200 Mr. Walker’s Experiments on the 
as a convenient method of afcertaining when the mercury was 
congealed; for if, after being fubjedted to the cold of a frigo- 
rific mixture, the thermometer glafs fhould be taken out and 
inverted, and the mercury found to remain completely fuf- 
pended in that half of the bulb now uppermoft, no doubt can 
remain of the fuccefs of the experiment ; an hydrometer, with 
its lower bulb half an inch in diameter, and three-fourths full of 
mercury, was likewife provided, in cafe any accident fhould 
happen to the other. 
It may be proper to premife here, that in all experiments of 
this kind I remove each veffel, when the liquor it contains is 
fufficiently cooled, out of the mixture in which it is immerfed 
for that purpofe, immediately previous to adding the fnow or 
falts with intention to generate a ftill further increafe of cold ; 
and likewife prefer adding the fnow or powdered falts to the 
liquor, inftead of pouring the liquor upon thefe : it is neceffary 
alfo to ftir about the fnow or falts, whilft cooling in a frigorific 
mixture, from time to time, otherwife it will freeze into a hard 
mafs, and fruftrate the experiment. 
A half-pint glafs tumbler, containing two ounces and a half 
of the above-mentioned diluted mixture of acids, being im- 
merfed in mixtures of nitrous acid and fnow, until the liquor 
it contained was cooled to — 30°, was removed out of the 
mixture and placed upon a table ; fnow, likewife previoufly 
cooled in a frigorific mixture to — 1 5 0 , was added by degrees to 
the liquor in the tumbler, and the mixture kept ftirring until a 
mercurial thermometer lunk to — 6o°, where it remained fta- 
tionary ; the hydrometer was then immerfed in the mixture 
(the thermometer glafs having been broken in the courfe of 
the experiment), and ftirred about in it for a fhort time, and 
on taking the hydrometer out, and gently fhaking it, I 
1 perceived 
