The third method of producing cold which we propos ed to consider, is liquefaction induced by chemical action. NVe have already explained (see CHEMISTRY, page 680) the general principles upon which the reduction of tem perature by liquefaction depends. We shall now give a more detailed and practical view of the subject.
The solution of salts in water, by the transition to the fluid state, is always accompanied with a considerable diminution of temperature. When nitre is added to water, at the ordinary temperature of the atmosphere, until the water is saturated, the temperature is reduced 15 or 16 degrees ; and a still greater degree of cold is obtained by the solution of muriate of ammonia. But of all the salts, nitrate of ammonia seems to undergo the greatest reduction of temperature during its solution. If it be mixed, in the state of a fine powder, with an equal weight of water, the temperature is reduced from 50° to 4°.
The greatest number of experiments on this subject, have been made by Mr Walker or Oxford. Being aware that water, saturated with one salt, was still capa ble of dissolving a portion of another, he inferred, that a greater reduction of temperature would be obtained, by dissolving different salts in succession, than by the solution of any one of them separately. The conjecture was confirmed by experiment. We have already men tioned, that nitrate of ammonia being dissolved in an equal weight of water, reduces the temperature of the ingredients from SO° to 4° ; but if an equal weight of carbonate of soda be added, the temperature is reduced from to 7°, though the latter salt, when employed alone wit water, has but little frigorific effect.
As the production of cold, by freezing mixtures, de pends partly upon the rapidity with wl.ich the substances employed pass into the liquid state, and partly upon the increase of capacity for caloric which is acquired by that transition, it might be expected that a greater reduction of temperature would be obtained, by dissolving salts in acids than in water. Accordingly, by dissolving a pound and a half avoirdupois of Glauber's salts (sulphate of soda) in 12 ounces of nitrous acid, previously diluted with 6 ounces of water, Mr \\Talker obtained a diminu tion of temperature from 50° to —1'; when sulphuric acid, diluted in the same proportion, was employed, the reduction was from 50° to ; and three parts of the same salt by weight reduced to a tine powder, and dissolved in two parts of muriatic acid, sunk the thermometer from 50" to zero. The reduction of temperature is still great
er, when different salts are employed at the same time. Thus, nitrate of ammonia 6 parts, phosphate of soda 9 parts, and diluted nitrous acid 4 parts, all by weight, reduce the temperature of the mixture from 50° to —21°. This is a diminution of 71°, and is the greatest degree of cold that can be procured by a single mixture yet known. Mr Walker succeeded in freezing mercury, by cooling, in a succession of these mixtures, the ingredients by whose liquefaction the congelation was ultimately to be produced, though the temperature of the air at the commencement of the experiment was 45°. Thus, let the mixtures be represented by A, B, C, D : the ingre dients for the mixture B were cooled by A ; those for C by B ; and lastly, those for D by C ; so that the ingre dients for the last mixtures were at a very reduced tem perature, before they were exposed to mutual action with one another.
When ice or snow is dissolved in acids, the solution takes place very rapidly, and is accompanied with a great reduction of temperature. The degree of cold obtained varies with the acids, and even with the state of concentration in which they are employed. I.owitz has ascertained, that, in the undiluted state, muriatic acid produces the greatest cold, nitrous the next, and sul phuric acid the least ; the first reducing the temperature from 32' to-29°, the second to —22°, and the third to-11° of Fahrenheit. Sulphuric acid, however, when it is diluted with water, has a more powerful effect than either of the other two. Contrary to what we should have expected, if the acids be cooled so low as to be congealed, they produce, when added in that state to snow, a smaller reduction of temperature than they pro duce when liquid. The diminution of effect is proba bly to be ascribed to the solution taking place more slowly.