CHAP. XIV. 
COLOUR AND SMELL. 
357 
sively feeble as to be wholly insufficient to overcome the blue- 
ness of the carbon if it ivere blue. The fact is, that the cause 
of carbon in the system of vegetation being green belongs to 
that numerous class of facts of which no explanation can be 
given in the existing state of human knowledge. 
"Although we are justified by the mass of evidence in assert- 
ing that the green colour of plants is owing to the fixation of 
carbon in their tissue, in consequence of the power that light 
possesses of decomposing their carbonic acid, yet there are 
some exceptions that deserve attention. Humboldt found 
Poa annua and compressa, Plantago lanceolata, Trifolium 
arvense. Wall-flower, and the Rhizomorpha verticillata, green 
in the subterranean galleries of the mines of Freyberg, al- 
though born in total darkness, but in atmosphere highly 
charged either with hydrogen or nitrogen. Ferns and 
mosses, again, will be green where other plants are blanched ; 
and Humboldt found near the Canaries a fucus which was 
bright grass-green, although it had grown at the depth of from 
25 to 32 fathoms (190 feet). Now, as light, according to the 
experiments of Bouguer, after traversing 180 feet, is weakened 
in the proportion of 1 to 1477*8, this Fucus must have been 
illuminated when growing by a power 203 times less than 
that of a candle at a foot distance. Are we to suppose that 
this feeble degree of illumination was sufficient to decompose 
the carbonic acid of such a plant, or was not the decomposi- 
tion rather owing to the operation of some unknown cause ? 
" Leaves, which, as we very well know, are usually green, 
may assume different colours in special cases. It is common 
to see in the autumn this green change to yellow, as in the 
Lombardy poplar, &c. ; or to red, as in the barberry, the 
sumach, the Virginian creeper, and many kinds of oaks. It 
has been ascertained by Macaire that in such cases the leaf, 
shortly before this change takes place, ceases to exhale oxygen 
in sun-light without ceasing to absorb it at night ; whence he 
infers that the chromule is oxydized, which at first brings on 
a yellow and afterwards a red colour, for the most decided red 
always begins by a change to yellow. It is remarked that red 
colours are most common in leaves which contain some kind 
of ^cid, as the vine, the pear, the viburnum, the sorrel, &c, 
A A 3 
