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swept away, animal life would soon become extinct; and if all animal existence was brought to a close, the forest would fall, and the flowers of the field, which now clothe the earth with gladness, perish in the utterness of a lamentable decay. It has been supposed that the vegetable world was called into existence long previous to the creation of animals, and to this period is referred the formation of the coal strata. There might have been an epoch when the disturbed condition of the earth - its earthquake shocks, and volcanic strugglings, may have poured so large a quantity of carbonic acid into the atmosphere, as to have rendered this planet unfit for the habitation of animals, until a teeming and most gigantic vegetation, by exhausting it for their own supply, purified the air, and rendered the more quiet earth a fitting abode for creatures endowed with reason and with instinct. But the hypothesis is unsupported by facts, and it is not within the range of probabilities that the animal or vegetable kingdoms can ever have an independent existence.

(409.) The animal kingdom is constantly producing carbonic acid, water in the state of vapour, nitrogen, and, in combination with hydrogen, ammonia. The vegetable kingdom continually consumes ammonia, nitrogen, water, and carbonic acid. The one is constantly pouring into the air what the other is as constantly drawing from it, and thus is the equilibrium of the elements maintained.

Plants may be regarded as compounds of carbon vapour, oxygen, hydrogen, and nitrogen gases, consolidated by the all-powerful, all-pervading influences of the solar ray; and all these elements are the produce of the living animal, the conditions of whose existence is also greatly under the influence of those beams, which are poured in unceasing flow from the centre of our system. Can any thing more completely display a system of the loftiest design, and most perfect order, than these phenomena ?

(410.) The most casual observer could not fail to remark the peculiar influences of the solar agencies, at different seasons of the year. In spring, a fresh and lively green pervades the field and forest; this in summer assumes a darker hue, and in the autumn passes gradually into a russet brown. In a very early stage of my photographic researches, I discovered a remarkable difference in the chemical action exerted by the solar rays an hour or two before noon, or an hour or two after it. I was convinced at an early period, and the continued observations of many years prove, that similar differences are to be detected between the solar emanations of the vernal and the autumnal periods. The change in the colour of the leaves appears to be entirely dependent upon the absorption of oxygen, which all the green parts of plants have the power of absorbing, almost without intermission. This true case of chemical affinity, it would appear, goes on equally with the spring or the summer leaves; but during these periods the vital force, under the stimulus of the Light, is exerted in producing the assimilation of the oxygen for the formation of the volatile oils, the resins, and the acids. In the autumn this exciting power is weakened; the summer sun has brought the plant to a certain state, and it has no longer the vital energy necessary for continuing these processes. Consequently, the oxygen now acts in the same manner on the living plant, as we find in experiment it acts upon the dried green leaves, when moistened and exposed to its action. They absorb gas and change colour.

Sir John Herschel observes, in reference to the action of Light on the juices of plants: “ The earlier flowers of any given species reared in the open air, are more sensitive than those produced, even from the same plant, at a late period in its flowering, and have their colours more completely discharged by Light. As the end of the flowering period comes on, not only the destruction of the colour by Light is slower, but residual tints are left which resist obstinately.” These residual tints are the same which produce the brown of the autumnal leaf; INFLUENCE OF LIGHT ON PLANTS.


and the same agent may be traced in the production of photographs upon papers spread with expressed juices, and on the changing colours of flowers and of leaves.

(411.) A remarkable example of the influence of Light upon the juice of plants, is the Cacalia ficoides, cited by Liebig. During the hours of darkness, this plant, like others, assimilates oxygen, and in the inorning it is as acid to the taste as the sorrel. By the influence of the morning sun it loses this oxygen, and at noon it is tasteless; and by the continued action of the Light still more is abstracted, and the plant is positively bitter in the evening.

(412.) Experiments have been instituted with a view of ascertaining if any particular ray of the spectrum had the power of inducing, more powerfully than others, the progress of plants towards the Light, a phenomenon which is strikingly exhibited by the potato. It would appear that the yellow — luminous— rays exert this influence with the greatest force; the blue rays exerting, as might have been expected from their small illuminating power, no influence — and the red rays causing the plant to bend in an opposite direction.





(413.) Nothing having been done in this section of the inquiry since this chapter was first published, it stands without alteration

It may appear to many at first, that this inquiry is not quite in place in the present volume, it having been shown that the conclusions arrived at by M. Moser, who first called particular attention to the phenomena in question, are in all probability erroneous. The subject is, however, so intimately united with those agencies whose powers we have been considering, that this treatise would be incomplete, did it not contain a particular account of the discovery, the discussion to which it has given rise, and record all those experiments of interest which bear upon these mysterious actions.

(414.) In a memoir “ On Vision and the Action of Light on all Bodies," * M. Ludwig Moser first announced the following fact: If a surface has been touched in any particular parts by any body, it acquires the property of precipitating all vapours which adhere to it, or which combine chemically with it on these spots, differently to what it does on the other untouched parts."

In a memoir entitled “ Some Remarks on Invisible Light,† and in another “On the Power which Light possesses of becoming latent," I his views were still further

* Poggendorff's Annalen, vol. lvi. p. 177. No. 6. 1842. † Poggendorff's Annalen, vol. lvi. p. 569. No. 8. 1842. $ Poggendorff's Annalen, vol, lvii. p. 1. No. 9. 1842.

Those papers have been translated by Henry Croft, Esq., and published in Taylor's Scientific Memoirs, vol. iii. part xi. February, 1843: from these translations all my quotations will be made.



developed, and I shall now endeavour to place them in as correct a light as possible.

(415.) In the Daguerreotype process those parts of the iodised silver plate upon which the Light has acted with most power, receive, when the plate is exposed to the vapour of mercury, the largest quantity of that vapour over their surfaces, and the gradations of Light are marked very beautifully by the thickness of these mercurial films. Now if we write with a piece of steatite on a looking-glass, the writing is invisible until we breathe upon it, when it appears distinctly. If we place coins on a plate of glass or metal, and allow them to remain for some few hours in contact, although no change will be visible when they are removed, we may bring out beautiful images of the coins by breathing on the plate, or exposing it to any vapour. Upon these experiments M. Moser has based his hypothesis, That Light of a peculiar degree of refrangibility is absorbed by all bodies, and that they radiate it again in darkness.

(416.) We must, however, observe all the phenomena which M. Moser has brought before the scientific world. These effects are produced by writing on glass or metal with any substance whatever. “ We may first breathe uniformly over the whole plate, and then write on it, either with blotting-paper, a brush, or any thing else; the characters will become visible whenever the plate is breathed on, and this phenomena lasts for some time. Not only is glass applicable to this purpose, but every other polished body exhibits the same appearances: I have tried it with metals, resins, wood, pasteboard, leather, &c. Even fluids may be used; if we take a clean and still surface of mercury, hold over it a body, and breathe on the other parts, or, what is better, breathe on the whole surface first, and then remove the moisture by any gentle means from particular parts, they will again become visible when breathed on, even after several days, if the mercury remains undisturbed. Moreover, absolute contact with the

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