732 



COLD. 



on. 



water by 



i . c v :: i- 



rituuxed 

 receiver. 



then placed in/a beer gl-us containing about a cubic inch 

 of ether ; if in this stair, the whole be included in the re- 

 crirer of an air-pump, and the air be exhausted slowly 

 and gently, the ether will pass into a violent ebullition, 

 and toon be converted into vapour. But in ord<-r to un- 

 dergo that change of condition, the ether mint derive the 

 necrmry supply of caloric, cither from itielf, or the con- 

 tiguous bodiw. Thi abstraction of caloric soon reduce* 

 the temperature of the ether, and that of the bodies in 

 contact with it ; and a: length a degree of cold is pro- 

 duced sufficient to freeze the water inclosed in the tube. 

 It it necessary, however, to continue the process of ex- 

 haustion 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. 



^ r ke'l' e h* 8 inggMted a beautiful variety of this ex- 

 periment. It occurred to him, that, by a sufficient de- 

 gree of exhaustion, water itself might be converted into 

 vipontiun vapour at the ordinary temperature of the atmosphere ; 

 un *' er * n and that the quantity of caloric necessary for that pur- 

 pose 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 

 tlie 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 wa- 

 ter, by being gradually deprived of its caloric, was at 

 last reduced to the freezing point, and parsed 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 absorbing 

 the vapour as rapidly as possible. The cup for holding 

 ihe water, should be made of unglazed earthen ware, 

 very flat, and placed at such a distance from the sulphu- 

 ric acid, as not to be affected in any creat degree by the 

 increase of temperature which it suffers from absorbing 

 the vapour. And lastly, the receiver should not be larger 

 than is barely sufficient for containing the apparatus tor 

 the experiment. 



The vessels for containing the water and the sulphuric 

 mini of the ac j ( j i are disposed in the following manner: The cup 

 apparatus. w j,j c j, con tains 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, determi- 

 ned by experiment, that when equal weights of sulphuric 

 , and various deliquescent salts, are placed under a 

 receiver, and the air exhausted, the sulphuric acid ab- 

 itncts moisture from all of them which we have tried. 



If the air pump he good, and the sulphuric acid suf- 

 ficiently concentrated, the congelation of the water ge- 

 nerally take* place in two minutes, even when the tem- 



Arrange- 



perature of the room is iO or 60 of Fahrenheit's scale. 

 We have had an opportunity of performing the cxperi- 

 m'-nt with a very powerful air-pump, constructed by 

 Miller and Adie, which brings the gage to within ^ of 

 zn inch of the barometrical column, and have sometime* 

 succeeded in a minute and a half from the commencement 

 of the exhaustion, though the temperature of the water 

 was then 60. When the exhaustion is nearly comple- 

 ted, 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, if the exhaustion 

 be less perfect, and carried on without exposing the wa- 

 ter to much agitation, the temperature is frequently re- 

 duced 10 below the freezing point without congelation 

 taking place ; but upon giving a suddrn blow to the air- 

 pump, and thua 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. Thui 

 let the temperature of the water at the commencement 

 of the experiment be 50 : since water, in its transition to 

 vapour, absorbs as much caloric, as in a sensible state 

 would raise its temperature about 900, in order that it 

 may be reduced 18, or from 50 to .32, a quantity of 

 the whole must be evaporated equal to 7 ' z 8 -> or rs' 



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. Thelatter was 

 affixed to the un'der side of a brass plate, which was in- 

 tended to support the receiver and the freezing appara- 

 tus. 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 exprl 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. When 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 boil- 

 er 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 un- 

 screwed from the stop-cock of the boiler. The process 

 of expelling the air from the boiler by steam was repeat- 

 ed, and after the vapour was condensed, a communication 

 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 sufficiently great for the 

 success of the experiment. When the receiver was small, 

 three or four times were sufficient. 



We are disposed to think, that this experiment might 

 also be repeated very conveniently, by means of a Tor- 

 ricellian vacuum obtained by water. 



As it was well known that ice suffered a gradual dimi- 

 nution by evaporation, even when the temperature was 

 considerably below the freezing point, Mr Leslie conclu- 

 ded, that this evaporation would go on still more rapidly 

 in vacua, and that a great reduction of temperature would 

 be obtained, by placing a quantity of ice under a recei- 

 ver, in circumstances similar to those we have already 



Cold. 



The uroe 

 experiment 

 performed 

 by means 

 of the va- 

 cuum pro- 

 cured by 

 Mearru 



Freezing of 

 mercury 

 by the eva- 

 poration of 

 ice under 

 an exhaust- 

 ed recei- 

 ver. 



