Mr. Leslie also found, not only the same effect in this experiment, but that the action ofa cold radiating surface upon the tin reflector produced exactly the same proportional effect upon the differ ential thermometer as the hot radiating surface, only in the opposite direction of the scale. The differential thermome ter, which is always at zero when both bulbs are equally heated, is beatifully calculated to shew this striking experi went. Thus, if the difference of teNro rature, between the heat-radiating sub stance and the atmosphere be 60 de grees, and if this raises the thermometer 45 degrees, the same difference between the cold radiating substance and the atmo sphere will sink the thermometer 45 de grees, and so in proportion ; so that a cold of 16 degrees will sink the ther mometer 12 degrees ; for 60 : 45 : : 16 : 12.
Great degrees of cold are produced by mixing together those substances which dissolve rapidly . The reason of this will appear, by recollecting what has been said of the absorption of caloric, when a solid body is converted into a fluid. Mixtures to produce artificial cold are generally made of the neutral salts dissolved in water ; of diluted acids and some of the neutral salts; and of snow or pounded ice with some of these salts. A great number of experiments were made upon this subject by Mr. Walker ; also by Professor Lowitz, of Peters burgh ; by Fourcroy and Vauquelin ; and by Guyton. The following table exhi bits he results of some of these experi ments.
When any of these substances are to be employed as freezing mixtures, the salts should be used fresh crystalized, and reduced to fine powder ; and it will perhaps be found most convenient to ob serve the proportions which are set dowu in the table. Suppose it is wanted to produce a degree of artificial cold equal to-30°, which is the temperature pro duced from 32° by the seventh freezing mixture. The substances employed, namely the muriate of lime and the snow, must be previously cooled down to the temperature of 32°, or any de gree below it. This may be done by placing them separately in the third freezing mixture,, the sulphate of soda. and diluted sulphuric acid, which re duces the temperature from 50° to 30°; or in the fourth freezing mixture of snow and common salt, which reduces the temperature from 32° to 0°. The materials, thus cooled down, are then to be mixed together as quickly as possi ble, when, if the experiment succeed, the temperature will fall from 32° to — se, as in the seventh freezing mixture. The vessels which are employed for these processes should be very thin, and made of the best conductors of heat. Vessels of tin plate answer the purpose, and when acids are to be used they may be lined with wax, which will secure them sufficiently against their action. They should be of no larger dimensions than just to contain the materials.