The Atmosphere 



It must, therefore, be regarded as an essential con- 

 stituent of the atmosphere, and can only be regarded 

 as an impurity when present in quantities considerably 

 greater than the normal amount, as it frequently is in 

 large cities, or in crowded rooms, owing to its being 

 formed by the burning of wood and coal and by the 

 respiration of animals. 



Most plants have the power of absorbing the 

 carbonic acid from the atmosphere, and, with the aid 

 of sunlight, of decomposing it, using up the carbon 

 which it contains to build up their woody tissues, and 

 giving back the oxygen, at least in part, to the 

 atmosphere. 



Carbonic acid is soluble in water, and is, therefore, 

 caught up by falling raindrops, and it is this dissolved 

 gas in rain - water which brings about many of the 

 changes alluded to above as weathering, while it is 

 also responsible, as will be seen later, for the formation 

 of caverns in limestone countries. 



From such observations as have been made, it 

 would appear that the quantity of carbonic acid in the 

 lower parts of the atmosphere is much greater than 

 that at higher levels, and this fact, as the sequel will 

 show, has a very important bearing on the temperature 

 of the upper and lower portions. 



The water vapour which is in an invisible form is 

 very variable in amount. The quantity of water vapour 

 which air is capable of holding depends upon the 

 temperature. The higher the temperature the more 

 water vapour it can hold. If, therefore, a quantity of 

 warm, moist air is cooled, some of the water will appear 

 in the liquid form, as cloud, fog or dew. 



