632 



FARMERS' REGISTER. 



[No. II 



a small quantity of water was daily supplied to 

 the frrass by means of the stop-cock.* Every 

 day likewise a certain quantity of water was re- 

 moved by a syphon, and water saturated with car- 

 bonic acid gas was supplied in its place; so that it 

 may be presumed that a small quantity of carbonic 

 acid gas was constantly present in the receiver. 

 On the 7th of July, 1807, the first day of the ex- 

 periment, the weather was cloudy in the morninof, 

 but fine in the afternoon; the thermometer at 67, 

 the barometer 30.2: towards the evening of this 

 day aslight increase of the gas was perceived: the 

 next three days were bright; bui in the morning 

 of the 11th the sky was clouded; a considerable 

 increase of the volume of the gas was now ob- 

 served: the 12ih was cloudy, with gleams of sun- 

 shine; there was still an increase, but less than in 

 the bright days: the 13th was briiiht. About 

 nine o'clock a. m. on the 14th, the receiver was 

 quite full; and considering the original quantity in 

 the jar, it must have been increased by at least 30 

 cubical inches of elastic fluid: at times during this 

 day globules of gas escaped. At ten on the morn- 

 ins" of the 15th, I examined a portion of the gas; 

 it contained less than -^\- of carbonic acid gas; 100 

 parts of it exposed to the impregnated solution 

 left only 75 parts; so that the air was (bur per 

 cent. pJrer than the air of the atinosphere. 



I shall detail another similar experiment made 

 with equally decisive results. A shoot from a 

 vine, having three healthy leaves belonging to it, 

 attached to its parent tree, was bent so as to be 

 placed under the receiver which had been used in 

 the last experiment; the water confining the com- 

 mon air was kept in the same manner impreg- 

 nated with carbonic acid gas: the experiment was 

 carried on from August 6ih, till August 14tb, 

 1807; durmg this time, though the weather h;id 

 been generally clouded, and there had been some 

 rain, the volume of elastic fluid continued to in- 

 •crease. Its quality was examined on the morning 

 •of the 15th; it contained j\ of carbonic acid gas, 

 and 100 parts of it aflx)rded 23.5 of oxygen gas. 



These facts confirm the popular opinion, that 

 when the leaves of vegetables perform their healthy 

 functions, they tend to purify the atmosphere in the 

 common variations of weather, and changes ii^om 

 light to darkness. 



In germination, and at the time of the decay of 

 the leaf, oxygen must be absorbed ; but when it 

 is considered how large a partof the surface of the 

 earth is clothed with perennial grasses, and that 

 half of the globe is always exposed to the solar 

 light it appears by far the most probable opinion, 

 that more oxygen is produced than consumed 

 during the process of vegetation; and that it is this 

 circumstance which is the principal cause of the 

 uniformity of the constitution of the atmosphere. 



Animals produce no oxygen gas during the ex- 

 ercise of any of their functions, and they are con- 

 stantly consuming it; but the extent of the animal, 

 compared to that of the vegetable kingdom, is very 

 small ; and the quantity of carbonic acid gas pro- 

 duced in respiration, and in various processes of 

 combustion and fermentation, bears a proportion 

 extremely minute to the whole volume of the at- 

 mosphere: if every plant during the progress of its 

 life makes a very small addition of oxygen to the 

 air, and occasions a very small consumption ofcar- 



* See Fig. 17. 



bonic acid, the effect may be conceived adequate 

 to the wants of nature. 



It may occur as an objection to these views, 

 that if the leaves of plants purify the atmosphere, 

 towards the end ofaulumn, and through the winter, 

 and early spring, the air in our cliinates must be- 

 come impure, the oxygen in it diminish, and the 

 carbonic acid gas increase, which is not the ca^^e; 

 but there is a very satisfactory ansvi^er to this ob- 

 jection. The dift'erent parts of the atmosphere are 

 constantly mixed together by winds, which, when 

 they are strong, move at the rate of (i-om 60 to 

 100 miles in an hour. In our winter, the south- 

 west gales convey air which has been purified by 

 the vast forests and savannahs of South America, 

 and which passing over the ocean, arrives in an 

 uncontaminated stale. The storms and tempests 

 which often occur at the beginning, and towards 

 the middle of our winter, and which generally blow 

 from the same quarter of the globe, have a salutary 

 influence. Ey constant agitation and motion, the 

 equilibrium of the constituent parts of the atmo- 

 sphere is preserved; it is fitted for the purposes 

 of life ; and those events which the superstitious 

 formerly referred to the wrath of Heaven, or the 

 agency of evil spirits, and in which they saw only 

 disorder and confusion, are demonstrated, by 

 science, to be ministrations of Divine Intelligence, 

 and connected with the order and harmony of our 

 system. 



I have reasoned, in a former part of this lecture, 

 against the close analogy which some persons 

 have assumed between the absorption of oxygen 

 and the formation of carbonic acid gas in germi- 

 nation, and in the respiration of the foetus. Simi- 

 lar arguments will apply against the pursuit of 

 this aiialogy, between the functions of the leaves 

 of the adult plant, and those of the lungs of the 

 adult animal. Plants grow vigorously only when 

 supplied with light; and most species die if de- 

 prived of it. It cannot be supposed that the pro- 

 duction of oxygen fi-om the leaf, which is known 

 to be connected with its natural color, is the exer- 

 tion of a diseased I'unction. or that it can acquire 

 carbon in the day-time, when it is in most vigor- 

 ous growth, when the sap is rising, when all its 

 powers of obtaining nourishment are exerted, 

 merely for the purpose of giving it ofl^ again in the 

 night, when its leaves are closed, when the mo- 

 tion of the sap is imperfect, and when it is in a 

 state approaching to that of" quiescence. Many 

 plants that grow upon rocks, or soils, containing 

 no carbonic matter, can only be supposed to ac- 

 quire their charcoal from the carbonic acid gas in 

 the atmosphere; and the leaf may be considered 

 at the same time as an organ of absorption, and 

 an organ in which the sap may undergo different 

 chemical changes. 



When pure water only is absorbed by the roofs 

 of plants, the fluid, in passing into the leaves, will 

 probably have greater power to absorb carbonic 

 acid from the atmosphere. When the water is 

 saturated with carbonic acid gas, some of this sub- 

 stance, even in the sunshine, may be given off by 

 the leaves ; but a part of it likewise vvill be always 

 decomposed, which has been proved by the expe- 

 riments of M. Sennebier. 



When the fluid taken up by the roots of plants 

 contains much carbonaceous matter, it is probable 

 that plants may give off carbonic acid from iheir 

 leaves even in the sunshine. In short, the func- 



