200 M. R. Bnusen on the Lata of Absorption of Gases. 



metre, or about 9 square feet of land, only 1*293 litre or 2*569 

 grms. of carbonic acid. For every temperature above 0° this 

 amount of course decreases. The carbonic acid carried down 

 with rain from the atmosphere is therefore so inconsiderable, 

 that it appears to be quite inadequate for the nourishment of 

 plants. Still it must not be forgotten that this amount of car- 

 bonic acid originally present in the rain-water, in proportion as 

 it is absorbed by the plants, is again taken up from the passing 

 currents of air by the moisture of the leaves and ground. Dew, 

 rain, and even the juices of plants serve then, as means by which 

 the nourishing substances of the atmosphere arc transferred to 

 the bodies of the plants. The supply of substances capable of 

 assimilation is therefore regulated by the law of absorption ; it 

 varies with the temperature and barometric pressure. The 

 volume of carbonic acid which is contained in the unit-volume 

 of the sap of the plant and moistening surface of water at the 

 mean barometric pressure, as obtained from formula (3), varies 

 with the temperature in the following ratio : — 



6 C. Volume. 



0-000744 



5 0000600 



10 , 0-000490 



15 ..... . 0-000415 



20 0000373 



The plant therefore finds in a volume of the water with which 

 it is moistened at 15°, the same amount of carbonic acid which 

 a like volume of air contains ; above this temperature, however, 

 there is less, and below more. 



It is scarcely possible to overlook in these phamomena an en- 

 deavour of nature to assist the tardy vegetation of the cold north 

 by a richer nourishment, and by a sparing supply to keep the 

 luxurious growth of the tropics within limits. 



Similarly to carbonic acid, oxygen plays an important part in 

 these phenomena, for under the influence of the decomposing 

 organic constituents of the ground it is converted into carbonic 

 acid. The mixture containing the greatest amount of carbonic 

 acid which can thus be formed is — 



Nitrogen 79-007 



Carbonic acid 2CH)93 



lOOOOO 



If we suppose this mixture of gases to remain constant by 

 continual additions from without, according to formula (3), the 

 unit-volume of the moisture contained in the ground surround- 

 ing the roots will take up of carbonic acid,-— 



