358 Dr, blagden’s Hiflory of 
effectually prevented the water from being cooled kelow the 
freezing point, as ice conftantly formed on the bottom, per- 
haps in confequence of the early generation of fome minute 
portions of it in the crack. But independently of thefe cir- 
cumftances, neither ffirring, agitation, a current of frefh air 
on the furface, nor the contact of any extraneous body not 
colder, would caufe the water to {hoot into ice, even after it 
was cooled many degrees below the freezing point, notwith- 
ftanding the repeated aflertions of authors to the contrary. 
How far thefe facts may be applicable to the above men- 
tioned inftanc.es, where the thermometer froze before the 
quickfilver in which it was immerfed, can fcarcely be deter- 
mined unlefs more particulars were known. They {hew, how- 
ever, that the congelation would not neceflarily be brought on 
by ffirring the quickfilver, as praCtifed by Dr. guthrie; and 
point the way to various conjectures upon this difficult pheno- 
menon, the difeuffion of which muff be referved for a future 
opportunity. 
This fource of error in the method for fettling the point of 
mercurial congelation, mnveafily be obviated by a fmall change 
in the apparatus. Nothing further is necefi'ary than to employ 
thermometers made of alcohol, eflential oils, or fuch other 
fluids as will bear the requiflte cold without freezing. Pro- 
bably the former of thefe will be found moff convenient : 
and although the contraction of other fluids does not exactly 
keep pace with that of quickfilver, yet as the relative propor- 
tions can be readily determined, experiments with them may at 
all times be reduced to the mercurial ffandard, being not onlv 
the moff familiar, but hkewife that which feems to correfpond 
beft with equal increments and decrements of heat. 
There is one way all'o in which mercurial thermometers may 
he employed to afeertain the freezing point of quickfilver; I mean 
by 
