cooling of Water below Its freezing Point . 127 
attended with a fimilar refult, and have fufficiently convinced 
me, that, other things equal, boiled water may be cooled a 
greater number of degrees below the freezing point, without 
congealing, than water which, not having undergone that 
operation, retains the air it naturally imbibes. 
As a further proof that the prefence of an aerial fluid in 
water rather lefTens than increafes its quality of being cooled 
below the freezing point, I found that diftilled water, which 
had been for that purpofe impregnated with fixed air, generally 
fhot into ice at a lefs degree of cold than the fame water in its 
ordinary ftate. I fufpedl, however, that it is ufually by the 
admixture of other aerial fubftances, fuch as dephlogifticated 
air, phlogifticated air, or perhaps both, and not of fixed air, 
that water is inclined to congeal loon after it has palled the 
freezing point ; for, as will be feen hereafter, acids rather im- 
prove than diminifh the quality in water of refitting conge- 
lation. 
To determine the effect of other extraneous fubftances, I 
took fome very hard pump-water, fuch as is found in the 
northern parts of London, and let it in the frigorihc mixture. 
In general it congealed fooner by one or two degrees than un- 
boiled diftilled water ; that is, at 25 0 or 24 0 of the thermo- 
meter ; and as there was fome variation in this refpedf, I was 
led to remark, that the greateft cooling ufually took place when 
the water was moft clear and tranfparent. With a view to this 
circumftance I took fome New-River water, which happened 
at that time to be confiderably turbid, and tried it in the frigo- 
rihc mixture ; when I found, very unexpe&edly, that it was 
not in my power to cool any of it below the freezing point ; 
a cruft of ice always forming round the fides and at the bottom 
of the vettel, whilft the thermometer, fufpended about the 
2 middle 
