FREEZING MIXTURES. Freezing is the solidification of fluid bodies by the ab straction of the heat necessary to their fluid form. Freezing mixtures are such as produce cold by the liquefaction of their solid ingre dient. Such mixtures reduce the temperature of substances immersed in them on the prin ciple of the transfer of heat, which always takes place from hotter to colder bodies when exposed to each other.
The process used by confectioners for pro ducing cold is by the mixture of ice and com mon salt, both of which in liquefying absorb so much heat, or, in other words, produce as much cold, as will reduce the thermometer from the usual temperature to zero of Fahren heit; or even rather below it. If however freshly fallen snow be used instead of ice, then the fluidity is more suddenly produced, and the cold is more intense. It is found that ice or snow, though exceedingly convenient sub stances for the production of artificial cold, are by no means necessary to it. Any salts which dissolve rapidly in water, finely pow dered, are powerful freezing mixtures. Mix tures of muriate of ammonia and nitrate of potash, and of these with phosphate of soda, act strongly in this way; and so do various other salts.
The following are useful freezing mixtures, with the temperature which they produce :— —Salt 1 part, snow 2 parts — Fehr. —Salt 2, mur. of ammonia 1, snow 5-12° „ —Salt 10, snow 24, mur. of ammo
nia 5, nit. of pot. 5 —18° „ —Snow 3, dilute sulph. acid 2 —23° „ —Snow 5, concen. mur. acid 8 —27° „ —Snow 4, concent. nit. acid 7 —30° „ --Snow 5, chloride of calcium 4 . —10° „ —Snow 2, chloride of calcium 3 . „ —Snow 3, fused potash 4 —51° t, —Snow 8, dilute sulphuric acid 10-91° „ These negative temperatures indicate so many degrees below zero of Fahrenheit. some cases they depend on the temperature of the substance before the freezing process. Solid carbonic acid is an agent in producing still greater degrees of cold.
In order to produce the congelation of wa ter by a rapid evaporation from its own sur faceunder the exhausted receiver of an air pump, Sir John Leslie introduced in the re ceiver a shallow vessel containing highly con centrated sulphuric acid, above which was placed the vessel containing the water. The air being extracted as quickly as possible, the vapour, which, in consequence of the removal, of the pressure, escaped continually from the water, was absorbed by the acid as fast as it rose ; and the remainder of the water was speedily frozen. Dry potash, muriate of lime, or powder of basalt, will do instead of sulphu ric acid. Leslie succeeded in freezing mer cury by the absorption of caloric from a coat ing of ice in which the bulb of a thermometer was immersed.