Freezing Mixtures. When matter passes from the solid to the liquid, or from the liquid to the gaseous (or vaporous) state, a considerable quantity of heat in general disappears or is rendered 'latent,' owing to the fact that, energy being required to effect these changes of molecular state, it is taken from the energy in the form of heat already existing in the substance; and this abstraction of heat causes a fall of temperature (see HEAT; MATTER; GAS AND GASES). This property is taken advantage of for the production of low temperatures. The solution of a salt in a liquid, and the liquefaction of two or more solid substances when mixed, are both examples of the change from the solid to the liquid state, accompanied by a lowering of temperature, unless this be neutralised by heat developed by some purely chemical action. The following table gives the composition of freezing mixtures commonly in use. The solid materials should be finely powdered and intimately mixed together in a vessel of low thermal conductivity. The first column of the table gives the components in each mixture; the second their relative proportions by weight; in the third the resulting temperature (Centigrade) of the mixture, assuming that, with the exception of snow or ice, the temperature of the materials previous to mixture is 10° C. The fourth column gives the diminution of temperature in degrees Centigrade:
| Components. | Relative proportion by weight. | Temperature of mixture. | Diminution of temperature. |
|---|---|---|---|
| Water..... | 1} | 16° | 26° |
| Ammonium nitrate..... | 1} | ||
| Snow or pounded ice..... | 5} | .. | 20° |
| Common salt..... | 2} | ||
| Snow or pounded ice..... | 1} | .. | 45° |
| Calcium chloride, crystals..... | 2} | ||
| Ammonium nitrate..... | 1} | 14° | 29° |
| Sodium carbonate..... | 1} | ||
| Water..... | 1} | 29° | 39° |
| Sodium phosphate..... | 27} | ||
| Ammonium nitrate..... | 18} | .. | 28° |
| Fuming nitrous acid..... | 8} | ||
| Water..... | 4} | 18° | 28° |
| Sodium sulphate..... | 8} | ||
| Hydrochloric acid..... | 5} | 32° | 42° |
| Snow..... | 8} | ||
| Hydrochloric acid (at 0° C.)..... | 5} |
Such mixtures are only applicable where a low temperature is required for a short space of time, and are of no use where a continuous process of refrigeration is necessary. For this latter purpose the low temperature produced by the expansion of gases and vapours is chiefly used, the principal substances employed being water, ether, ammonia, and carbon bisulphite. A full account of these processes, and of the apparatus connected with them, will be found under REFRIGERATION. Low temperatures are also obtained by the evaporation of a liquid, either under artificially diminished pressure, or where its vapour pressure is constantly small; the energy necessary to effect the change of state being taken from the heat in the substance itself. The evaporation (and consequent cooling) of water from the surface of porous earthenware vessels, called water-coolers, is due to this principle. The same is true of the formation of ice in shallow lakes at night in tropical climates.
The extremely low temperatures reached by Natterer and others, in their experiments on the liquefaction of gas, were obtained by a mixture of solid carbonic anhydride and ether; or of solid nitrous oxide and carbon bisulphide. By such means the temperature of C., the lowest yet attained, was reached. See also COLD, FROST, ICE, THERMOMETER, WATER.