92 NATURE STUDY 



evaporating substance like gasoline or bisulphide of carbon a vefy 

 low temperature can be obtained. Drops of these on the hand 

 produce a marked cooling effect as they evaporate. 



Applications of the facts learned about evaporation will 

 readily suggest themselves in regard to the drying up of the 

 ground after a rain, the filling of the air with moisture from the 

 sea, lake and rivers. 



Boiling. 



The studv of boiling may well follow that of evaporation. 

 For this purpose the glass flask which has already been used is 

 excellent as it allows the phenomenon to be seen. Fill the flask 

 two-thirds with water and use an alcohol lamp, as considerable 

 heat is needed. Have the whole process watched very carefully. 

 The following points are to be successfully made out: first, the 

 carrents of the water from the heating, then the formation of bub- 

 bles on the sides of the flask. These are bubbles of air which have 

 been dissolved in the water which the heat expands. They will 

 rise to the top and disappear. They have nothing to do with 

 boiling. Later some bubbles will form just over the flame. These 

 will rise and disappear within the liquid, or only reach the top as 

 very much smaller than when starting. These are bubbles of 

 real steam, but as the water is not yet well heated throughout, 

 the3' wholly or partially condense in the cooler water as they rise. 

 Their collapse makes a tinkling noise in a glass flask. Many of 

 them sounding together make the singing noise preceding the 

 actual boiling. In a short time the water is so well heated that 

 the bubbles of steam reach the top in full size and push out the 

 air. The steam now pours out at the top where reaching the cool 

 air it condenses into a white cloud. This usuall}' is called the 

 steam, but it consists of minute particles of water condensed from 

 the steam. The steam is the transparent vapor over the water in 

 the flask. 



Clouds, fogs and the white masses which arise from steam 



