292 HOW KAIN IS FORMED. 



case of the glass tumbler containing ice. Any one may try the experi- 

 ment for himself. To produce hoar-frost, it is only necessary to cool 

 the condensing surface below the freezing point, which may be done by 

 crushing some ice and mixing it with salt. A tin pot is better than a 

 glass to make this experiment. 



When not only the ground, but also the air to a considerable height 

 above it, is cooled in like manner, we have the production of fog, fog 

 being the form in which the vapor is first condensed, and consisting of 

 water in drops too minute to be separately visible. The formation of 

 fog is very much aided if the air be laden with smoke. Smoke consists 

 of extremely minute particles of unburnt coal or other fuel, and these 

 cool faster than the air at night, and so cool the air in contact with 

 them. Each one of them, too, condenses wat^r on its surface, and 

 being thus weighted they sink and form that dense fog that Londoners 

 know so well. 



Clouds are essentially the same as fog, but formed high up in the air. 

 But in their case, and that of rain, snow, and hail, another and differ- 

 ent cooling agency comes into play, and this will require some prelimi- 

 nary explanation. 



I dare say that some of you maj^ at some time or other have charged 

 an air-gun. And if so, you will be aware that when so charged the 

 reservoir becomes pretty warm. Now this heat is produced, not, as 

 might be supposed, by the friction of the piston in charging, but is due 

 to the fact that work has been done upon the air by compressing it into 

 a very small space : in other words, work has been converted into heat. 

 If the compressed air be allowed to escape at once, its heat is recon- 

 verted into work. It has to make room for itself by thrusting aside 

 the atmosphere into which it escapes, and when thus expanded it is no 

 warmer than before it was compressed. Indeed, not so warm, for it 

 will already have parted with some of its heat to the metal chamber 

 which contained it. And if when compressed it is allowed to cool down 

 to the ordinary temperature, and then to escape, it will be cooled below 

 that temperature just as much as it was heated by compression. Thus, 

 if in being compressed it had been heated 100°, say from 60° to 100°, 

 and then allowed to cool to 60°, on escaping it will be cooled 100° below 

 60°, or to 40° below zero, which is the temperature at which mercury 

 freezes. This is the principle of the cold air chambers now so exten- 

 sively employed on ship-board for the transport of frozen provisions 

 from New Zealand and Australia. 



Bearing in mind, then, this fact — that air in expanding and driving 

 aside the air into which it expands is always cooled, — let us see how this 

 applies to the case before us, the production of cloud and rain. 



The volume of a given weight of air — in other words, the space it oc- 

 cupies — depends on the pressure to which it is subject ; the less this 

 pressure the greater its volume. If we suppose the atmosphere divided 

 into a number of layers superimposed on each other, the bottom layer 



