148 ELEMENTS OF SCIENCE 



Air has lately been reduced, by Professor Dewar, to a 

 liquid less blue than liquid oxygen. 



The hotter the air, the greater the amount of aqueous 

 vapour it can contain, and vice versa. The maximum 

 quantity which air contains at 50 F. is about T |^ of its 

 weight, but at 82 air will contain ^ of its weight. 



If air, containing much aqueous vapour, be suddenly 

 cooled, the latter is thereby forced to condense itself 

 into particles of liquid water, which may appear as mist, 

 cloud, rain, or dew. A familiar example of such con- 

 densation may be seen when a decanter of iced water is 

 brought into a hot room; then aqueous vapour will 

 immediately condense upon its exterior. 



Dew is occasioned by the radiation * of heat from the 

 earth's surface which, when the sky is clear, i.e., when 

 there are no clouds to reflect it back again, rapidly cools 

 that surface, and so forces the air immediately in con- 

 tact with it, to condense its vapour and part with it in 

 the form of dew. Dew does not fall, but, as it were, 

 grows upon the surface it coats, just as on the surface 

 of the decanter of iced water above mentioned. Hoar 

 frost is vapour frozen into crystals of ice. 



Since the warmer the air, the greater the amount of 

 aqueous vapour it may, but by no means must, contain, 

 it follows that the greater amount of cooling air can 

 stand without shedding its dew, the drier it must be 

 the less the proportion of aqueous vapour in it. In 

 England it rarely needs 30 of cooling, but in some hot 

 climates it may need more than twice this amount. 



The main essential characters of air, in as far as it is a 

 gaseous body, have already been stated, t as well as the 

 downward force it exercises, and the consequent compres- 



* See ante, p. 96. t See ante, pp. 78 and 147. 



