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quinine in dilute sulphuric acid. On the first half the lavender rays became vivid apple green, while, on passing to the other half, they instantly changed to a equally vivid lavender colour. These rays often darken the surface throughout their whole length; sometimes they acquire a powerful bleaching action, and sometimes they have no effect, as evidently appears from the following experiments.

(357.) "The juice of fresh-gathered petals of double flowering pomegranate in alcohol afforded an example of this. Paper washed with this juice became rich crimson, and on being exposed wet to the condensed spectrum, a narrow line of deep crimson was formed at the junction of the green and yellow rays, or perhaps in the most refrangible yellow, surrounded by a whitish lozenge-shaped border (a). On again washing with the juice, instead of the white border, which had vanished, there was a crimson flame-shaped image, curved at the lower edge of the yellow rays, and tapering upwards to the violet; its colour was darker than that of the ground, though paler than the narrow line which maintained its intensity; and although the latter increased in width, it did not become as broad as the image in question. At the upper end of the violet another little dark image was formed, apparently owing to the action of the lavender rays, having exactly their form: the orange and red rays, especially the red, had no effect, though at the distance of about half the length of the spectrum beyond the red, two distinct spots were formed of deep crimson, which I believe to be the heat spots which you discovered. After some time a bleaching appearance surrounded the whole image from the red upwards, probably owing to rapid evaporation from the heat of the spectrum (b). Exterior bleaching frequently took place in the course of the ex

α

b

C

periments, permanent in some instances, while in others it vanished as the surface dried. When water was used with the juice instead of alcohol, the general character of the image was similar to that described, except that the small figure beyond the violet was more distinct, and seemed to bear the same proportion to that formed by the rest of the spectrum which the length of the lavender rays bears to the length of the sum of the others. The bleached part round the whole was more extended, and a faint crimson haze encompassed the dark spots, which were very distinct (c).

(358.) "The following are some of the cases in which the simultaneous effect was produced. For example, paper washed with the juice of the petals of Globe Amaranthus in distilled water, on exposure to the spectrum, acquired a delicate pink tint which was soon bleached to whiteness from the upper edge of the green to the end of the lavender rays, while at the same time a perfectly circular spot of equal whiteness was seen under the red rays and a little way below them, which had the appearance of being an image of the sun. After more washing with the juice, the two bleached parts were united by a long white neck, which speedily vanished, and was succeeded by a dark crimson image, whose greatest intensity of colour was under the yellow rays. At some distance below the red rays two crimson spots were strongly marked, especially the uppermost, both surrounded by a paler halo.

(359.) "The juice of the petals of pale blue Plumbago auriculata, in distilled water imparted its tint to writing paper, which after exposure to the action of diffused Light acquired a pale yellowish green hue. The part under the lavender and violet rays of the spectrum, repeatedly washed with the juice, assumed a pale brown colour. The indigo rays seemed to have no effect, although, from their lowest edge to the distance of half the length of the spectrum below the red rays, a lavender

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blue image was formed. Under the orange rays a minute indigo-coloured spot appeared, and also a larger spot of the same colour under the yellow, which were soon blended into one, forming a single oblong figure of maximum intensity, surrounded by a halo of paler indigo. An isolated disk of the same colour as the halo, with two dark spots in its centre, appeared at some distance below the red rays.

(360.) "The juice of the Beet-root, in a strong solu tion of common salt, imparted a pink colour to the paper, and the most refrangible rays acquired a power ful bleaching energy; the pink ground was whitened under the lavender, indigo, and blue; a deep crimson spot was formed under the yellow, with a rosecoloured halo, elongated to the bleached part on one side, and to the end of the orange on the other, while a hazy rose-coloured disc was visible at a distance below the red. The crystallisation of the salt on this figure was, in proportion to the intensity of colour, most on the crimson spot and its halo, and on the coloured disk, but scarcely any on the bleached portion."

(361.) The great number of instances now adduced, in which we have distinct evidence of chemical change under the influence of the sun's rays appear sufficient to support the position I have long maintained, that the solar rays are continually acting upon matter-it signifies little in what form it may be presented to its influence. Although for photographic purposes we can only select those compounds which exist in a state of "tottering equilibrium," at least, in the present state of our knowledge, yet we have distinct evidence, that a sunbeam cannot fall upon any solid body without leaving permanent traces of its action.

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CHAPTER VI.

THE INFLUENCE OF THE SOLAR RAYS UPON VITAL

ORGANISATION.

(362.) THE surface of our earth is rendered beautiful by the almost countless forms of vegetable life which adorn it. On the bare surface of the wind-beaten rock, the mysterious lichen finds a sufficient amount of those elements which assimilate and form its structure, to support it through all the stages of its growth; and at length, having lived its season, it perishes, and in its decay forms a soil for plants, which stand a little higher in the scale of vegetable life. These again have their periods of growth, of maturity, and of dissolution; and by their disintegration, form a soil for others, which pass through the same changes, until at length the once naked rock is covered with a garden, and the flowering shrub and the enduring tree wave in loveliness above it.

(363.) In a short time we find the almost microscopic seed, placed in a few grains of earth, springing into life, developing its branches, unfolding its leaves, and producing flowers and fruit. Although it has become a stately plant, we shall not discover much diminution of the soil from which it grew, and from which it would, at first, appear it derived all those solid matters of which its structure is composed. Experiments have been made in the most satisfactory manner, and it has been proved, that a very small amount only, of the soluble constituents of a soil are taken up by the roots of a plant; we have then to look to other sources for the origin of the woody matter, of the acid and saccharine juices, of the gums and of the resins, yielded by the vegetable world. These are all, it will be found, formed by some mysterious modifications of

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a few elementary bodies. The plant in virtue of its vitality, and under the excitement of the solar rays, effects the assimilation of these elements; and these are the phenomena which it is the business of this section to examine.

(364.) The conditions necessary to germination are moisture, a moderate temperature, and the presence of oxygen gas. The experiments of Ray, Boyle, Scheele, Achard, and Humboldt, all show that the presence of atmospheric air is necessary. Germination cannot take place at the freezing point of water, and at 212° all vitality is destroyed. If seeds are kept quite dry, they will not germinate, although the other conditions are fulfilled. All seeds do not germinate at the same seasons, some requiring a more elevated temperature than others, which fact explains the cause of the different periods at which we find the plants springing from the soil.

It has already been remarked, that Michellotti proved Light to be injurious to germination, and Ingenhouz and Sennebier found that seeds germinated more rapidly even beneath the soil in the shade than in sunshine. This fact has been now established beyond all doubt.

(365.) Priestley's Experiments* on the influence of growing plants upon the air are most instructive; and since they are not generally known, it is thought advisable to give an abstract of them in this place. "Without Light," says Priestley, "it is well known that no plant can thrive; and if it do grow at all in the dark, it is always white, and is, in all other respects, in a weak and sickly state. Healthy plants are probably in a state similar to sleep in the absence of Light, and do not resume their proper functions but by the influence of Light, and especially the action of the rays of the sun."

Again, arguing that the green matter which forms in water grows and gives off gas by the influence of Light

* Experiments and Observations relating to various Branches of Natural Philosophy, with a continuation of the Observations on Air. By Joseph Priestley, LL.D., F.R.S. Birmingham, 1781. Vol. II.