small that it can produce durable effects, in any degree indicative of its extreme intensity. Mr. Robert Harrup appears to have investigated this subject with much care, and he determined the fact, that several of the salts of mercury were reduced by the Light, and not by the heat of the sun's rays.

(18.) In 1775 B. C. Méese first published some experiments upon the influence of Light on plants; and in 1779 Dr. Priestley gave his experiments on the same subjects to the world. There are few examples in the records of science in England, which exhibit more perfectly the advantages of the inductive system than those well-conducted experiments by Priestley. He determined the problem of vegetable respiration-he showed, that carbonic acid was absorbed by the plant, that under the influence of Light it was decomposed, and that its oxygen was again liberated. Upon the publication of Priestley's researches, a great number of naturalists and chemists took up the inquiry, and many important facts, all of them confirming Priestley's discoveries, were published by Senebier, Ingenhousz, Decandolle, Saussure, and Ritter. †

(19.) The general result of the investigations on the chemistry of vegetation, up to this point, was, that Light was essential to healthful vegetation; but that the decomposition of the carbonic acid by the plant took place

Nicholson's Journal, August, 1804.

+ Priestley, Experiments and Observations on different Kinds of Air, and other branches of Natural Philosophy. Printed at Birmingham,

1790.

Senebier, Expériences sur l'Action de la Lumière Solaire dans la Végétation. Paris, 1788,

Ingenhousz, Expériences sur les Végétaux. Philosophical Transactions, 1782.

Decandolle, Mémoires des Savans Étrangers, vol. i. Saussure, Recherches Chimiques sur la Végétation. Chimie, vol. i.

Ritter, Gehlen Journal der Chem, vol. vi.

Annales de

INFLUENCES ON VEGETATION.

15

more decidedly under the influence of the most refrangible range of the spectrum, than of those which possessed superior illuminating power. In 1801, Labillardière communicated to the Philomathic Society his discovery that Light was necessary to the development of pores in plants; and about the same time Victor Michellotti, of Turin, published a statement, at that time doubted, but the truth of which has since been confirmed, to the effect "that Light has a decided action on those germs which are exposed to it,—that this action is prejudicial to them, and it manifests its action by retarding their expansion if the Light be weak or a reflected Light, or by total extinction of their life if it be very intense, as that which comes directly from the sun. In connection with this section of the inquiry, M. Macaire Prinseps observed, "that sheltering leaves from the action of Light prevents their change of colour in the autumn; that if the entire leaf was placed in the dark it fell off green; if only a part, the rest of the parenchyma changed colour, and the covered portion retained its original colour."† Researches of this character were continued by different investigators, a list of whom is given in a report furnished to the British Association in 1850.

(20.) Böckman found that, by exposing phosphorus in nitrogen and other gases to sunshine, there was deposited upon the side of the glasses nearest the Light a coloured powder, whilst no such effect was produced upon the parts in shadow. He also appears to have observed that the two ends of the solar spectrum produced dissimilar effects on phosphorus.‡

(21.) In 1801, Ritter, of Jena, repeated the experiments of Scheele, and rose the question of the existence

Experiments and Observations on the Vitality of Germs. Journal de Physique Ventose, p. 9.

+ Mémoires de la Société de Physique et d'Histoire Naturelle de Genève, tom. iv. p. 1.

Voigt's Magazine, vol. iv.

of solar rays possessing very powerful properties in producing chemical change, which do not act sensibly upon the organs of vision, or which, in other words, are not Light-giving rays. Ritter found that the chloride of silver darkened rapidly beyond the violet extremity of the prismatic spectrum; in the violet ray it was less darkened; still less in the blue; below which ray the power of darkening diminished quickly. He also stated that the red ray had the power of restoring darkened chloride of silver to its original colour; and hence concluded that there are two sets of invisible rays, one on the red side of the prismatic spectrum, which favours oxygenation, and the other on the violet side, which assist disoxygenation. The error into which Ritter fell here was only one of degree, the inequality of action, and the apparently opposite effects produced by the most and by the least refrangible rays has been fully proved. Ritter also states, that he found phosphorus to emit white fumes in the invisible red rays, but that no such effect was produced by the invisible violet rays; this was merely the effect of heat.

(22.) Dr. Wollaston in 1802, examined the chemical action of the rays of the spectrum; and in his Memoir* he says, "This and other effects usually attributed to Light are not in fact owing to any of the rays usually perceived." About the same period Desmortiers observed that the sun's rays produced a decolouration of Prussian blue; at a somewhat later period Dr. Wollaston showed that cards moistened with tincture of gum guaiacum acquired a green colour in the violet rays, which colour was rapidly destroyed by the red rays. In 1802, M. Saget noticed that crystals of ruby arsenic effloresced in the Light.

(23.) In the Philosophical Transactions for 1804, Dr. Young gives an interesting experiment, which, although Philosophical Transactions, 1802, p. 379. Journal de Physique, 1802.

DR. YOUNG'S EXPERIMENTS.

17

of a different character, confirms the results obtained by Ritter and Wollaston: "In order to complete the comparison of their properties (the chemical rays) with those of visible Light, I was desirous of examining the effect of their reflection from a thin plate of air, capable of producing the well-known rings of colours. For this purpose I formed an image of the rings, by means of the solar microscope, with the apparatus which I have described in the Journals of the Royal Institution; and I threw this image on paper dipped in a solution of nitrate of silver, placed at the distance of about nine inches from the microscope. In the course of an hour, portions of three dark rings were very distinctly visible, much smaller than the brightest rings of the coloured image, and coinciding very nearly, in their dimensions, with the rings of violet Light, that appeared upon the interposition of violet glass. I thought the dark rings were a little smaller than the violet rings, but the difference was not sufficiently great to be accurately ascertained: it might be as much as or of the diameters, but not greater. It is the less surprising that the difference should be so small, as the dimensions of the coloured rings do not by any means vary at the violet end of the spectrum so rapidly as at the red end. The experiment in its present state is sufficient to complete the analogy of the invisible with the visible rays, and to show that they are equally liable to the general law, which is the principal subject of this Paper," that is, the interference of Light.

(24.) In 1806, Vogel exposed fat, carefully protected from the influence of the air, to Light, and it became in a short time of a yellow colour: it acquired a rancid penetrating smell and a bitter taste, producing a burning sensation in the throat; whereas that which was open to the air, during exposure, always became acid. The same

observer found that ammonia and phosphorus exposed to the sun's rays were rapidly converted into phosphuretted hydrogen and a black powder-phosphuret of ammonia.

Vogel also noticed that the red rays of the prismatic spectrum produced no effect upon a solution of corrosive sublimate (bichloride of mercury) in ether, but that the blue rays rapidly decomposed it. He also observed that the decomposition of several metallic compounds was gradually brought on by the same class of rays. Dr. Davy much more recently repeated these experiments, and he found that corrosive sublimate in crystals was not changed by exposure, but that the "liquor hydrargyri oxymur." of the London Pharmacopoeia quickly decomposed in sunshine, depositing calomel.

(25.) In the Transactions of the Royal Society of London for 1800, Dr. Herschel's Memoirs on the heating Power of the Solar Spectrum will be found. Previously to this time it was supposed that each ray contributed its proportional share to the intensity of the heat which is produced by the concentration of the sun's rays in the focus of a burning-glass. Dr. Herschel was, however, led to suspect that this was not the fact, from the following circumstances:-"In a variety of experiments," says this philosopher, "which I have occasionally made, relating to the method of viewing the sun with large telescopes to the best advantage, I used various combinations of differently coloured darkening-glasses. What appeared remarkable was, that when I used some of them, I felt a sensation of heat, though I had but little Light; while others gave me much Light, with scarce any sensation of heat. Now, as in these combinations, the sun's image was also differently coloured, it occurred to me that the prismatic rays might have the power of heating bodies very unequally distributed among them." These experiments, having an important bearing on the chemical phenomena which form the principal subject of investigation, will be especially noticed in the next chapter. Herschel also describes his experiments to determine the illuminating powers of the different rays. He discovered that

* Annales de Chimie, vol. lxxv. fig. 225.