136 Mr. Walker's Experiments on 
The above table is divided into two classes. The first class 
consists of mixtures of salts and ice, in which the temperature 
of mixing is of no consequence, the effect produced being the 
same at any temperature of the air : the salts should be in the 
state of powder. Ice pounded small may be substituted, with 
equal effect, for snow. 
-The second class consists of such mixtures as will produce 
an effect greater, the colder the temperature is at which the ma- 
terials are mixed, but in a diminishing ratio ; ceasing entirely at 
that degree of cold at which the composition itself freezes.* 
The salts should be in the state of fine powder. 
N. B. The figures after the salts, or acids, and ice, express the 
proportions, by weight , to be used. 
In the above table, the ordinary effect of snow, or ice- 
powder, is given ; but, if the latter be prepared (ground) with 
a sharp instrument, using light pressure, the effect will be 
somewhat greater, the ice being then reduced to an impal- 
pable powder : hence, ice powder, thus obtained from a block of 
ice, may at any time be substituted for snow. 
Cold is produced by mixing various other chemical sub- 
stances with ice : in the above table, such only are retained as 
produce that effect in a remarkable degree. 
As the new nomenclature is now generally adopted, I have 
used it in this paper. 
* The materials may be cooled, previously to mixing, when required, by a frigo- 
rific mixture taken freftuthe table : for this purpose, either of the mixtures in Class I, 
are convenient ; particularly the first, consisting of muriate of soda and snow. 
