1G4 
Dll. DAUBENY ON THE ACTION OF LIGHT UPON PLANTS, 
contrived an apparatus, in which the quantity of air should be so large, that the 
healthy functions of the plant might be as little as possible interfered with ; in which 
the constitution of the air could be examined as frequently as I pleased; and in which 
a regular supply of carbonic acid could be kept up, without disturbing the plant, or 
suspending the progress of the experiment by its introduction. 
The apparatus consisted of a large bell-glass jar, containing in one case 600 , in the 
other 800 cubic inches of air*, and suspended by pulleys. Its edges dipped into 
quicksilver, contained in a double iron cylinder of corresponding dimensions to the 
jar, which being closed at bottom, constituted a well of about six inches in depth, 
calculated to receive a fluid, and to admit of the glass vessel moving freely in it. 
The inner margin of this hollow cylinder was cemented air-tight, according to cir- 
cumstances, either to a plate or a pot of iron, upon 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 external atmosphere was cut off, and the effect 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 through a tube, com- 
municating with its interior, and introduced at its outer extremity under a pneu- 
matic trough, wherein the air might be collected and examined. By connecting 
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 propor- 
tion of this gas and of oxygen present was carefully registered. The amount of car- 
bonic acid was determined by a solution of potass, that of oxygen by the rapid com- 
bustion of phosphorus with a portion of it in 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 corresponding semicircular plates of iron, which were cemented air-tight, to 
the inner margin of the iron cylinder on the one hand, and to the stem of the branch 
on the other. In this manner, when the jar came to be placed over them, and to dip 
beneath the surface of the mercury, the external air was as effectually excluded, as it 
had been when the whole of the plant was inclosed. 
The results of several experiments conducted after this plan will be given in a ta- 
bular form ; but it may be well in the first instance to specify one of the most satis- 
factory of those undertaken. In this case the jar itself contained about COO cubic 
inches of air, and the plant experimented on was the common Lilac ( Syringa vulgaris). 
' Larger jars, containing from 1200 to 1300 cubic inches, were latterly employed. 
