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CHAPTER VII.

THERMOGRAPHY. -A PARTICULAR EXAMINATION OF ALL THE PHENOMENA CONNECTED WITH THE SUPPOSED RADIATION OF LIGHT IN ABSOLUTE DARKNESS.

(413.) NOTHING having been done in this section of the inquiry since this chapter was first published, it stands without alteration.

It may appear to many at first, that this inquiry is not quite in place in the present volume, it having been shown that the conclusions arrived at by M. Moser, who first called particular attention to the phenomena in question, are in all probability erroneous. The subject is, however, so intimately united with those agencies whose powers we have been considering, that this treatise would be incomplete, did it not contain a particular account of the discovery, the discussion to which it has given rise, and record all those experiments of interest which bear upon these mysterious actions.

(414.) In a memoir "On Vision and the Action of Light on all Bodies,' "* M. Ludwig Moser first announced the following fact: "If a surface has been touched in any particular parts by any body, it acquires the property of precipitating all vapours which adhere to it, or which combine chemically with it on these spots, differently to what it does on the other untouched parts."

In a memoir entitled "Some Remarks on Invisible Light," and in another "On the Power which Light possesses of becoming latent," his views were still further

* Poggendorff's Annalen, vol. lvi. p. 177. No. 6. 1842. Poggendorff's Annalen, vol. lvi. p. 569. No. 8. 1842.

Poggendorff's Annalen, vol. lvii. p. 1. No. 9. 1842.

Those papers have been translated by Henry Croft, Esq., and published in Taylor's Scientific Memoirs, vol. iii. part xi. February, 1843:. from these translations all my quotations will be made.

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developed, and I shall now endeavour to place them in as correct a light as possible.

(415.) In the Daguerreotype process those parts of the iodised silver plate upon which the Light has acted with most power, receive, when the plate is exposed to the vapour of mercury, the largest quantity of that vapour over their surfaces, and the gradations of Light are marked very beautifully by the thickness of these mercurial films. Now if we write with a piece of steatite on a looking-glass, the writing is invisible until we breathe upon it, when it appears distinctly. If we place coins on a plate of glass or metal, and allow them to remain for some few hours in contact, although no change will be visible when they are removed, we may bring out beautiful images of the coins by breathing on the plate, or exposing it to any vapour. Upon these experiments M. Moser has based his hypothesis, That Light of a peculiar degree of refrangibility is absorbed by all bodies, and that they radiate it again in darkness.

(416.) We must, however, observe all the phenomena which M. Moser has brought before the scientific world. These effects are produced by writing on glass or metal with any substance whatever. "We may first breathe uniformly over the whole plate, and then write on it, either with blotting-paper, a brush, or any thing else; the characters will become visible whenever the plate is breathed on, and this phenomena lasts for some time. Not only is glass applicable to this purpose, but every other polished body exhibits the same appearances: I have tried it with metals, resins, wood, pasteboard, leather, &c. Even fluids may be used; if we take a clean and still surface of mercury, hold over it a body, and breathe on the other parts, or, what is better, breathe on the whole surface first, and then remove the moisture by any gentle means from particular parts, they will again become visible when breathed on, even after several days, if the mercury remains undisturbed.

Moreover, absolute contact with the

extraneous body is not necessary, mere juxtaposition producing similar effects. If we hold over a polished body a screen, part of which has been cut out according to pleasure, but without allowing it to touch, and then breathe on the whole, and allow the water to evaporate, we shall find that, on breathing on it again, we shall be enabled to distinguish fully the figure of the excised parts: and still farther, it does not require a polished body, inasmuch as dull glass exhibits the same phenomena."

(417.) These phenomena were produced in a great many ways. "An engraved metallic plate was warmed, and then held for about half a minute on a well polished piece of silver foil, or a clean mirror plate. When the plates were cold, they were breathed on, and exhibited the above-mentioned appearances in a much more perfect manner; for not only were the outlines of the body visible, but also the individual figures, letters, &c., and all with the greatest distinctness." Frequently silver or other metallic plates were made warm, and cold bodies, variously cut stones, figures of horn, pasteboard, cork, coins, &c. allowed to remain on them for some time. The phenomena were all the same." Mercurial vapour was found to act in the same manner as the vapour of water, and the vapour of iodine the same as that of mercury. An iodised silver plate, having some of these bodies placed upon it, "was introduced into the vapours of mercury, and then the perfect image became visible, that is to say, Daguerre's phenomenon was produced without the intervention of Light, for the experiments succeeded just as well by night as by day."

(418.) Moser argues from these experiments, "that contact is capable of imitating the action of Light," and he considers the following experiments to prove this clearly: "A silver plate was iodised during the night, and even without the Light of a candle; a cut slab of agate, an engraved metallic plate, and a ring of horn, &c. were then laid upon it, and the plate was afterwards introduced into

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the vapours of mercury. A good, clear picture of all the figures, of the stones, the letters of the plate, and of the ring, was obtained. A plate which had been treated in the same manner was exposed to day or sunlight, and similar pictures were produced. Other plates of the same kind were placed under coloured glasses-yellow, red, and violet; under the first two only a trace of the image was evident; but under the violet glass it was clearly defined." Upon these experiments, Moser remarks, "the violet rays continue the action commenced by contact." He then proceeds to examine the action of Light upon simple or difficultly changeable bodies:" the results are certainly exceedingly curious and instructive.

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(419.) "A new plate of silver was cleaned and polished as well as possible. A surface with various excised characters was suspended over it without touching, and the whole was exposed to the sun for some hours, and directed towards it; after the plate, which of course did not exhibit the least change, had been allowed to cool, it was held over mercury heated to about 60° Reaum. A clear image of the screen was produced; those parts where the sunlight (which had been very weak) had acted, had caused the deposition of a quantity of mercury." Plates of copper and of glass were treated in the same manner, and with the same results. "If we compare the remarkable fact of the action of Light upon surfaces of silver, with the above-mentioned phenomena produced by contact, we can no longer doubt that Light acts on all bodies, modifying them so that they behave differently in condensing the vapours of mercury." Moser then proposes the following general expression of the fact: "Light acts on all bodies; and its influence may be tested by all vapours that adhere to the surface, or act chemically upon it."

(420.) If through one of the excised screens we breathe upon a plate of metal or glass, and then removing the screen, allow the vapour to pass off, and then again breathe upon the plate, the vapour will adhere to those

parts which have been protected, and the portions on which the breath was at first condensed will appear dark: the vapours of mercury act in a similar manner. Hence, Moser concludes, "That the same modification is produced in plates when vapours are condensed, as when Light acts on them" (419).

(421.) "If an iodised silver plate be allowed to remain too short a time in the camera obscura, it afterwards exhibits no image when exposed to the vapours of mercury; a Light film of mercury is deposited over the whole plate, which is not only the case with this, but with a plate of the pure metal and the blackened iodide of silver. If the plate remain a longer time in the camera obscura, a picture certainly is produced, but in which only the brightest parts are depicted, and, which is here of importance, the light parts are of a white colour, i. e. they condense the most mercury. If the plate remains still longer in the camera obscura, a picture with all its details is formed; but the bright parts have lost a portion of their whiteness, and appear grey, i. e. they do not condense so much of the mercurial vapour. If it be left still longer, on taking it out no picture at all is to be seen; if now inserted into the mercurial vapours, a negative image is produced, or, in other words, these bright parts do not condense any mercury."

(422.) "If Light acts on iodide of silver" (I still quote from the "Memoir on Vision," &c.) " it imparts to it the power of condensing mercurial vapours in an increasing ratio; but if it acts beyond a certain time, it then diminishes this power, and at length takes it away altogether, and this happens before the yellow iodide has changed its colour. The vapours of mercury have been seen to do the same, and if in the last-described phenomenon they produce a negative image, it is only what Light would have done had it been allowed to act still longer. Iodide of silver is found to blacken under the influence of the solar rays, and the vapour of mercury also renders it