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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 inaximum 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.





(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



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 alone, Priestley gives the following experiment: “Having a large trough of water, full of recent green matter, giving air very copiously, so that all the surface of it was covered with froth, and jars filled with it, and inverted, collected great quantities of it, and very fast; I filled a jar with it, and, inverting it in a basin of the same, I placed it in a dark room. From that instant no more air was yielded by it, and in a few days it had a very offensive smell, the green vegetable matter with which it abounded being then all dead and putrid.” Dr. Priestley then instituted a series of experiments to prove that the green matter, and not the water, produced the air. Rumford imagined that any porous body, as cotton, wool, silk, and even threads of glass, would separate air from the water: Priestley's experiments were singularly conclusive on this point.

* 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.

(366.) Priestley continued his experiments with the higher order of plants :—“Having by this means fully satisfied myself that the pure air I had procured was not from the water, but from the green vegetating substance assisted by Light, I concluded that other aquatic plants must have the same effect; and going to a piece of stagnant water, the bottom of which was covered with such plants, I took five or six different kinds promiscuously. Then having put them into separate jars of the water in which they were growing, and inverted them in basins of the same, I placed them in the sun, and I found that all of them, without exception, were immediately covered with bubbles of air, which, gradually detaching themselves from the leaves and stalks where they had originated, rose to the surface of the water; and this air, being examined, appeared to be, in all the cases, very pure, though not quite so pure as that which was before procured from the green matter."

(367.) It must be remembered that Carbonic acid was unknown to Priestley and those who laboured in the same field with him. Dr. Ingenhousz*, for example, says, “The

* Ingenhouz's Experiments on Vegetables.




air obtained from the leaves is by no means air from the water, but air continuing to be produced by a special operation carried on in a living leaf exposed to the daylight, and forming bubbles, because the surrounding water prevents this air from being diffused through the atmosphere.

* It is wonderful that this green matter seems never to be exhausted of yielding dephlogisticated air, though it has no free communication with the common atmosphere, from which the most part of other plants seem to derive their stock of air. Does this vegetable matter imbibe this air from the water, and change it into dephlogisticated air? This does not seem to me probable. I should rather incline to believe that the wonderful power of nature, of changing one substance into another, and of promoting perpetually the transmutation of substances, which we may observe everywhere, is carried on in this

green vegetable matter in a more ample and conspicuous way.”

(368.) Dr. Priestley, with his usual ingenuity, very soon determined that the plants separated some gas from the water, which they decomposed, and that, after a time, they ceased to give out air in water. He says, “ I put a handful of these water plants, without distinguishing their kinds, into a receiver containing eighty ounce measures of water, inverted in a basin of the same; and when they had yielded between six and seven ounce measures of air, I examined it, and found that, with two equal quantities of nitrous air, the measures of the test were 0.8. But the air had been minishing about three days, so that I believe there had been eight ounce measures in all, or one-tenth the capacity of the jar, and certainly purer than it was now found to be. It was evident, therefore, that no more air would have been produced by these plants in water, though placed in the sun.

It is also a proof that the proper origin of all the air produced in these circumstances is not the plant and the Light, and that these are only agents to produce that effect on so:nething else; that


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