Acid. 
Ail irregularity of a remarkable kind occurred in trying two 
of thefe acids ; namely, when the undecanted part was melted 
and again made to congeal, its freezing point was found to be 
much lefs cold than that of the decanted part, and the dif- 
ference was much greater than could be attributed to the 
difference of Strength. This feems to have happened only in 
the two ftrongeft acids, namely, N° i. and 2. and in great 
meafure confirms the fuppofition which I formed from Mr. 
M c Nab’s former experiments, that the congealed part of oil 
of vitriol differs from the reft, not merely in ftrength, but 
alfo in fome other refpeft, which I am not acquainted with. 
It fhould feem, however, that this property does not extend to 
weak oil of vitriol. 
It perhaps may be fufpedted, that this property takes place 
in the nitrous acid alfo, and was the caufe of the flow melting 
of the ice taken notice of in p. 172. But I think it more 
likely, that that phenomenon proceeded from the caufes there 
afligned. 
Some fmaller irregularities occurred in trying the vitriolic 
acid, the caufe of which I believe was, that when this acid 
has been cooled below the freezing point, and begins to freeze, 
the congelation proceeds but flowly ; fo that a considerable 
tifrie elapfes before it riles to the true freezing point. Something 
of the fame kind feems to take place in the nitrous acid alfo, 
though in a lefs degree ; for the decanted liquors ufually con- 
tinued to freeze and depofit a fmall quantity of ice, for a few mi- 
nutes after they were poured off, though their cold, at leaft in 
fome inftances, was found rather to dimiuifh during that time. 
Vol. LXXVIII. A a " It 
