834 LOWERING OF TEMPERATURE BY EVAPORATION. 



of temperature caused by its own rapid evaporation. 

 A shallow vessel half filled with concentrated sulphuric 

 acid is placed under the receiver of the air-pump. In 

 the acid a very small vessel cut out of a piece of cork, 

 and covered inside with lampblack, is supported by 

 three legs of glass. About 1 or 2 CC of water is poured 

 into the cork vessel, and the receiver is exhausted as 

 perfectly as possible. Fig. 403 gives a section of the 



FIG. 403 (i real size). 



apparatus .containing the liquids. As soon as the air is 

 removed, a rapid evaporation of the water commences, 

 and the receiver would soon be iilled with vapour of 

 water, which would have again to be pumped out, in 

 order that the evaporation might continue, were it not 

 for the sulphuric acid, which has the property of eagerly 

 absorbing aqueous vapour. The vapour of water, after 

 the air is removed, is absorbed by the sulphuric acid as 

 rapidly as it is formed. After some time the tempera- 

 ture of the water falls to 0, and it begins to freeze. 

 Frequently the solidification is retarded; the tempera- 

 ture sinks below 0, and the whole mass freezes all at 

 once. 



The air-pump used for this experiment must be a good one and 

 in working order, while the preceding experiment will succeed even 

 if the exhaustion is not very perfect. The tension of aqueous 

 vapour at is 4 mui> 6 (see page 799) ; it follows that the pressure 

 of the air under the receiver must be reduced to at least 4 mm '6 if 

 the evaporation is to continue after the temperature has fallen to 



