As liquids assume the state of vapour, at a very reduc ed temperature, when the pressure of the atmosphere is removed from their surface, it might be expected, that a more rapid as well as a greater production of cold would be obtained by inclosing them in the receiver of an air-pump,and then exhausting the air. Accordingly, if a small tube, about a quarter of an inch in diameter, and four or five inches long, be nearly tilled with water, and then placed in a beer glass containing about a cubic inch of ether ; if in this state, the whole be included in the rOcciver of an air-pump, and the air be exhausted slowly and gently, the ether will pass into a violent ebullition, and soon be converted into vapour. But in order to undergo that change of condition, the ether must derive the necessary supply of caloric, either from itself, or the contiguous bodies. This abstraction of caloric soon reduces the temperature of the ether, and that of the bodies in contact with it ; and at length a degree of cold is produced sufficient to freeze the water inclosed in the tube. It is necessary, however, to con tinue the process of exhaustion during the whole of the experiment, otherwise the vapour of the ether would soon act by its elasticity on the surface of the liquid part of it, and produce the same mechanical effect as the atmosphere.
Mr Leslie has suggested a beautiful variety of this experiment. It °mu-red to him, that, by a sufficient degree of exhaustion, water itself might be converted into vapour at the ordinary temperature of the atmos phere ; and that the quantity of caloric necessary for that purpose being abstracted from the water, might reduce its temperature so much as to cause it to freeze. In order to supersede the necessity of removing the vapour by continued exhaustion, he inclosed a quantity of highly concentrated sulphuric acid under the same receiver with the water subjected to experiment, in a separate vessel. The acid, by its strong attraction for humidity, absorbed the vapour of the water as fast as it was disengaged, and thus left the vacuum as complete as before, while the water, by being gradually deprived of its caloric, was at last reduced to the freezing point, and passed into the state of ice. We shall state, very briefly, the circum stances to be attended to in performing this very interest ing experiment. The air-pump must possess an exhaust ing power, capable of bringing the gage within at least half an inch of the barometrical column ; the sulphuric acid should be highly concentrated, and should present as large a surface as the receiver will allow, for absorb ing the vapour as rapidly as possible. The cup for hold ing the water should be made of unglazed earthen ware, very flat, and placed at such a distance from the sul phuric acid, as not to be affected in any great degree by the increase of temperature which it suffers from absorb ing the vapour. And lastly, the receiver should not be larger than is barely sufficient for containing the appara tus for the experiment.
The vessels for containing the water and the sulphuric acid, arc disposed in the following manner : The cup which contains the acid is placed on the plate of the air pump, and a flat vessel of the shape of a beer-glass is then placed in it, to support the porous cup which con tains the water, and keep it at a sufficient distance from the acid. By this arrangement, the cup containing the water is protected from any radiation that might take place from the sulphuric acid, while the glass, being a bad conductor, suffers very little heat to be conveyed by transmission. Instead of sulphuric acid, any other sub stance that has a strong attraction for humidity may be employed, as dry potash, muriate of lime, &c. though
sulphuric acid, when it is highly concentrated, appears to answer the purpose best. We have, indeed, determin ed by experiment, that when equal weights of sulphuric acid, and various deliquescent salts, are placed under a receiver, and the air exhausted, the sulphuric acid ab stracts moisture from all of them hich we have tried.
If the air-pump be good, and the sulphuric acid suf ficiently concentrated, the congelation of the water gen erally takes place in two minutes, even when the tem perature of the room is 50° or 60° of Fahrenheit's scale. We have had an opportunity of performing the experi ment with a very powerful air-pump, constructed by Miller and Adie, which brings the gage to within of an inch of the barometrical column, and have sometimes succeeded in a minute and a half from the commence ment of the exhaustion, though the temperature of the water was then 60°. When the exhaustion is nearly completed, the water is thrown into a state of violent ebullition, and occasionally large bubbles are arrested before they have time to burst, and suddenly converted into thin hemispheres of ice. At other times, it' the ex haustion he less perfect, and carried on without exposing the water to much agitation, the temperature is frequently reduced 10° below the freezing point without congela tion taking place ; but upon giving a sudden blow to the air-pump, and thus communicating a tremulous motion to the apparatus, the water is instantly frozen, while the temperature starts up to 32°.
We may determine by an approximated calculation, how much of the water must be evaporated, to reduce it from some given temperature to the freezing point. Thus let the temperature of the water at the commence ment of the experiment be 50° : since water, in its tran sition to vapour, allsorbs as much caloric, as in a sensi ble state would raise its temperature about 900°, in order that it may be reduced le, or front 50° to 32°, a quantity of the whole must be evaporated, equal to or We have succeeded in performing the experiment of Mr Leslie, with the partial vacuum procured by steam. The boiler we employed for the purpose was pretty large, and had an iron lid soldered to it, in the middle of which was a stop-cock, having an internal screw adapted to the external screw of another stop-cock. The latter was affixed to the under side of a brass plate, which was intended to support the receiver and the freezing ap paratus. The boiler was placed on a brisk fire, with a little water in it, and the stop-cock was left open, to allow the air to escape. After the water had boiled a sufficient time to expel the air, the stop-cock was turned, and at the same instant the boiler was removed from the fire, and plunged into a vessel of cold water. NN' hen the steam was sufficiently condensed, the brass plate with its apparatus was screwed to the stop-cock of the boiler, a collar of leather being interposed between the screws, to make the junction air tight. The stop-cock of the boiler was then turned, and a communication being thus opened between the receiver and the boiler, the air rush ed by its elasticity from the latter to the former, and produced a partial vacuum in the receiver. The stop cock connected with the plate was then turned, and unscrewed from the stop-cock of the boiler. The pro cess of expelling the air from the boiler by steam was repeated, and after the vapour was condensed, a com munication was again opened as before between the receiver and the exhausted boiler. This was repeated, until the exhaustion of the air in the receiver was suffi ciently great for the success of the experiment. When the receiver was small, three or four times were suffi cient.