best Methods of producing artificial Cold. 2 75 
quently the upper part of the tube not surrounded, as it ought 
to be, by the frigorific mixture. The dissolution of the ice 
and salts may, if required, be hastened by adding occasionally 
a little water ; but then the cold produced will be less intense, 
and not so durable. 
That particular form of the vessel, in which the ice is made 
and reduced to powder, is chosen, because it subjects the pow- 
dered ice in the tube to the constant action of the freezing 
mixture, without which it would be less fit, particularly in 
warm weather, for the intended use, and because in it the 
ice is not liable to be impregnated with the salts of the mix- 
ture, by which it would be utterly spoiled : and that for cool- 
ing the nitrous acid, and making the second mixture in, be- 
cause it is steady, and is besides insulated as it- were from the 
external warm air, and surrounded in its stead by an atmo- 
sphere much colder. 
It is scarcely necessary to add, that when snow which has 
never thawed can be procured, it may be cooled in this appa- 
ratus by a mixture of snow (instead of the pounded ice), and 
the salts, and the trouble of reducing the ice into powder saved. 
I prefer the red fuming nitrous acid, becau e, as I have ob- 
served in a former paper, it requires no dilution. Being un- 
der the necessity at one time of using the pale nitrous acid, 
I found it required to be diluted with one-fifth its weight 
of water. The best and only way of trying or reducing any 
acid to the proper strength, is by adding snow, as Mr. Ca- 
vendish directs, or the powdered ice to it, until the thermo- 
meter cease to rise ; then cool the acid to the same tempe- 
rature of the snow again, add more snow, which will make 
the thermometer rise again, though less ; cool it again, and 
