274 Mr . Walker’s Observations on the 
rounded by its frigorific mixture) added to the acid as quick 
as possible ; when the thermometer sunk to near — 50°, and 
the mixture soon froze the quicksilver in the glass bulb. In 
this experiment, 18 minutes were required to freeze the water 
perfectly solid ; and 15 to reduce the ice, by moderate labour, 
to very fine powder. The experiment was over in 55 minutes ; 
and the temperature of the preparatory cooling mixture then 
found to be — io°. 
I had a spirit thermometer by me, but a mercurial thermo- 
meter being much more sensible, and consequently descend- 
ing much quicker, I prefer it in experiments made merely to 
freeze quicksilver; knowing from experience how the congela- 
tion is going on, from the irregular descent of the mercury 
when a few degrees below its freezing point ; and from hav-. 
ing usually found that the quicksilver in the thermometer 
glass begins to freeze, as soon as the mercurial thermometer 
reaches — 4,0°. 
Whenever ! have occasion to use ice in summer for this pur- 
pose^ usually pound together first some ice and salt in a stone 
mortar, about two parts of the former to one of the latter ; 
throw this away, and wipe the pestle and mortar perfectly 
dry ; the mortar being thus cooled, the ice may afterwards be 
pounded small without melting. 
And as a mixture made of snow, or ice in powder, and salts, 
does not give out its greatest cold till it is become partially li- 
quid, by the action of the ice and salts on each other ; it is 
necessary that the whole be stirred well together, till it is be- 
come of an uniformly moist pulpy consistence, especially since 
in becoming liquid the mixture shrinks so much, that if this 
be not attended to the vessel will not be near full, and conse- 
