THE TEN-WEEKS' STOCKS.

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completely whitened. The colour seems to resist the first impression of the Light, as if by some remains of vitality, which being overcome, the tint gives way at once, and the discoloration, when commenced, goes on rapidly. It does not even cease in the dark when once begun; hence photographic images received upon papers prepared with this juice slowly fade out.

(337.) Prismatic Analysis.-Exposed to the spectrum, in about fifteen or twenty minutes the colour is totally destroyed, and the paper whitened in the whole region of the green, blue, and violet rays, to which, therefore, the most energetic action is confined. If the action of the spectrum be prolonged, a much feebler whitening becomes sensible in the red, and a trace of it also beyond the violet into the "lavender" rays. By keeping papers thus impressed, terminal spots were detected beyond the red extremity, and also beyond the violet, they having gradually developed themselves.

(338.) Common Ten-weeks' Stocks. Mathiola annua. The colour imparted to alcohol by the double variety of this flower, in the height of its flowering, is a rich and florid rose red; when fresh prepared, papers stained with it are sensibly discoloured in a few hours, and completely whitened in two or three days. Exposed to the spectrum, the rays chiefly active in operating the discoloration, are found to be those extending from the yellow to the less refrangible red, beyond which rays the action terminates abruptly. Above the yellow, it degrades rapidly to a minimum in the blue, beyond which it recovers somewhat, and attains a second but much feebler maximum in the violet rays.

(339.) Sulphurous acid whitens this paper, but it resumes its original colour after a little time, which is materially quickened by the aid of Light. Papers thus completely discoloured, when exposed to the spectrum, were restored to their original colour, by rays complementary to those which destroy it in the natural state of the

paper; the violet rays being chiefly active, the blue almost equally so, the green little, and the yellow, orange, and most refrangible red not at all. Sir John Herschel attributes, and I think rightly so, the power of the sulphurous acid in inducing a dormant state of the colorific principle to a partial deoxidisement, unaccompanied, however, with disorganisation of its molecules.

(310.) It has been noticed that alcohol in many cases weakens the colours of vegetable juices, and in some entirely masks them. With the Papaver orientale this is very strikingly shown. The colour of the flower is a brilliant orange, "the colouring matter of which is only extractable by alcohol, and then only in a state so completely masked, as to impart no more than a faint yellowish or pinkish hue to paper, which it retains when thoroughly dry, and apparently during any length of time, without perceptible increase of tint." This paper is immediately rendered a vivid scarlet colour when an acid is applied to it. If paper covered with this alcoholic extraction is exposed for a long period to the influence of the Light, it undergoes some disorganisation, so that, on being exposed to the vapours of muriatic acid, a dormant picture supposing it to have been covered with an engraving — is gradually developed in a soft and pleasing style. The time required to produce these pictures extends over from twenty to thirty days.

(341.) Papers covered with an alcoholic tincture of turmeric are slowly acted upon. It is whitened by the blue and violet rays. If it is browned by carbonate of soda it is somewhat more sensitive, especially when wet, and an abruptly terminated action is perceptible in the red region.

(342.) Bulbine bisulcata and two other species from the Cape of Good Hope were found by Sir John Herschel to yield from the green epidermis of their leaves and flower stalks a bright yellow juice, which darkens rapidly on exposure to Light, changing at the same time to a ruddy

brown. Exposed to the spectrum, the less refrangible rays are found inoperative either in inducing a change of tint, or in preserving that portion of the paper on which they fall from the influence of dispersed Light. A darkening commences about the mean yellow ray, but it continues very feeble through the green ray, above which it darkens more strongly, arriving at its maximum in the blue, but extending to a considerable distance beyond the violet with some degree of intensity.

(313.) Cheiranthus cheiri, Wallflower. "A cultivated double variety of the flower, remarkable for the purity of its bright yellow tint, and the abundance and duration. of its flowers, yields a juice, when expressed with alcohol, from which subsides, on standing, a bright yellow finely divided fæcula, leaving a greenish-yellow transparent liquid, only slightly coloured supernatant. The fæcula spreads well on paper, and is very sensitive to the action of Light, but appears at the same time to undergo a sort of chromatic analysis, and to comport itself as if composed of two very distinct colouring principles, very differently affected. The one on which the intensity and sub-orange tint of the colour depends, is speedily destroyed, but the paper is not thereby fully whitened. A paler yellow remains as a residual tint, and this on continued exposure to Light, so far from diminishing in tone, slowly darkens to brown. Exposed to the spectrum, the paper is first speedily reduced nearly to whiteness in the region of the blue and violet rays. More slowly, an insulated solar image is whitened in the less refrangible portion of the red. The exposure continuing, a brown impression begins to be perceived in the midst of the white streak, which darkens very slowly over the region between the lower blue and the extreme violet rays. It never attains any great intensity, but presents a singular appearance in the midst of the white train previously eaten out."

(344.) The common Marigold yields an insoluble fæcula, which appears identical with that produced by the wall

flower and that of the Corchorus japonica, and it is found to be quite as sensitive to Light; but photographs procured upon it cannot be preserved, the colour is so fugitive. The juice of the Mimulus Smithii affords a yellow die, which is similarly affected. "The Ferrarea undulata, a dark-brown flower, yields, when expressed, a dull green juice, which, spread on paper and dried, turns very speedily blue, under the influence of the blue and violet rays of the spectrum, owing to the destruction of this yellow principle, which, mingling with the substratum of blue (itself a much more indestructible tint), gives its natural tinge of green. The brown colour of the French marigold, Tagetes patula, passes rapidly in sunshine from brown to green, probably from the destruction of the same yellow principle. And bees' wax, it is well known, is bleached by Light, from the presence of a similar fugitive principle.

(315.) The Viola odorata yields to alcohol a rich blue colour, which it imparts in high perfection to paper. Exposed to sunshine it fades pretty rapidly, but a residual blue tint remains, which resists the action of Light for a long time, even for weeks. When carbonate of soda is added to this solution it becomes green, and a slip of paper stained with this fluid, exposed to the spectrum, is changed yellow under the orange and red rays; a slight discoloration is perceived in the indigo-blue rays, but not the slightest alteration under the green rays. The colouring matter of the purple iris shows this in a still more marked manner. In these instances the blue constituent of the green is destroyed by the solar rays. (Herschel.)

(346.) A variety of Sparaxis from the Cape of Good Hope gave to paper a dark olive-green colour, which was nearly insensible to Light. The addition of carbonate of soda changes this colour to a good green, which is tolerably sensitive to solar influence. A photograph impressed on a paper prepared with it is reddened by muriatic acid fumes. If then transferred to an atmosphere of ammonia, and when supersaturated, the excess of

alkali allowed to exhale, it is fixed, and of a dark-green colour. (Herschel.)

(347.) The Red Poppy.-Papaver Rhoas yields a very beautiful red colour, which is entirely destroyed by Light. When perfectly dried on paper the colour becomes blue. This blue colour is speedily discharged by exposure to the sun's rays, and papers prepared with it afford very interesting photographs. The purple juice of the Senecio splendens, the double purple groundsel, imparts a beautiful colour to paper. It requires, however, an exposure of some weeks to daylight before the original whiteness is restored, which it at length is in the most perfect manner.

(318.) The juices of the leaves of a great number of plants have been examined by the author; and the juices from leaves of the laurel, the vine, the common cabbage, and the grasses, have been found to be sufficiently sensitive, when spread upon paper, to give very good copies of engravings in an hour, provided the atmosphere was clear and the sun bright. The action of the prismatic spectrum upon those I have examined, agrees very nearly with results published by Sir John Herschel, as obtained upon the juice of the elder leaf. The red rays have a decided action, and give a ruddy yellow impression. It appears to me, this change is dependent upon the calorific agency merely, as similar changes are brought about by artificial heat. On the upper edge of the yellow ray, the space between the red and it being unaffected, a very faint image begins to be formed; this action goes on increasing up to the mean blue rays; it then declines, and ceases altogether within the limits of the visible violet ray.

(349.) From an examination of these researches on the colouring matters of plants, it will be seen that the action of the sun's rays is to destroy the colour, effecting "a sort of chromatic analysis, in which two distinct elements of colour are separated, by destroying the one and leaving the other outstanding." The action is confined within the visible spectrum, and thus a broad distinction is exhibited.