the Point of Congelation . 297 
lity may in general be afcertained, merely by trying the greatest 
cold it will produce with fnow. 
Notwithstanding this ufual correfpondence between the 
greateSt quantity of a hilt that could be diflolved, and the 
greatefl degree of cold it would produce, compared with the 
freezing point of its folution, I faw reafon to believe that, with 
fome halts at leaf!:, an irregularity took place in this refpetl. 
The Rochelle fait, mixed with fnow, did not fink the ther- 
mometer lower than 24 0 ; yet the freezing-point of a folution 
of one part of it in two of water appears, by the numbers in 
the table, to have been 21°. It is remarkable, however, that 
in the experiment with a hill Stronger folution, where fome ol 
the fait was depofited, the freezing point came down to 24 '. 
Likewife the folution of white vitriol, whofe freezing point 
was 28°!, depofited a copious fediment, and yet produced 
with fnow a cold of 20°. This fediment did not look like cry- 
stallized fait, but gave a turbid appearance to the whole folu- 
tion. The mod remarkable circumftance of this nature, I 
think, occurred with the purified common fait ; which, when 
mixed with fnow, funk the thermometer only to - 4 0 . By 
experiment I found that one part of this fait would difl'oive in 
about 2°i of water. Now, if the proportions of 1 to 4 give 
the freezing point at + 4 0 , as by the table; 1 to 2\ would, 
upon the general analogy, give it between - 1 2" and -13'’; 
that is, 8 or 9 degrees lower than the greatefl cold the fait 
would produce with fnow. This circumftance leads to a ful- 
picion, that however uniform the preceding tables appear, yet in 
realitv fomething of a decreasing ratio exifts, and that each 
fubfequent addition of a fait does not deprels the freezing point 
quite fo much as the preceding. I11 thole falts where the 
whole quantity of depreffion is but fmall, and in the upper 
part 
