101 
Dr Gilby on the Respira tion of Plants^ 
shade. In the shade, the air is vitiated by the disappearance of 
the oxygen, and the formation of carbonic acid ; on the con- 
trary, under insolation, the carbonic acid, if any be present, is 
rapidly decomposed, and the air is again improved by the res- 
titution of the oxygen. 
The rapidity with which this process takes place is truly as- 
tonishing, as the following experiment, among many I have 
made with this view, will demonstrate. I filled a glass jar, hold- 
ing 21 cubic inches, with a mixture of common air and carbonic 
acid, in the proportion of 70 parts of the former to 30 of the lat- 
ter. I then introduced into the jar a bundle of fresh grass, which 
displaced, by previous measurement, exactly 2 cubic inches of 
air, and submitted the whole to the full light of the sun during 
four hours, the inverted jar being surrounded by mercury in a 
saucer. At the close of this time, I found, that only 2 parts 
out of the 100 were subtracted by lime-water, while not less 
than 41 parts were consumed by phosphorus. In estimating, 
therefore, the proportion of carbonic acid and oxygen before and 
after the experiment, it appears, that 26.3 of oxygen were added 
in this short space of time, over and above the 14.7 originally 
existing ; or if we compute the quantity of each gas before and 
after the experiment, this will be the proportion : 
At the begin-' 
Of Nitrogen, 
Cubic In. 
10.507 
At the close of j 
1 Of Nitrogen, 
Cubic In. 
10.507 
ning of the 
Of Carbonic Acid, 5.7 
the Experi- 
Of Carbonic Acid, 37 
Experiment 
»Of Oxygen, 
2.793 
ment there < 
^Of Oxygen, 
7.79 
there were 
in the Jar, ' 
19.000 
were 
1 
18.677 
Very few, I imagine, would hesitate in admitting, that this 
conversion of carbonic acid into oxygen, is entirely the result of 
the chemical action of the sun’s rays. I thought, however, it 
would be interesting to establish this point by experiment. 
Mr Ellis had suggested to me the idea of confining plants in 
an artificial atmosphere of carbonic acid and common air, under 
jars of different coloured glass, and thus of insulating them. 
Finding it impossible, however, to procure jars of the required 
colours, I adopted the following expedient in performing the 
experiment. I procured three phials, holding exactly 3J cubic 
inches, into each of which I thrust a bundle of grass, occupying 
