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CHAPTER V.

ON THE ACTION OF THE SOLAR RAYS ON NON-METALLIC SUBSTANCES.

(321.) RESINS.-Heliography.-By this name M. Niepce distinguished, in 1827, the first process by which the images of the camera obscura were rendered permanent, after having impressed themselves upon prepared tablets. Although the process of the philosopher of Chalons is not likely to attract much attention from the photographer, who is now in possession of processes which infinitely surpass it in sensitiveness, yet, as it develops some important operations of the solar rays, it could not be allowed to pass without notice. This process is now becoming more important, since it is employed for the purpose of obtaining impressions upon metal plates and lithographic stones, which can be etched, or prepared for printing. M. Niepce has given directions, which are essentially as follows:

(322.) Into a glass is put a small portion of asphaltum, upon which is dropped essential oil of lavender till the asphaltum is impregnated with it, and as much additional oil is added as will cover it to a slight depth. The mixture is then submitted to a gentle heat, until the whole of the essential oil is saturated with the colouring matter of the bitumen. A highly polished plate of silver is procured, and with a soft roll of skin some of this varnish is applied, in a very thin and equal coating; the plate is then placed upon heated iron, and when the varnish has ceased to simmer, it is withdrawn, and left to cool and dry in a gentle temperature, secured against any mois

ture.

The plate thus prepared is placed in the camera,

and in bright summer sunshine a period of four or six hours is necessary to produce anything like the proper effect. The images are exceedingly faint at first, but they are brought out by the action of a solvent, which removes from the plate, or renders perfectly transparent, those parts upon which the solar rays have not acted. This solvent consisted of one part, by volume, of essential oil of lavender, poured upon ten parts, by measure also, of oil of white petroleum. The varnished tablet is placed in a proper vessel, which has been filled with the solvent, and the operator, by reflected Light, watches the development of the images, and removes the plate when the proper effect is produced.

The process is then completed, by placing the plate upon an inclined plane, and washing it with very clean water, to remove all the softened parts of the varnish which may still adhere to it. This varnish may be spread upon metal, glass, or stone. Engravings are more easily copied by this method than pictures from nature can be procured.

(323.) Niepce appears to have advanced this process considerably; but his partner in this inquiry, M. Daguerre, suggested the use of materials by which the operation was greatly improved, as it regards sensitiveness and general effect. These improvements consisted in applying the residuum obtained by the evaporation of the essential oil of lavender to the plates, instead of the asphaltum; and instead of dipping the plate, after exposure, into a solvent, it is so placed that the vapour of petroleum acts upon it, by which the portions of the varnish that have been acted on by the Light are rendered transparent.

(324.) Daguerre remarks that all bitumens, all resins, and all residua of essential oils, are decomposable by sunshine in a very sensible degree. To exhibit this action, very thin coatings of them should be spread over fitting surfaces; and it is a curious fact, and well worthy the inquiry of chemists, that different solvents act differently

upon these resinous plates after they have undergone the action of solarization. If alcohol is used, the parts on which the Light acted are dissolved off; but if an essential oil is employed, the parts in shadow are those effected by the solvent.

I have tried nearly all the gum resins in general found in the shops of druggists, &c., and these are, I find, acted upon in the same way as the pure resins, and indeed the gums give some indications of losing or acquiring solubility by exposure to sunshine.

(325.) GUAIACUM.— The colour of this peculiar resin is yellowish brown, but upon exposure to the sun's rays it becomes green. Dr. Wollaston first pointed out that this change was brought on by the violet rays; that the original colour was restored if it was exposed to the red rays, and that the same change was effected by the application of artificial heat.

Dr. Wollaston did not observe any change on exposing pieces of card, covered with an alcoholic solution of this gum, in the prismatic spectrum. But, taking a lens of seven inches in diameter, and having covered the central part of it, so that a ring of one-tenth of an inch only was left at its circumference, he could collect the rays of any colour in a focus-the focus for yellow Light being 24 inches. By this arrangement it was found, as above stated, that the violet and blue rays changed it to green, that the yellow rays produced no effect, and that in the red rays the green colour was destroyed. When the guaiacum was placed in carbonic acid, it could not be rendered green at any distance from the lens, but was rapidly restored from green to yellow by the red rays. Thence he inferred that the rays of greatest refrangibility favour disoxygenation, but that the least refrangible ones favour oxygenation. These changes have been investigated by Sir John Herschel, the results of whose inquiries I shall now give.

(326) This resin dissolved in alcohol, spread evenly

upon paper, gives nearly a colourless ground. Exposed to the spectrum, a blue colour is impressed upon the paper, over the spaces between the least refrangible green rays, and a point situated upon the invisible rays a few lines beyond the violet rays. The action of dispersed Light changes the paper to a pale green; but over the region of the red rays the original pale colour of the paper is preserved.

(327.) If a paper thus prepared is exposed to the action of the solar rays, which have permeated a deep blue fluid, until it assumes a uniform pale blue tint, it will be found, that under the influence of the prismatic spectrum, a restoration of the original yellow colour takes place over the region of the green, yellow, orange, and red rays, the blue colour being entirely removed by the orange ray. It will be remembered that Dr. Wollaston attributed this restoration of colour to the action of heat. Had this been the true interpretation, we might expect to find the change most evident in that part of the spectrum where the heat was greatest, which we see is not the case.

(328.) Sir John Herschel exposed a portion of this paper to the action of chlorine considerably diluted with common air, by which it acquired a pale, dirty, greenishyellow hue. Transferred immediately to the spectrum, it was impressed with faint tints nearly corresponding to the natural ones: the red was evident, the yellow dilute, and nearly white; the blue, a fine sky blue; while beyond the violet succeeded a train of somewhat greenish dark

ness.

(329.) If a paper prepared with the alcoholic solution of guaiacum, is placed in an aqueous solution of chlorine, it acquires a beautiful and pure celestial blue colour. "This paper is very sensitive, and may be used for copy-ing engravings, which it does with this singularity, that the picture penetrates the paper, and appears on the back with nearly the same intensity as on the face." Under

the influence of the less refrangible rays, the blue colour is changed into a pale reddish yellow, but simply whitened over the more refrangible region of the spectrum. Photographs or spectra received on this paper speedily fade. (Herschel.)

(330.) Several experiments were tried, with a view of determining if the conclusions arrived at by Dr. Wollaston, as to the influence of artificial heat in producing these changes of colour were correct, and if they explained the restoration produced by the least refrangible rays of the solar spectrum. It has already been noticed that the rays of greatest calorific power produce no change upon the paper, whereas the decoloration is brought about by the rays in the region of the red, orange, yellow, and green spaces. It was found by Sir John Herschel that an artificial heat between the limits of 180° and 280° soon changed guaiacum from a green to a yellow state if moist, but that no such change was produced if absolutely dry. It was thought desirable to try the effect of the spectrum upon papers prepared as above, whilst, at the same time, they were subjected to a temperature sufficiently high to produce this effect had the paper been moist; this artificial heat being meant to assist the power of the calorific rays: this was done by holding a hot iron at the back of the paper. It was found, however, that, although it quickened the action over the luminous spaces, which were not at all changed in their character, that no perceptible effect was produced by those rays which possess the greatest heating power. From these researches it would appear that this restoration of guaiacum to its original colour is not dependent upon the heating power of the sun's rays, but upon some influence strictly analogous to that exerted by the "chemical rays," but modified by the combined influences of Light and heat.

(331.) COLOURED JUICES OF FLOWERS. The action of Light on the juices of plants has been carefully studied by M. Chevreul; but as his experiments were made with