. 
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79 
were equally powerless, while the bright green Amacharis Canadensis 
or the U/va datissima would supply oxygen enough for the lungs of the 
inhabitants of the fresh and salt water respectively. Daubeny had 
ascertained by reliable experiments, that a leafy plant would add 
18 per cent. of oxygen to the air of a jar. The question now occurred, 
by what agency was the decomposition of the carbonic acid effected ? 
By the leaf, the light, or both. His own impression was, that the light 
was the sole agent, the leaf merely affording a suitable nidus. 
He had already spoken of the difficulty of direct decomposition. 
The chemist effected it by three agencies of light, heat, and electricity, 
By heat, he separated oxygen from mercury or lead ; by electricity 
from hydrogen ; while light dissolved the union between silver and 
chlorine. 
A series of experiments, conducted independently by Draper and 
Cloez and Gratiolet showed that the yellow ray of the spectrum pro- 
duced the most rapid evolution of oxygen, exceeding that of the com- 
pound ray. The red, green, and blue took the succeeding ranks, while 
no effect whatever was produced by the violet and indigo, or by the 
extra spectral-space beyond them. It was remarkable that the rays 
which produced the effect were the illuminating, not the heating or 
chemical rays. It might be objected to his view that light produced 
no effect on carbonic acid contained in a jar; but even supposing 
that fact to be proved, allowance had still to be made for the sub- 
division caused by the cellular tissue. He did not, however, claim for 
his opinion any authority beyond that of a suggestion, as it was not 
susceptible of proof. 
The second statement—that plants exhaled carbonic acid, came to 
us recommended by a great weight of evidence, and fortified by ex- 
periments made by Garreau, who actually collected the carbonic acid 
in lime water, and tabulated the results. From one plant, the Rheum 
undulatum, he obtained, after five hours’ exposure at I1I degrees 
Fahrenheit, ‘025 of carbonate of lime, or ‘024 of carbonic acid, an 
amount almost too small for appreciation, and attributable in his 
opinion to other causes. His own belief was that no such respiration 
ever took place, and this opinion was favoured by the high authority 
of Mr. Haseldine Pepys, whose long and careful experiments have 
been confirmed by Messrs. Cloez and Gratiolet. A plant placed in 
lime water during the hours of darkness usually deposited some car- 
bonate of lime before morning. His explanation of this circumstance 
was that the absorption of carbonic acid during the day was caused by 
the action of heat on the leaves, while its decomposition was due to 
