COLD FROM EVAPORATION. 185 



some, is a compound of chlorine and hydrogen and 

 soda, each of which is a poison, and when perfectly 

 pure a very deadly one, nor are we acquainted with 

 any real chymical combination in which the proper- 

 ties of all the ingredients are not suspended and 

 new ones produced, while the combination subsists. 

 That, indeed, is just what is meant by a chymical 

 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 reason 

 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 surface than that of water 

 at the temperature of freezing, or even at a tem- 

 perature a little higher than that, perhaps too as 

 high as about forty-two degrees of the common 

 thermometer, the temperature 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, without any internal motion 

 upwards or downwards. 



The slower evaporation that takes place from ice 

 than from water is the reason why, in walking 

 abroad, one feels so much more warm and comfort- 

 Q2 



