RELATION OF VOLUME IN THE GASES ABSORBED AND EVOLVED. 



conclusions are devoid of that degree of precision which the advance of chemistry, in 

 all its departments, enables us to attain. The conclusion to which these earlier phi- 

 losophers came was, that plants had the power of absorbing carbonic acid from the air, 

 and rendering oxygen in return by elaboration from their vessels ; and this they regard- 

 ed as the great means employed by nature to maintain the integrity of the composition 

 of the atmosphere. A similar view has been taken of this subject by almost every 

 philosopher who has since examined it. Professor BURNET, to accommodate the the- 

 ory to the observed facts, infers that plants exercise two functions, the one of breathing, 

 the other of digestion, the latter only occurring during the stimulant action of the sun- 

 shine. This phenomenon is, however, unquestionably, one depending on the exalted 

 capillary action of a tissue when radiant matter impinges on it ; and the evolution of 

 nitrogen, or of some other gaseous or vaporous matter, is, therefore, an essential part of 

 the process. 



435. The analyses made in the foregoing sections show that the volume of gas 

 which remains after action is complete, is exactly the same as the volume of carbonic 

 acid first operated on. The best method of proving this directly is to take a tube, the 

 diameter of which may be half an inch or upward, which is graduated into inches and 

 decimal parts. Fill it with water, from which all gaseous matter has been expelled by 

 long-continued boiling ; place a few vegetable leaves in it, carefully removing any bub- 

 bles of air which may be attached to them ; invert the tube in a vessel of water, and 

 pass into it as quickly as possible a measured quantity of pure carbonic acid, and trans- 

 fer it to a mercurial trough. This arrangement is seen fig. 60. Conduct the experi- 

 ment first in a cool, dark place ; absorption will rapidly go on, and in a short time all, 

 or the greater part of the carbonic acid will disappear, a column of mercury, e e, rising 

 in the tube to replace the gas. It is to be remarked that it is not always easy to pro- 

 cure the entire absorption of all the gas, a little bubble remaining in the upper part of 

 the tube, containing the impurities that may have existed in the gas, and also any re- 

 mains of the carbonic acid, for the amount absorbed depends upon several circumstan- 

 ces, as the relative proportion of the volume of gas to the volume of water, the height 

 of the mercurial column suspended in the tube, the temperature of the arrangement, &c. 

 Then, on exposure to the solar rays, gas is copiously given off", the quantity continually 

 decreasing until farther exposure ceases to evolve any more. On making the usual 

 corrections for temperature and pressure, the aggregate of evolved gas will be found 

 precisely the same as the volume first operated on. 



436. Sometimes, however, the volume is increased by an amount varying from -10 

 downward, due chiefly to a certain amount of gas given off from the leaves extrane- 

 ously, and partly to the capillary action of the whole system upon the elements of at- 

 mospheric air, which are transferred by slow degrees to the water operated upon, should 

 there be a film of that fluid between the mercury and the sides of the glass tube ; but, 

 by making allowance for these disturbing actions, the proportion of equality will be 

 found to be rigidly observed by the absorbed and the evolved quantities. 



437. We find, therefore, that the evolution and decomposition of carbonic acid by 

 the solar ray are due to that part of it exciting heat ; that the chemical ray has no di- 



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