DEWPOINT: ASPIRATORS. 



53 



the two readings, first when the dew began to form and second 

 when it just clears off, may be tak^n as the true dew-point. For 

 instance, suppose that the temperature of the room was 18 centi- 

 grade to begin with, and that the thermometer showed 10 when 

 the dew first became visible and 10 *5 when the glass became 

 clear again, then the true dew-point would be the average of 10 

 and 10*5 = 10'25 centigrade. This shows that the air of the 

 room was not saturated with moisture at the temperature of 18, 

 but that as much moisture was present as would saturate it at the 

 temperature of 10 '25. 



In order to avoid the taste of the ether-vapour being sucked into 

 the mouth, a contrivance called an aspirator may be conveniently 

 used ; this may be simply made from a tin can with a tap soldered 

 in near the bottom (fig. 27), and a tightly-fitting cork in the neck 

 through which passes a short piece of glass tubing on to which the 

 india-rubber tube is slipped ; the cork is taken out and the can 

 filled with water ; the cork is then fixed in its place and the tap 



Fig. 27. Tin Can Aspirator. Fig. 28. Glass Aspirator. 



turned on, when as the water runs out air will be sucked in in its 

 place. This contrivance is often used in laboratories to produce a 

 current of air through tubes, &c. Fig. 28 represents a glass 

 aspirator consisting of a bottle with a Second neck near the bottom 

 into which a tap is fitted by means of a cork. 



The cork of the aspirator ought to fit air-tight ; if this is the 

 case, on tightly pinching the india-rubber tube the water will cease 

 to flow. If the cork is not quite air-tight it may be made so by 

 means of sealing-wax melted over it, or with a little putty or 

 linseed-meal paste smeared over it so as to fill up leaks and 



