o 
re 

. 
INFLUENCE OF CARBONIC ACID GAS ON HEALTH OF PLANTS. 59 
Accordingly the process went on, whether fibrous substances were placed 
in the water or not, although in the latter case somewhat less rapidly, the 
presence of such bodies serving to disentangle the particles of gas from their 
adhesion to the water more easily than would happen otherwise. 
There cannot therefore be a doubt, that the common opinion, which regards 
the emission of oxygen from the surfaces of leaves, whether placed in water 
or in air, as a vital phenomenon, is the correct one, and hence it is quite con- 
sistent with analogy, that, as we have already seen, some one proportion of 
carbonic acid in the air should be more favourable to the exercise of this 
function, than any other one more considerable in amount would prove. 
I was therefore encouraged to proceed in my inquiry as to the quantity of 
carbonic acid contained in air, which was decidedly prejudicial to the health 
of ferns. 
With that view specimens of the same five species as before were selected 
for experiment, and these were placed under the jar which contained about 
2800 cubic inches of air cut off from the external atmosphere by water. To 
‘this air 1 per cent. of carbonic acid was at first added, and a daily increase 
to the same amount in the quantity present was kept up, until the propor- 
tion reached ZO per cent. This same quantity was then maintained in the 
jar for twenty days, by successive additions to compensate for the ascertained 
amount of leakage, now found to be inconsiderable, and the appearance of 
the plants was from time to time examined and noted. 
It was not till the 13th day that any sensible alteration for the worse was 
perceptible, when we observed, that in Péeris longifolia the fronds had be- 
come very brown; in Nephrodium molle and in Gymnogramma chrysophylla 
two or three of the lower fronds showed signs of yellowness; that those of 
the Adiantum looked in general very sickly, but that Pteris serrulata did 
not appear injured. The experiment was however continued seventeen days 
longer, when it was found, for the first time, that the amount of carbonic acid 
present in the jar, as ascertained in the usual way by potass, exceeded what 
had been added; proving more decisively than before, that decay had com- 
menced. The plants were accordingly taken out, and the following notes 
respecting their condition were entered in the Minute-Book. 
Peeris longifolia.—All the old fronds are now dead, but the vitality of the 
rhizoma is not destroyed, for young fronds are putting out, and appear at pre- 
sent to be healthy. 
Pteris serrulata even now appears but slightly damaged, its fronds being 
only more yellow than is natural. 
pinta molle seems in the same condition nearly as Pteris longi- 
ia. 
* Gymnogramma chrysophylla.—tits old fronds slightly damaged and yellow, 
but young ones are putting out. 
_ Adiantum cuneatum.—All the fronds have died down. 
_ Thus it appears that this large amount of carbonic acid, even if gradually 
added, would in time prove fatal to plants of the above description, although 
operating upon them with various degrees of intensity, and apparently not 
exerting any specific influence upon the stem.and roots. 
'. That the effect however was attributable; not to the diminution in the pro- 
portion of oxygen consequent upon the addition of so large an amount of 
carbonic acid, but to something positively deleterious in the latter gas itself, 
was inferred, by exposing the plants to air impregnated with ZO per cent. of 
hydrogen, which in the course of ten days appeared to exert no sensible in- 
fluence upon their health. 
There did not appear to be any very material difference in the action of 
