the Congelation of Vitriolic Acid. 271 
different flrengths, remaining fluid, although they had been 
expofed to a much greater cold than was neceflary for the con- 
gelation of that acid liquor which had frozen, I was led to 
believe, that there mud: be fome certain flrength at which the 
vitriolic acid was more dilpofed to freeze than at any other, 
greater or lefs. I knew that the fpecific gravity of the acid which 
had frozen was nearly to that of water as 1800 to 1000, and that 
of the flronger acid, which had not frozen, was as 1846 to 
1000 ; which laft is the ufual denflty of the oil of vitriol coinr 
monly fold in England. I knew alfo, that the acid which had 
frozen was in no refpedt hut in flrength different from the 
flronger acid which had retained its fluidity ; having myfelf, 
fome weeks before, taken the former acid from the bottle con- 
taining the latter, and diluted it with water till it was reduced 
to the fpecific gravity of 1800. 
Although from the above obfervations I was convinced of 
the propofition generally, that the vitriolic acid is moft dif- 
pofed to freeze when at a certain flrength, and then it is fuf- 
ceptible of congelation by means of much lefs cold than has 
been hitherto imagined ; yet, as only part of my acid had 
frozen, I could not with certainty know the flrength of the 
frozen part, and I therefore was not able to ftate, with any 
accuracy, the degree of flrength mofl favourable to congela- 
tion, nor the limits of flrength within which the acid may be 
congealed by fuch moderate cold. In the following winter I 
had not leifure to purlue the fubjeft ; but fince the commence- 
ment of the prefent year, I have verified my former obferva- 
tion with more attention to the exaft denfities of the acids ; 
and I have found, that the point of flrength mofl favourable 
to congelation is very determinate, and that a very fmall varia- 
tion above or below that point renders the acid incapable of 
freezing without a confiderable augmentation of cold. As the 
R r 2 acid, 
