58 4-4 - REPORT—1849, 
three days, besides being subjected for seven days to about 5 per cent. of the 
same. At the end of this time only two of the ferns appeared at all damaged, 
namely Pteris longifolia and Nephrodium molle, the fronds of both which were 
rather discoloured, those of the other three species remaining as before. 
The same description of experiment was made upon a species of Pelargo- 
nium, which after having been during twenty-seven days exposed to the ac- 
tion of 10 per cent. of carbonic acid contained in the air of a large jar, ap- 
peared in exactly the same condition as a corresponding one placed under 
glass in a vessel free from any abnormal mixture of that ingredient. 
From these, and from the experiments of the preceding year, it might be 
inferred that plants in general are tolerant of a much larger volume of car- 
bonie acid gas than exists in the atmosphere at present; but it did not 
therefore follow, that the amount of carbonic acid decomposed, and of © 
oxygen exhaled, would bear any proportion to the quantity with which their 
leaves were brought into contact. 
From several trials indeed which I made as to the per-centage of oxygen 
present in the jar at different stages of the experiment, I was led to infer that 
the amount of the latter was not increased in the degree which might have 
been expected; but, as a more easy way of determining the same point, I 
introduced a certain number of fresh leaves of an Helianthus, in each case 
exposing exactly the same amount of surface, into jars filled with water con- 
taining different proportions of carbonic acid gas. In No. 1, for instance, the 
proportion of gas to water was only as 1 to 12; in No. 2 as 1 to6; and in 
No. 3 as 1 to 3. Now it was found, that, instead of the oxygen disengaged 
by the leaves keeping pace with the supply of carbonic acid, only 0-7 of a 
cubic inch was given off from No. 3, whilst No. 2 had disengaged 4 cubic 
inches, and No. 1 3°3 cubic inches; and in another experiment only 0-1 was 
emitted by No.1; 4°5 by No.2; and 2:0 by No. 3, the other circumstances, 
as to time, exposure to light, &c., being in all cases the same. If therefore 
the disengagement of oxygen from leaves be, as is generally admitted, the 
result of their vital action upon the carbonic acid in contact, under the 
stimulus of light, it would follow, that where the carbonic acid exceeds a 
certain amount, that action is in a great degree suspended, 
There is however an experiment of Count Rumford’s, originally reported 
in the Philosophical Transactions for 1786, and alluded to by one of my co- 
adjutors in these investigations, I mean Mr. Hunt, in his late work entitled 
‘The Poetry of Science,’ which would seem to imply that the decomposition of 
carbonic acid by plants was not a vital phenomenon, and consequently could 
not be influenced by any such circumstance as the application of a super- 
abundant portion of this gas to the surfacesvof their leaves. 
Count Rumford states, that the property of causing water to emit oxygen in 
the sun, is possessed, not only by living plants, but likewise by threads of silk, 
by wool, and even by spun glass; in which case the decomposition of carbonic 
acid would seem to be simply the effect of light, the plant merely serving, by 
the surfaces it exposes to the water, to disengage from it, more rapidly than 
would otherwise happen, that oxygen which had been obtained without its — 
direct agency. 
On repeating this experiment, however, I found, as might have been anti- 
cipated, that at first no such effect took place when wool, cotton, silk, or spun 
glass were introduced into the water, but that after some days it occurred 
abundantly in every one of these cases—the disengagement of the gas 
however being always coincident with the appearance in the liquid of green 
conferve, to the action of which doubtless this decomposition of carbonic 
acid was to be attributed. 7 

