282 Mr. Walker's Observations on the 
That it may likewise be frozen, by mixing expeditiously 
together snow and nitrous acid, when the temperature of 
each is -}- 7 0 : 
Or by mixing ground ice and nitrous acid at -f io°. 
Hence it follows, that the cold of this climate offers occa- 
sionally opportunities of freezing quicksilver, without pre- 
viously cooling by art the materials to be mixed ; for I have 
once seen the thermometer at -j- 6°, and others, I believe, have 
seen it lower. 
I expected an opportunity would have offered this winter, 
but the lowest point I saw my thermometer at, this season, 
was only io°; at this temperature, I mixed nitrous acid 
(cooled out of doors to the temperature of the air) and snow, 
on January 23d last; but the cold produced was not quite suf- 
ficient to freeze the quicksilver, although very near it, as indi- 
cated by a thermometer. From what I have observed since 
these latter experiments were made, I think it may be reason- 
ably expected, that powdered ice and nitrous acid at -f- 14 0 , or 
snow at -|- i°°j will succeed, if mixed expeditiously. 
Strong spirit of vitriol, whose specific gravity is 1,848, re- 
quired to be diluted with half its weight of water, and pro- 
duced with snow at the temperature of -f 30°, abouf eight de- 
grees less than with nitrous acid, sinking the thermometer to 
— 24 0 ; four parts of the diluted vitriolic acid required, at that 
temperature, six parts of snow. 
It perhaps will be remarked, that I have taken no notice be- 
fore of the vitriolic acid. The reason is, because the freezing 
point of quicksilver being 39 0 , it may be frozen tolerably hard 
by a mixture of nitrous acid with snow, or ground ice, though 
