FRAUNHOFER'S LINES.

145

above it appears of a dark semi-metallic purple shaded by a dusky brown border. It is quite impossible to represent these delicate differences between the respective portions. by a wood engraving. The accompanying figure is, therefore, to be regarded as representing with accuracy the general character of the impression, showing the limits of greatest and of least action, and little more.

(235.) By operating with a well-defined spectral image of a small slit, formed by the knife-edges, throwing this spectrum upon a large lens of long focus, and receiving the image at the distance of several feet from the lens on the collodion plate, we obtain a spectrum showing most of the dark lines of Fraunhofer, particularly those which are discovered at the most refrangible end of the spectrum, and over the space beyond the luminous rays, which is rendered luminous by the fluorescent rays of Professor Stokes, these being so many inactive spaces, which are in the first impression represented as so many white lines. A positive copy being taken from the negative spectrum, these are then represented correctly as dark lines, with an inversion of the character of the chemically darkened spaces. This, however, proves only that the modes of motion of Light and actinism are the same, it does not prove the identity of these forces. (See frontispiece).

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CHAPTER IV.

SOLAR ACTION ON VARIOUS METALLIC COMPOUNDS.

(236.) GOLD. It has been long known, that a solution of gold in ether decomposes by exposure to the sun's rays, metallic gold being deposited on the side of the vessel nearest the Light. Charcoal saturated with a solution of the chloride of gold, and exposed to the sun's rays, is speedily covered with a very fine film of the revived metal. Ivory may also be gilded by washing it with the same solution and exposure. Most of the salts of gold, indeed, are reduced by solar agency.

(237.) Sir John Herschel, in the Philosophical Transactions, Part I. for 1810, has given some very interesting particulars respecting some aurated preparations:

Papers washed with CHLORIDE OF GOLD, freed from an excess of acid, are slowly changed under the influence of the solar beams, a regularly increasing darkness takes place, and the paper at length becomes purple (Herschel). I have observed that the first action of the Light is to whiten paper, which has been rendered a pale yellow by the wash of chloride of gold. If papers are removed from the Light when thus bleached, it will be found that a darkening action will gradually come on, and eventually develop the picture impressed on the paper. This process is much quickened by placing the paper in cold water, and, however slight the exposure may have been, the process of darkening continues until all the salt of gold is decomposed.

(238.) Chloride of gold with nitrate of silver gives a precipitate of a yellow brown colour, possibly metallic double salts, in which the gold as well as the silver is in the state of chloride. On glass this precipitate is but very

CHLORIDE OF GOLD.

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slightly sensitive; on paper it is blackened somewhat more speedily (Herschel).

(239.) If paper impregnated with oxalate of ammonia be washed with chloride of gold, it becomes, if certain proportions be hit, pretty sensitive to Light, passing rather rapidly to a violet purple in the sun (Herschel). I have found it exceedingly difficult to arrive at the best proportions generally speaking, thirty grains of the oxolate of ammonia, and a saturated solution of the chloride of gold, has been the most successful in practice. These papers must be dried in the dark without heat. After the picture has been obtained, I have succeeded in fixing it, by soaking it in cold water, and then washing it over with the ferrocyanate of potash.

(240.) Paper impregnated with the acetate of lead, when washed with perfectly neutral chloride of gold, acquires a brownish-yellow hue, and a sensibility which, though not great, is attended with some peculiarities highly worthy of notice. The first impression of the sun's rays seems rather to whiten than to darken the paper, by discharging the original colour, and substituting for it a pale greyish tint, which by slow degrees increases to a dark slate colour; but if arrested, while yet not more than a moderate ash grey, and held in a current of steam, the colour of the part acted on by the solar rays (and of that only) darkens immediately to a deep purple: the same effect is produced by immersing it in boiling distilled water. If plunged into cold water, the same change comes on more slowly, and is not completed till the paper is dried by heat. A dry heat, however, does not operate this singular change (Herschel).

(241.) Prismatic Analysis. -Chloride of Gold. The maximum effect is produced by the mean blue ray, and the influence is exerted but a little way below the green; indeed, it is doubtful if it can be said, that any visible effect is produced below the green itself. Above the blue, the action is carried on, but with declining energy, through

the indigo and violet rays, beyond the most refrangible edge, of which no action can be detected.

To distinguish the following processes from the Chrysotype, which will be described in another section, I propose to designate them as Aurotypes.

(242.) Aurotypes. Protocyanide of potassium and gold, prepared according to Himly's method*, was washed over paper and dried; then it was washed with a solution of nitrate of silver, and again dried. This paper darkens with considerable rapidity, and this blackening proceeds steadily in the dark. Good photographs result from this preparation. The pictures are best fixed by soaking in a little salt and water, and then washing with a weak solution of the hyposulphite of soda.

(243.) It will be found that several of the combinations of the oxide of gold with cyanogen yield very interesting pictures, which promise to be of some importance in the photographic art. A few of these may be briefly men

tioned.

(a) Protocyanide of potassium and gold, with a weak wash of nitrate of silver, changes with tolerable quickness, and presents a good contrast of light and shadow.

(b) Protocyanide of gold, formobenzoic acic, and nitrate of silver, give very beautiful results, and are tolerably quick in changing, although as yet no paper has been prepared, sufficiently sensitive for use in the camera obscura. The darkened portions are exceedingly intense, the impression being made nearly through the entire substance of the paper; hence affording very perfect photographs, from which copies of exceeding sharpness may be pro

cured.

(c) Protocyanide of gold, formobenzoate of ammonia, and nitrate of silver, give results of an exceedingly pleasing kind. Papers thus prepared do not appear to be quite so sensitive as those which are prepared with the formo

Appendix No. I.

PRISMATIC ANALYSIS OF GOLD SALTS.

149

benzoic acid, but they are sufficiently so for copying engravings in good sunshine.

(d) Nitrate of silver, protocyanide of potassium, and gold. A very delicate picture results from a short exposure to sunshine, which continues to darken without the aid of Light as long as any portion of gold remains undecomposed. It is a peculiar property of all the salts of gold, that the darkening process once set on foot is carried on in the dark as long as any gold remains.

(244.) Prismatic Analysis. The following results, obtained upon several different preparations, will serve to exhibit most of the peculiarities which mark the influence of the solar beam or auriferous preparations.

On paper prepared as above (243. a). The maximum of action is far down in the blue rays, nearly on the verge of green. After a few minutes, the action is extended through the green ray to the very centre of the yellow ray. Towards the most refrangible end, the action is tolerably uniform through the whole of the blue and indigo rays; it slowly declines through the violet; but it extends with some considerable power over a space beyond the visible spectrum, equal to a third of its entire length.

A paper washed with the protocyanide of potassium and gold, between two washes of the nitrate of silver, exhibited the maximum of intensity on the very edge of the least refrangible blue ray. The darkening process was carried on, down to the edge of the orange ray; below which no action could be detected. Above the blue rays the influence became gradually weaker, and faded away entirely at a point beyond the violet rays, distant from the visible ray about the width of the violet rays themselves. No action could be detected at the least refrangible end of the spectrum.

A paper prepared as described (243. b) was acted upon with much energy. The maximum of action was in the mean blue ray; and a well-defined line marked the least refrangible edge of the rays. The influence was, however,