HOW RAIN IS FORMED. 291 



obviously, since water can evaporate at all temperatures, so we sliould 

 expect that it may condense at all temperatures. On what then does 

 the condensing point depend '? 



I mentioned just now that air at the temperature of (10^' can contain 

 as much as 5"^ grains of vapor, and at 80° rather less than 11 grains in 

 each cubic foot. Obviously then if air at 80°, containing this maxi- 

 imum quantity, be cooled to 60°, it must get rid of more than 5 grains, 

 or nearly half its vapor, and this excess must be condensed. 1 speak 

 of air containing these quantities, but in point of fact it makes no ap- 

 preciable diiierence 'vhether air be present or not. An exhausted glass 

 vessel of one cubic foot capacity can hold 5'^ grains of vapor at 60° and 

 no more, and nearly II grains at 80'° and no more ; and if, when thus 

 charged at 80°, its contents be cooled to C0°, more than 5 grains will 

 be condensed. If however it contain only 5.f grains at 80°, none will 

 condense until the temperature falls to 60°, but any further cooling 

 produces some condensation. Thus then the condensing point depwids 

 on the quantity of vapor present in the air, and is the temperature at 

 which this quantity is the maximum possible for that temperature. 



This preliminary point being explained, we may now proceed to in- 

 quire what means Nature employs to condense the vapor in the air, 

 producing at one time dew and hoar-frost, at another time fog and 

 cloud, and at another, rain, hail, and snow. 



Let us take the case of dew and hoar-frost tirst, as they are compara- 

 tively' sirai)le. And in connection therewith I may relate a little inci- 

 dent that took place at Calcutta some years ago. A gentleman, who 

 had not much acquaintance with physical science, was sitting one even- 

 ing with a glass of iced brandy and water before him. It was in the 

 rainy season, when the air, though warm, is very ilamp, and he had a 

 large lump of ice in his tumbler. On taking it up, he noticed to his 

 surprise that the glass was wet on the outside, and was standing in 

 quite a little pool of water on the table. At first he thought his tum- 

 bler was cracked, but i)uttiug his finger to his tongue he found the fluid 

 tasteless. " Very odd," he remarked ; " the water comes through the 

 glass but the brandy doesn't." 



Now however, with our present knowledge, we may be inclined to 

 smile at the simplicity of this remark, it so happens that up to the end 

 of the last century very much the same explanation was popularly held 

 to account for dew. It was supposed to be a kind of persi)iration emit- 

 ted from the earth, and no satisfactory explanation of the phenomenon 

 had been arrived at by the physical philosophers of the day. It re- 

 mained for Dr. Wells to prove, by a long series of observations and 

 experiments, which have been quoted by Sir .lohn Herschel and Mr. 

 John Stewart Mill as a typical instance of philosophical iiupiiry, that 

 the cold surface of grass and shrubs condenses the vapor previously 

 held in susi>ension in the air, these surfaces being cooler than the air, 

 and below its point of condensation. And such of course is also the 



