WHAT OTHERS SAY. 445 



own bulk of this compound, but like other gases it may be greatly com- 

 pressed; so that water may be made to hold several times its own bulk 

 of it, as long as the pressure is maintained. 



Carbonic acid is about one-half heavier than common air, hence as- 

 cends much more slowly than the elementary gases, and exists in largest 

 quantities near the surface of the earth, and spread by the winds over 

 great areas, constantly entering into the composition of the air. Though 

 on an average it constitutes only the one twenty-five hundredth part of 

 the atmosphere yet it may in some localities, as when forests are burned, 

 form a greater portion 



In a pure state, carbonic acid is fatal to all plant life, yet, existing 

 as it does in the air, it constitutes the greater portion of all plant food, be- 

 ing constantly imbibed by the leaves and roots. The experiments of 

 De Saussure, as to the effect of carbonic acid, in excess of what is usu- 

 ally contained in the air, on plant life, are instructive. The growth of 

 plants was stimulated, in the sunshine, when the quantity of this com- 

 pound was increased so as to constitute about one-twelfth of the air; 

 when it formed two-thirds it ceased to grow, and speedily died when it 

 was made one-half. In the shade any increase in this gas in the air 

 proved injurious. 



Carbonic acid combines with the alkaline elements, lime, potash, 

 soda, etc., to form the carbonates which are in somewhat common use 

 as fertilizers. Hence, this compound of the organic elements is indis- 

 pensible to plant growth as one of the active forces in the preparation 

 of plant food. Whence comes the supply of this.' Science says it is 

 produced by fermentation of vegetable sub.stances, generated in the de- 

 cay of animal and vegetable matter; perhaps the larger portion results 

 from the daily burning of cords of wood, and tons of coal by the 

 million. 



AMMONIA. 



Another compound consisting of one of nitrogen and three of hy- 

 drogen, is known as ammonia, which exerts a powerful effect on plant 

 growth. Water can be made to contain 670 times its own bulk of this 

 gas, and this proportion is known also as hartshorn. The pure gas has 

 and acrid alkaline taste, and not only kills growing plants, but disor- 

 ganizes them. 



All vegetable substances absorb ammonia rapidly, as do even the 

 clays of our sub soils when in contact with it. Charcoal will absorb 

 about 95 times its own bulk of this compound, and light friable soils, 



