VAPORIZATION. 



61 



carry off the condensing water, may be made to pass through 

 openings in the corks, which they fit, as represented in Fi- 

 gure 3. 



Evaporation in vacuo. Water rises rapidly into vapour in a 

 vacuous space, without the appearance of ebullition, at all tem- 

 peratures, even at 32, and greatly lower. Its elastic force in- 

 creases as the temperature is elevated, till at 212, it is equal to 

 that of the atmosphere, or capable of supporting a column of 

 mercury thirty inches in height. Various other solid and liquid 

 substances emit vapour in similar circumstances, such as cam- 

 phor, alcohol, ether, and oil of turpentine. Such bodies are 

 said to be volatile, and other bodies, such as marble, the metals, 

 &c. which do not emit a sensible vapour at the temperature of 

 the air, are said to be fixed. All bodies which boil at low tempe- 

 ratures belong to the volatile class. An accurate estimate of 

 the volatility of different bodies is obtained by determining the 

 elastic force of the vapour which they emit in the vacuous space 

 above the column of mercury of the barometer. If we pass up 

 a bubble of air into the vacuum of the barometer, above 

 the mercurial column, standing at the time at a height of 30 

 inches, the mercury is depressed, we may suppose to the level of 

 29 inches, or by one inch. This would indicate that the air, by 

 rising above the mercury, has been ex- 

 panded into thirty times its former bulk, 

 or that the elastic force of this rare air is 

 equal to a column of one inch of mercury. 

 I The elastic force of vapour is estimated in 

 the same manner. A few drops of the 

 liquid operated upon are passed up into 

 the vacuum above the mercurial column, 

 which is depressed in proportion to the 

 elastic force of the vapour. The depression 

 produced by various liquids is very diffe- 

 rent, as illustrated in the annexed figure, 

 representing four barometer tubes, in 

 which the mercury is at its proper height 

 in No. 1 ; is depressed by the vapour of 

 water of the temperature 60 in No. 2 ; 

 and by alcohol and ether at the same 

 temperature in Nos. 3 and 4 respec- 

 tively. 



