SCIENTIFIC AMUSEMENTS. 



crevices. The same remark applies to the cork in the test-tube. 

 The instrument thus made is termed " Regnault's Hygrometer," 

 or apparatus for measuring the amount of moisture in the air. In 

 order to see the point where dew first forms and finally clears off 

 more readily, the bottom of the glass test-tube should be silvered 

 outside, or fitted into a closely-fitting thin polished silver case like 

 a thimble. 



India-rubber stoppers, perforated with one, two, or more holes 



(fig. 29) may be purchased at the 

 instrument dealers ; these gene- 

 rally fit much better than ordinary 

 corks, and are more easily made 

 air-tight. Some little practice is 

 requisite in order to pass a glass 

 tube through the holes ; the tube 

 should not be so wide as to require 

 any great amount of force to pass 

 it through with a screwing motion, the tube being moistened with 

 water ; nor so narrow as to be leaky when the apparatus is used. 

 With thin glass tubes, the glass may break in the hand, giving a 

 nasty cut if incautiously forced in a direction inclined to its length. 

 The cutoff ends of the glass tube should be smoothed with a file 

 so as not to cut the rubber (vide Expt. 48). 



Instead of india-rubber perforated stoppers, tubed caps (fig. 30) 

 may often be conveniently employed. 



Fig. 29. India-rubber Stoppers. 



Fig. 30. India-rubber Tubed Caps. 



Expt. 46. To Freeze Mercury by the Evaporation of a Volatile 

 Liquid. By employing liquids more volatile than ether in a 

 similar fashion very high degrees of cold can be obtained. Mercury 

 may even be frozen by causing such fluids to evaporate quickly 

 by sucking a rapid current of air through the liquid contained in 



