284/ Mr. Walker's Observations on the 
If it be required to prepare the materials in a frigorific 
mixture, without the use of ice, a mixture of the proper 
strength may be chosen from the Table. 
It is immaterial, when the exact proportions of each are 
known, whether the powdered ice be added to the acid, or the 
’acid poured upon that, provided the powdered ice be kept 
stirred to prevent lumps forming, and the materials be mixed 
as quick as possible. But when the proportion is not known, 
it is better to be provided with more powdered ice than is ex- 
pected to be wanted ; and add it to the acid by degrees, until 
the greatest effect is produced, as shewn by a thermometer. 
The consistence is a pretty sure guide to those accustomed 
to mixtures of this kind ; viz. when fresh additions of snow or 
ice do not readily dissolve in the acid, though well stirred, 
and the mixture acquires a thickish flocculent appearance. 
Snow, or powdered ice, that have ever been subjected to 
a cold less than freezing are spoiled, or rendered much less 
fit for experiments of this kind. 
I prefer the method of adding the powdered ice or snow 
to the acid in a separate vessel, principally because the size 
of that vessel may be exactly adjusted to the quantity of mix- 
ture it is to contain. 
A mixture made of diluted nitrous acid, phosphorated soda, 
and nitrous ammoniac (by much the most powerful of any 
compounded of salts with acids), prepared with the greatest 
accuracy, is not quite equal to a mixture of snow and nitrous 
acid, each mixed at -f 30°, although very nearly so. 
Though quicksilver may be frozen by salts dissolved in acids, 
it is necessary that the materials be cooled, previously to mix- 
ing, much lower than when snow or ground ice are used. 
