DAUBENY ON THE CARBON OF PLANTS. 261 



a certain proportion of carbonic acid, the balance still con- 

 tinued to be in favor of the purifying influence of the veg- 

 etable. 



'*The apparatus I made use of consisted of a large bell- 

 glass jar, containing in one case 600, in another 800 cubic 

 inches of air,* and suspended by pulleys. Its edges dipped 

 into quicksilver, contained in a double iron cylinder of cor- 

 responding dimensions to the jar, which, being closed at 

 bottom, constituted a well of about six inches in depth, cal- 

 culated to receive a fluid, and to admit of the glass vessel 

 moving freely in it. The inner margin of this hollow cylin- 

 der was cemented air-tight, according as circumstances re- 

 quired, either to a plale of iron, or to a pot of the same 

 material upon or in which the plant operated on might be 

 placed ; and the jar was then let down upon it, until its 

 edges were sunk a little beneath the surface of the mercury. 



"Thus all communication with the external atmosphere 

 was cut off*, and the eflfect of the plant upon the air inclosed 

 in the jar was readily measured, by simply pressing down 

 the latter, and thus expelling a portion of its contents 

 throucrh a tube, communicating with its interior, and intro- 

 duced at its outer extremity under a pneumatic trough, 

 wherein the air might be collected and examined. By con- 

 necting this extremity with a vessel containing a measured 

 quantity of carbonic acid, and raising the jar a little in the 

 well of mercury, it was easy to draw in any proportion of 

 that gas, with which it was thought proper that the plant 

 should be supplied. A portion of the air was always tested, 

 immediately after the introduction of every fresh portion 

 of carbonic acid, and again after an interval of some hours, 

 and the proportion of this gas and of oxygen present was 

 each time carefully registered. The amount of carbonic 

 acid was determined by a solution of potass, that of oxygen 

 by the rapid combustion of phosphorus with a portion of it 

 introduced into a bent tube. 



" Such was the mode of procedure, when an entire plant 

 became the subject of experiment ; but some of the most 

 satisfactory trials were with branches of certain shrubs, 

 themselves too large to be admitted under the jar. These 

 branches, without being detached from the parent trunk, 

 were introduced through a hole in the centre of two corre- 

 sponding semicircular plates of iron, which were cemented 

 air-tight, to the inner margin of the iron cylinder on the 



** * Larger jars, containing from 1200 to 1300 cubic inches were lat- 

 terly employed." 



