VAPOR IN THE AIR. 41 



40. The air then always contains a regular propor- 

 tion of carbonic acid gas, which is constantly pro- 

 duced by the burning of combustibles, and in many 

 other ways, and as constantly decomposed by the 

 action of plants. As we are now only considering 

 the properties and nature of the air, we will, for the 

 present, pass over further consideration of carbonic 

 acid gas, to which we shall shortly return (108) when 

 studying the nature of carbon, and merely mention 

 now that it is of the greatest importance to the life 

 of plants, being the principal source from whence 

 they derive the carbon necessary for their growth. 



41. The air always contains dissolved in it some 

 water, or rather vapor, which varies in quantity ac- 

 cording as the air is hotter or colder. When it is 

 hot, a larger quantity of water is evaporated or con- 

 verted into vapor, and dissolved in the air, which in 

 consequence becomes more damp; whilst, on the other 

 hand, when the air becomes cold, the vapor in the air 

 is condensed, returning to the state of water, and the 

 air becomes drier. 



42. This, of course, is modified according to cir- 

 cumstances; thus, in dry barren countries, where the 

 ground contains but little moisture, the air, when it 

 becomes hot, remains comparatively dry; whilst in 

 moist or swampy countries, under similar circum- 

 stances, the air becomes damp from the abundance 

 of vapor given off; and thus some of the principal 

 differences of climate depend mainly upon the quan- 

 tity of water dissolved in the air. 



4* 



