best Methods of producing artificial Cold. 281 
to know how much room in a vessel the several materials take 
up separately, and when mixed, it will be right to observe, 
that snow, or ice in powder, at near o°, occupy in measure 
nearly two-thirds more than their weight ; that is, one ounce 
weight of water will, when in the form of snow, or ice ground 
to powder, nearly fill a vessel which holds three ounces wine 
measure ; powdered salts nearly double their weight ; strong 
nitrous acid about three-fourths its weight ; and a mixture 
made of salts and diluted nitrous acid, measures rather less 
than two-thirds of the weight of the ingredients. Without a 
previous knowledge of this, it is impossible to adjust the size of 
the vessels to the mixtures which are to be made ; because, in 
most nice experiments of this kind, the height to which a 
vessel will be filled is indispensably necessary to be known be- 
forehand. 
The long continuance of the late frost having afforded me 
opportunities of repeating these experiments in various ways, 
I shall mention briefly the result of such as appear to me to 
be material. 
I have found, that ice may be ground so fine as to be equal 
to frozen vapour, and the harder it is frozen the finer it is 
ground, but with more labour : 
That quicksilver may be frozen by cooling the nitrous acid 
only, saving the trouble and inconvenience of cooling the 
snow likewise; either by adding snow at -f 32 0 , to nitrous 
acid at — 29 0 ; or snow at 25 0 , to nitrous acid at — 20° ; 
or snow at -f 20° to nitrous acid at — 12 0 ; most winters offer 
an opportunity of doing it in this way ; the nitrous acid may 
be cooled in a mixture of snow and nitrous acid : 
