cooling of Water below its freezing Point. i 
be diftributed to the reft of the water ; but, I think, the 
above-mentioned effect takes place when no part of the fluid 
can be fuppofed to be many degrees below the freezing point. 
It has been alledged, that metal in contact, either with the 
outflde of the veflel containing the water, or with the w r ater 
itfelf, dilpofes it to freeze fooner after it is cooled below 3 2°. 
Though upon repeating this experiment I have found it poffible 
to cool water in a metal veflel many degrees below its freezing 
point, and even to touch it, when fo cooled, with metal 
equally cold, without producing congelation ; yet the metal 
certainly tends to haften the freezing, and, I believe, on the 
above-mentioned principle of too quick a change of tempera- 
ture, occafioned by its quality as a good condu&or of heat. 
For the fame reafon it is more difficult to cool water much 
below the freezing point in thin veflels than in thofe whofe 
bottom and fides are of confiderable thicknefs ; the latter 
tranfmitting the heat more flowly, and allowing it thereby to 
be diffufed more equably. 
In cooling water below its freezing point by frigorific mix- 
tures, it is of confequence to keep the mixture fome way 
below the upper edge of the water within the tumbler, other- 
wife the congelation quickly begins at that place. This very 
likely depends on the principle laft mentioned, that the thin 
edge of water riling up againft the fide of the glafs, being 
more in contaT with air than with the general mafs of water, 
does not fo eafily diftribute its cold, and therefore fufters a more 
rapid change of temperature by the adlion of the mixture. v 
Hence one of the moft eflential precautions for cooling water 
to the utmoft without congelation, is to perform the experi- 
ment in a warm room, that the air in contaft with the edges 
Vol. LXXVIII. T and 
