The best examples of latent heat are furnished by ice, water, and steam, and we are indebted chiefly to Dr. Black for the elegant and conclusive experiments demonstrating the important truths connected with the latent heat of these three conditions of matter. When various solids are heated, they frequently pass through certain intermediate conditions of softness, terminating in perfect liquidity; but ice and many other bodies change at once to the liquid state on the application of a sufficient quantity of heat. The process of melting ice is very slow, because every portion must absorb or render latent a certain quantity of heat before it can take the liquid state—hence the difficulty of melting blocks of ice when they are surrounded with non-conducting materials; and this fact the author has proposed to take advantage of in keeping water cool which is to be supplied to the ova of salmon whilst taking them to stock the rivers of Australia.
In order to prove that heat is rendered latent by the liquefaction of ice, it is only necessary to weigh a pound of finely-powdered ice and a pound of water at 212° Fahr. (boiling water), and mix them together; when the ice is all melted, the resulting temperature is only 52°, therefore the boiling water has lost 160° of temperature, of which 20° can be accounted for, because the resulting temperature of the melted ice is 52°; but in the liquefaction of the pound of ice, 140° have disappeared or become latent, or, as Dr. Black termed it, have become combined.
1 lb. of ice at 32° + 20° = 52°, the resulting temperature.
1 lb. of water at 212° - 52° = 160° - 20° = 140°, rendered latent.
140° represents the result obtained from innumerable experiments made by mixing equal parts of ice and boiling water, and it is this large quantity of latent heat required by ice and snow that prevents their sudden liquefaction, and the disastrous circumstances that would arise from the floods that must otherwise always be produced.
To put the fact beyond all doubt, it is advisable to mix together equal weights of water at 32° and boiling water at 212°, and the result is found by the thermometer to be the mean between the two, because half the extremes are always equal to the mean; and if the two temperatures are added together and divided by two, the result is a temperature of 122°, as shown below:—
1 lb. of ice water at 32° + 1 lb. of water at 212° = 244° ÷ 2 = 122°.
From similar experiments Dr. Black deduced the important truth, "that in all cases of liquefaction a quantity of heat not indicated by, or sensible to, the thermometer, is absorbed or disappears, and that this heat is withdrawn from the surrounding bodies, leaving them comparatively cold." At p. 79 it is shown how the sudden solution or liquefaction of certain salts produces cold, and hence numerous freezing mixtures have been devised. In olden times, when officials in authority did what they pleased, without being troubled with disagreeable returns, and colonels clothed their men, and were merchant tailors on the grand scale, gun cartridges were not confined to practice on the enemy, but they did duty frequently in the absence of ice as refrigerators of the officers' wine, in consequence of the gunpowder containing nitre or saltpetre; as a mere solution of this salt finely powdered will lower the temperature of water from 50° Fah. to 35°; whilst a mixture of four ounces of carbonate of soda and four ounces of nitrate of ammonia dissolved in four ounces of water at 60°, will in three hours freeze ten ounces of water in a metallic vessel immersed in the mixture during the liquefaction or solution of the salts.
Fahrenheit imagined he had attained the lowest possible temperature by mixing ice and salt together, and it is by this means that confectioners usually freeze their ices, or ice puddings; the materials are first incorporated, and being placed in metallic vessels or moulds, and surrounded with ice and salt placed in alternate layers, and then well stirred with a stick, they soon solidify into the forms which are so agreeable, and so frequently presented at the tables of the opulent. The temperature obtained is Fahrenheit's zero—viz., thirty-two degrees below the freezing point of water. According to the very wise police regulation observed in London, all householders are required to sweep or remove the snow from the pavement in front of their houses, and this is frequently done with salt; should an unfortunate shoeless beggar, tramp past whilst the sudden liquefaction is in progress, the effect on the soles of his feet is evidently very disagreeable, and the rapidity with which he retires from the zero affords a thermometric illustration of the most lively description.