PROPERTIES OF THJE AIR. 41 



and the even distribution of plant food ; but this will be 

 noticed more fully hereafter. 



The air contains, also evenly diffused through it, a very 

 small proportion of carbonic acid. This . averages about 

 one part by bulk to every 2500 of air ; and varies slightly 

 according to circumstances. This carbonic acid is of the 

 highest importance to the growth of plants, inasmuch, as it 

 is from this source chiefly, that, as is believed, plants derive - , 

 this necessary nutriment, from which is formed, the cellular 

 tissue, the starch, sugar, gum and fats, which they contain, 

 and which makes up so large a portion of their dry sub- 

 stance. 



The air also contains a varying quantity of watery vapor 

 diffused through it, amounting to 1 per cent, of its weight 

 on an average of season and locality. This proportion is 

 largest in summer, increasing with the temperature, and 

 least in the winter ; thus supplying the crops in their grow- 

 ing season out of the abundance gathered up for their sup- 

 port ; as well as to moderate the heat. This moisture is 

 condensed as the temperature is reduced, and forms clouds, 

 which, floating in the air, shade and protect the earth from 

 the too ardent rays of the sun, and at night they reflect the 

 heat which is radiated from the earth's surface, the loss of 

 which in cloudless countries causes intense cold in the night, 

 following equally intense heat at midday, when nothing in- 

 tervenes to intercept the burning rays of the sun. 



The air also contains varying quantities of matter con- 

 tributed by decaying organic substances and from the dry 

 soil; as ammonia evolved from putrefying plants; decaying 

 leaves, and decomposing dead animals, and the excrements 

 of living ones, and dust from this dried debris of animal 

 life. A ray of light or a sunbeam passing through a crev- 

 ice into a darkened room, shows this contribution to the 

 atmosphere which is not without its use and importance to 

 vegetable life in a myriad of particles which glisten as they 

 float in the light. 



Air, like all other matter, has weight. This was first 

 discovered about 200 years ago. A cubic foot of air weighs 



