246 MOISTURE OE THE AIR. 



dry, approaches its point of full-charge during the 

 radiation of the night. Thus it is that plants, 

 without any fall of wet taking place, but only by 

 virtue of the peculiar power with which nature has 

 endowed them, take up again in the night a part 

 of the moisture which they have lost during the 

 day. In the neighbourhood of great masses of 

 water, on the banks of rivers, on the coasts of seas, 

 where a brisk evaporation is always going on, the 

 herbage, even when there is no rain, cannot become 

 wholly dried up, while in inland districts, the want 

 of rain is very quickly felt on account of the 

 increasing dryness of the air which surrounds the 

 plants. 



If the air is by any cause cooled down below 

 that temperature, at which the moisture, which it 

 holds, will be its full charge, a part of its vapour 

 will be withdrawn from it in the form of liquid 

 water. Thus we see that cold bodies placed in the 

 open air very often become studded with drops of 

 water — Dew-drops — because they cool down the 

 surrounding air to a temperature below its point 

 of full charge, which is called the Dew-point. For 

 the same reason, when it is cold out of doors, 

 the windows of rooms which are dwelt in, and 

 which are therefore usually filled with moist air, 

 become wet. On cold winter days the moisture 

 thus set clown from the air hardens into ice. We 

 then say the windows are frozen. It is clear that 



