188 COLD FROM EVAPORATION. 



subsists. That, indeed, is just what is meant by 

 a chemical combination. 



But the water, when in the most minute state 

 of division, and ascending in air so very thin, that 

 the slightest cobweb would sink like a stone, is 

 in every one of its little and invisible drops, as 

 perfectly water as when it rolls in the flood of a 

 river, or spreads in the ocean; and it is just as 

 ready to obey all the laws of the water, in the one 

 situation as in the other. 



The evaporability of water is the principal rea- 

 son why it, and substances that are wet with it, 

 do not become so soon hot as substances that are 

 dry. When it is exposed to the air, it evaporates 

 not only as long as it remains liquid, but even when 

 it is frozen ; although the evaporation of ice when 

 it presents only one uniform surface to the air 

 is slower than that of liquid water ; because the 

 heat has to melt the ice, before it can turn the 

 water of that ice into vapour. Thus the cooling 

 influence of ice upon the atmosphere, is much less 

 and also more confined to the vicinity of the sur- 

 face than that of water at the temperature of 

 freezing, or even at a temperature a little higher 

 than that, perhaps too as high as about forty-two 

 degrees of the common thermometer, the tempera- 

 ture at which water has the greatest density, and 

 at which, when it is all cooled down to it, the 

 water in a pond or lake remains stationary, with- 

 out any internal motion upwards or downwards. 



The slower evaporation that takes place from 

 ice than from water, is. the reason why, in walk- 

 ing abroad, one feels so much more warm and 

 comfortable on a dry frosty day, a day even of the 

 keenest frost, than on those raw days which are 



