M. Regnault has laid before the Academy of Sciences, at Paris, some photographic specimens on paper, obtained by M. Blanquart-Evrard, by a modification of the usual process. In the preparations hitherto described, one part of the process presented serious difficulties, namely, that of the use of gallic acid in order to produce the impression. It happened frequently, that a proof taken in too mild a light, or of too large dimensions, could not receive the necessary force before disappearing, as it may be said, under the uniform colour produced by the mixture of the gallic acid with the aceto-azotate of silver, with which the paper is imbued. After having ascertained that the gallic acid produces this uniform colour on the impression, only because it is combined in small quantity with the aceto-azotate of silver, M. Blanquart-Evrard removes all the difficulty. After taking the proof from the camera obscura, he plunges it into a vessel of large dimensions, covered with a layer of one centimètre of gallic acid of cold saturation. The bath is agitated during the immersion; and the action may be thus prolonged until the impression has obtained the necessary force to secure a good result. The proof is then washed, and the gallic acid is replaced by a solution of bromure of potassium, or chloruret of sodium, in which it is left for about a quarter-of-an-hour.[H]
The chromatype, discovered by Mr. Hunt, consists in washing good letter-paper with the following solution:—
| Bi-chromate of potash | 10 grains |
| Sulphate of copper | 20 grains |
| Distilled water | 1 ounce |
Papers prepared with this are of a pale yellow colour; they may be kept for any length of time without injury, and are always ready for use. For copying botanical specimens or engravings, nothing can be more beautiful. After the paper has been exposed to the influence of sunshine, with the objects to be copied superposed, it is washed over in the dark with a solution of nitrate of silver of moderate strength. As soon as this is done, a very vivid positive picture makes its appearance; and all the fixing these photographic pictures require is, well washing in pure water.
M. Niepcé de St. Victor finds that, if a sheet of paper on which there is writing, printed characters, or a drawing, be exposed for a few minutes to the vapour of iodine, and there be applied immediately afterwards a coating of starch, moistened by slightly acidulated water, a faithful tracing of the writing, printing, or drawing, will be obtained. M. Niepcé has also discovered that a great number of substances, such as nitric acid, chlorurets of lime and mercury, act in a similar manner; and that various vapours, particularly those of ammonia, have the effect of vivifying the images which are obtained by photography.
In the words of a writer in the North British Review:—“While the artist is thus supplied with every material for his creative genius, the public will derive a new and immediate advantage from the productions of the solar pencil. The home-faring man—whom fate or duty chains to his birth-place, or imprisons in his fatherland—will, without the fatigues and dangers of travel, scan the beauties and wonders of the globe; not in the fantastic or deceitful images of a hurried pencil, but, in the very picture which would have been painted on his own retina, were he magically transported to the scene. The gigantic outline of the Himalaya and the Andes will stand self-depicted upon his borrowed retina—the Niagara will pour out before him, in panoramic grandeur, her mighty cataract of waters, while the flaming volcano will toss into the air her clouds of dust and her blazing fragments. The scene will change, and there will rise before him Egypt’s colossal pyramids, the temples of Greece and Rome, and the gilded mosques and towering minarets of eastern magnificence. But with not less wonder, and with a more eager and affectionate gaze, will he survey those hallowed scenes which faith has consecrated and love endeared. Painted in its cheerless tints, Mount Zion will stand before him, ‘as a field that is ploughed;’ Tyre, as a rock on which the fishermen dry their nets; Gaza, in her prophetic ‘baldness;’ Lebanon, with her cedars prostrate among ‘the howling firs;’ Nineveh made as a grave, ‘and seen only in the turf that covers it;’ and Babylon the great, the golden city, with its impregnable walls, its hundred gates of brass, now ‘sitting in the dust, cast up as an heap,’ covered with ‘pools of water,’ and without even the ‘Arab’s tent,’ or the ‘shepherd’s fold.’ But though it is only Palestine in desolation that a modern sun can delineate, yet the seas which bore on their breast the Divine Redeemer, and the everlasting hills which bounded his view, stand unchanged by time and the elements, and, delineated on the faithful tablet, still appeal to us with an immortal interest. But the scenes which are thus presented to us by the photographer have not merely the interest of being truthful representations: they form, as it were, a record of every visible event that takes place while the picture is delineating. The dial-plate of the clock tells the hour and minute when it was drawn, and with the day of the month, which we know, and the sun’s altitude, which the shadows on the picture often supply, we may find the very latitude of the place which is represented. All stationary life stands self-delineated on the photograph:—the wind, if it blows, will exhibit its disturbing influence; the rain, if it falls, will glisten on the house-top; the still clouds will exhibit their ever-changing forms; and even the lightning’s flash will imprint its fire-streak on the sensitive tablet.”
CHAPTER VII.
Heat, the cause of many wonders—Its universal diffusion and application—Story of a burning-glass—The Augustine friars and the Jesuits—Impostures as to the endurance of heat—Burning mirrors—The blow-pipe—The Giants’ Causeway—Application of currents of heated air—Travelling by steam.
Heat is everywhere present: every body that exists contains it in quantity to which we can assign no limits. The endless variety of forms which are spread over and beautify the surface of the globe, are to be traced to its influence. Without it, the land and the water would fall into one formless and impenetrable mass, and the air now essential to life, prove absolutely poisonous. We shall find in connexion with it, therefore, many extraordinary phenomena.
When Labat the Jesuit visited the Peruvians, he took the naked arm of one of them, and, concentrating on it the rays of the sun by means of a powerful lens, soon made him cry out with pain, while the others looked on with wonder, not unmixed with indignation. How could this effect be produced? was instantly the question; and, as promptly, the cause was declared to be infernal. In vain did Labat assert that it was merely natural. The Peruvians made many attempts to obtain possession of the lens in order to destroy it, and deliver themselves from the power of that which they regarded as able to bring upon them the vengeance of the gods.