130 CAUSE OF THE GREEN COLOUR OF PLANTS. 



up, framing a kind of hyperbola; on the anterior semi-circumference not a solitary one 

 was to be seen. The trough being now filled with sulphate' of copper and ammonia, 

 the arrangement of the crystals was found to be in every respect like the former. 



495. Supposing that this result might in some measure depend on the ray having 

 been subjected to reflexion before passing through the trough, I repeated the trials when 

 the sun's altitude was small enough to permit the rays to pass without requiring reflex- 

 ion, yet still the same results were uniformly obtained ; so that, whether the chemical 

 or the calorific rays were stopped, crystallization took place on the aphelion side of the 

 tube. 



496. May it not, therefore, be that this attractive force originates whenever the cal- 

 orific ray impinges on a surface I It does not necessarily follow from the phenomena 

 that any peculiar class of rays is emitted by the sun, which bring about this action ; but 

 if there are such, it is a question of interest to find what is the reason that good con- 

 ductors of electricity render their action nugatory. 



497. Botanical authors have long been aware of the important effects which solar 

 radiations exercise over the colour of vegetables. A plant which grows in the dark is 

 of a pale whitish colour, and of a transparent aspect, possessing none of that greenness 

 and vigour which are so characteristically developed on exposure to the sun ; its consist- 

 ency is watery, and although its growth may not be stunted, its appearance is very 

 sickly, its secretory actions are not duly performed, and all its vital operations are car- 

 ried on in a depressed way. There is no longer any evolution of nitrogen from the 

 leaves, and, consequently, no apparent production of oxygen gas. Light, which seems 

 to act merely as a stimulus on the green organs of vegetables, indirectly bringing about 

 the decomposition of carbonic acid, though accessory, is not, however, essential to growth. 

 Jn subterranean cavities, and places far removed from the direct solar ray, plants have 

 a colour of their own ; and in the abysses of the ocean, at depths to which no solar 

 beam can penetrate, and where there is a perpetual night, they are found flourishing. 



498. The green colour of leaves is presumed to be an immediate consequence of the 

 act of decomposing carbonic acid. It appears to me that there is some obscurity, if not 

 an actual error, in the view which botanists take of this matter. They suppose that, 

 by the stimulus of light, some portion of the green matter is enabled to decompose that 

 gas completely, or to accomplish its actual resolution into an equivalent volume of oxy- 

 gen, with the entire deposition of the carbon in the solid form ; that it is, moreover, 

 this carbon, so deposited, that gives origin to the green colour, seeing it forms the ckro- 

 mule verte itself. Much useless ingenuity has been thrown away by some chemists in 

 explaining how carbon, the colour of which is black or a deep blue, can produce a lively 

 green; and even if their supposition that the modifying action of a yellow tissue spread 

 over it were correct, of which there is much doubt, considering the thinness of that 

 tissue and the lightness of its tint, yet certainly we have no necessity to resort to any 

 such explanation. The deposite is not carbon chemically; it contains both oxygen and 

 hydrogen in unknown proportions. Of all the physical characteristics of a body, col- 

 our is the most uncertain: after uniting in a new mode, compounds never bear the col- 

 ours of their constituents ; nay. more, carbon itself is not essentially of a black colour, 

 as the diamond proves. 



