LOWERING OF TEMPERATURE BY EVAPORATION. 833 



tube should not be too narrow, but should allow a large quantity 

 of air to be blown through it without too much exertion. 



If the student has set up a reservoir, like that represented in fig. 

 176, and described on page 255, and has a very large bottle of about 

 10 litres capacity, the current of air may be produced in a different 

 and more convenient manner. The empty bottle is closed by a 

 cork, through which two tubes pass, as in fig. 401. The tube on the 

 left, which need not reach so far in the bottle in this case, is con- 

 nected with the reservoir ; water is allowed to enter the empty bottle, 

 and as it flows in, the air is forced out and passes in a strong current 

 through the efcher. 



In a vacuum ether evaporates at an exceedingly 

 rapid rate, and produces thereby a considerable dimi- 

 nution of temperature. A small ^ ..^^^ _ ^^_^ 

 wooden block is placed upon the 

 plate of the air-pump, a little 

 water is dropped on it, and a 



watch-glass is placed upon the FlG - 4 2 (I i ***) 

 water. Ether is poured into the watch-glass until it is 

 quite full, and the whole arrangement, which is shown 

 in fig. 402, is then covered by the glass receiver. 

 When the air is exhausted, a considerable portion of the 

 ether evaporates very rapidly, and after a few minutes 

 the water between the watch-glass and the wood be- 

 comes frozen. Air is then let in, the receiver is lifted 

 up, and the result of the experiment may be readily 

 inspected. 



It is advantageous to use a small and rather shallow receiver for 

 this experiment. A small glass cover, such as is frequently used for 

 domestic purposes, will serve very well, if its edge is previously 

 ground with emery powder upon a flat iron-plate, and afterwards, if 

 possible, upon a piece of plate glass,, until it is quite smooth and 

 flat all round. 



If a nearly perfect vacuum can be produced, even 

 water may be frozen, in consequence of the diminution 



