272 Mr. Walker's Observations on the 
may be formed. 'When the whole of the ice is thus reduced to 
powder, and the lumps, if any, broken, the frigorific mixture is 
to be let out quickly, by cutting or untying the string, and re- 
moving the bladder, &c. which confines it ; a communication 
made, by forcing a rod of glass or wood through the partition ; 
and the whole mixed expeditiously together. 
In this climate, a mixture much less expensive will be suffi- 
cient, viz. that composed of diluted nitrous acid, Glauber's 
salt, sal ammoniac, and nitre ; a mixture of this kind sinking 
a thermometer in the warmest weather to near o°. At the tem- 
perature of 70°, or a little higher, the quantity of diluted ni- 
trous acid may be about one-fourth less than is mentioned in 
the Table, for 50°. 
These methods are the most expeditious, and attended with 
the least trouble ; but as ice may be used with equal certainty, 
and with much less expence, I shall give a particular detail of 
an experiment made with the use of it, first mentioning a pre- 
paratory experiment, to which I was immediately led by the 
recollection that Sir Charles Blagden, in his paper “ on the 
“ point of congelation," (Phil. Trans. Vol. LXXVIII.) had 
found that common sal ammoniac and common salt, mixed 
with snow, produced a cold of — 12 0 , whereas the latter used 
alone with snow produces only — 5 0 . I used a mixed powder 
of equal parts of common sal ammoniac and nitre with the 
common salt, by which the thermometer sunk to — 18 0 ; and 
when I used nitrous ammoniac with common salt, to — 25 0 ; 
this cold I could not increase by the addition of any other salts, 
nor could I equal it by any other combination of salts : those 
I tried were Glauber's salt, salt of tartar, soda, and sal ca- 
tharticus amarus ; by several trials, I found the best proportions. 
