pose it, or evolve oxygen; and
the coloured parts, the flowers, fruits, etc., have an entirely opposite
effect, absorbing oxygen and giving off carbonic acid. The absorption of
carbonic acid and escape of oxygen has been proved by numerous direct
experiments by Saussure and others, in which both atmospheric air and
artificial mixtures containing an increased quantity of carbonic acid
have been employed. Saussure allowed seven plants of periwinkle (_Vinca
minor_) to vegetate in an atmosphere containing 7.5 per cent of carbonic
acid for six days, during each of which the apparatus was exposed for
six hours to the sun's rays. The air was analysed both before and after
the experiment, and the results obtained were--

Volume Carbonic
of the air. Nitrogen. Oxygen. Acid.
Before the experiment, 5746 4199 1116 431
After " 5746 4338 1408 0
---- ---- ---- ----
Difference, 0 +139 +292 -431

In this experiment the whole of the carbonic acid, amounting to 431
volumes, was absorbed, but only 292 volumes of oxygen were given off.
Had the carbonic acid been entirely decomposed, and all its oxygen
eliminated, its volume would have been equal to that of the acid, or
431, so that in this instance 139 volumes of the oxygen of the carbonic
acid have been retained to form part of the tissues of the plant. On the
other hand, the nitrogen is found to be increased after the experiment.
It might be supposed that the nitrogen evolved had been derived from the
decomposition of the nitrogenous constituents of the plant, but this
cannot be the true explanation, because in this particular case it
greatly exceeded the whole nitrogen contained in the plants
experimented on. Its source is not well understood, but Boussingault
supposes it to have existed in the interstices of the plant, and to have
escaped during the course of the experiment. Saussure found that the
oak, the horse-chesnut, and other plants, absorb oxygen and give off
carbonic acid in less volumes than the oxygen, while the house-leek and
the cactus absorb oxygen without evolving carbonic acid. The absorption
and decomposition of carbonic acid takes place only during the day, and
matters are entirely reversed during the night, when oxygen is absorbed
and carboni