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between the influence of the sun's rays on vegetable juices and on argentine compounds, the latter being most sensibly affected by the “invisible rays” beyond the violet.

(350.) “It may also be observed, that the rays effective in destroying a given tint are, in a great many cases, those whose union produces a colour complementary to the tint destroyed, or, at least, one belonging to that class of colours to which such complementary tint may be referred. For example, yellows tending towards orange are destroyed with more energy by the blue rays; blues by the red, orange, and yellow rays; purples and pinks by yellow and green rays.

(351.) I may here mention, that some very remarkable changes take place in the colours of many vegetable powders, in which we might least expect such alterations to occur. Experience has shown to the pharmacopolist the necessity of preserving the powdered leaves of the foxglove, the hemlock, the henbane, the aconite, and other green vegetable powders of active medicinal powers, in the dark. It is found that these powders do not merely lose colour, -passing slowly from a green into a slaty grey, and ultimately into a dirty yellow, — but they undergo some decomposition, by which, at the same time, they lose much of their medicinal activity, and indeed after a season they become nearly inert.

(352.) Few pharmaceutical articles suffer more in this respect than the powder of the jalap root ; the ipecacuanha also loses much of its emetic power by exposure to Light. This is entirely independent of any action of the air or moisture. I have observed these deteriorating influences on those powders, which have been kept in the most carefully closed bottles.

(353.) The powders of Cascarilla bark, of the Valerian root, and some others, particularly some of the varieties of rhubarb and the ginger root, are found to adhere with considerable firmness to the sides of the bottles next the Light, whereas the sides in shadow are left clear. I have also observed that a deposit will take in a similar manner on the



sides of bottles containing some of the vegetable tinctures. This of course depends upon the same function which occasions camphor to be deposited in crystals upon the side of the glass next the Light, and maintains them there; whereas, if that side is turned from the Light, the crystals will be gradually removed and again deposited on those parts upon which the rays of Light first impinge. These phenomena must have been long and often observed, yet we have not any satisfactory explanation of them. It does, however, appear, that we are advancing gradually towards the elucidation of these and many other matters, which have often excited the wonder of observers without leading to any particular inquiry.

(354.) In the “ Philosophical Transactions for 1844” is published an extract of a letter from Mrs. Somerville to Sir J. F. W. Herschel, dated Rome, September 20. 1845: On the Action of the Rays of the Spectrum on Vegetable Juices.

There is so much that is curious in this communication, that I have extracted a portion to show the character of the investigations in which that lady is engaged.

“ In the following experiments the solar spectrum was condensed by a lens of flint glass of 7} inches focus, maintained in the same part of the screen by keeping a pinhole, or the mark of a pencil, constantly at the corner of the red rays, which were sharply defined by using blue spectacles to protect my eyes from the glare of light, and the apparatus was covered with black cloth in order to exclude extraneous light.

“ Thick white letter paper moistened with the liquid to be examined, was exposed wet to the spectrum, as the action of the coloured light was more immediate and more intense than when the surface was dry. As I had not access to the morning sun, the observations were made between noon and three in the afternoon."

(355.) Mrs. Somerville approached very near to the discovery of the extra spectral rays of Stokes, as the following paragraph shows.

“ The lavender rays came vividly into view; under a condensed spectrum, on white paper washed with a solution of sulphate of quinine in dilute sulphuric acid, they were narrow, and their length by rough measurement was equal to the distance between the upper edge of the violet and the lower edge of the blue. They were very brilliant on black silk or other dark surfaces, and invariably of lavender colour; and even on paper stained with turmeric, the pale yellow rays which you had observed were tipped with lavender, on being washed with this liquid, though its duration was momentary, as it vanished as the surface became dry; but they were permanent in other instances.

(356.) “ The lavender rays changed their colour with a change of the liquid ; for instance, they are lavender colour on nitrate of silver discoloured by Light to a very pale brown, washed with a solution of sulphate quinine in dilute sulphuric acid ; whereas, on a similar surface of pale brown nitrate of silver washed with the juice of the petals of the pale blue Plumbago auriculata, in distilled water, to which sulphuric acid was added, they appeared of a vivid apple green, and acquired a tip of lavender colour, on the surface being washed with a solution of sulphate of quinine in dilute sulphuric acid of considerable strength. The effect, however, was transient. After several unsuccessful attempts to repeat this experiment next day, I at length discovered that its success depended upon the acid being strong enough to decompose the juice and give it a reddish orange hue, and even then the rays are not vivid till the paper has been frequently washed with the juice and become nearly dry; and the experiment is more successful when the liquid has been kept a night. The action of the surface in changing the colour of the lavender rays may be illustrated by passing the spectrum over paper coated with nitrate of silver brought to a clear yellow brown by exposure to the sun, one half washed by the liquid in question, and the other half with a solution of sulphate of



quinine in dilute sulphuric acid. On the first half the lavender


became vivid apple green, while, on passing to the other half, they instantly changed to a equally vivid lavender colour. These rays often darken the surface throughout their whole length; sometimes they acquire a powerful bleaching action, and sometimes they have no effect, as evidently appears from the following experiments.

(357.) “ The juice of fresh-gathered petals of double flowering pomegranate in alcohol afforded an example of this. Paper washed with this juice became rich crimson, and on being exposed wet to the condensed spectrum, a narrow line of deep crimson was formed at the junction of the green and yellow rays, or perhaps in the most refrangible yellow, surrounded by a whitish lozenge-shaped border (a). On again washing with the juice, instead of the white border, which had vanished, there was a crimson flame-shaped image, curved at the lower edge of the yellow rays, and tapering upwards to the violet ; its colour was darker than that of the ground, though paler than the narrow line which maintained its intensity; and although the latter increased in width, it did not become as broad as the image in question. At the upper end of the violet another little dark image was formed, apparently owing to the action of the lavender rays, having exactly their form: the orange and red rays, especially the red, had no effect, though at the distance of about half the length of the spectrum beyond the red, two distinct spots were formed of deep crimson, which I believe to be the heat spots which you discovered.

discovered. After some time a bleaching appearance surrounded the whole image from the red upwards, probably owing to rapid evaporation from the heat of the spectrum (6). Exterior bleaching frequently took place in the course of the ex


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periments, permanent in some instances, while in others it vanished as the surface dried. When water was used with the juice instead of alcohol, the general character of the image was similar to that described, except that the small figure beyond the violet was more distinct, and seemed to bear the same proportion to that formed by the rest of the spectrum which the length of the lavender rays bears to the length of the sum of the others. The bleached part round the whole was more extended, and a faint crimson haze encompassed the dark spots, which were very distinct (c).

(358.) “ The following are some of the cases in which the simultaneous effect was produced.

For example, paper washed with the juice of the petals of Globe Ama. ranthus in distilled water, on exposure to the spectrum, acquired a delicate pink tint which was soon bleached to whiteness from the upper edge of the green to the end of the lavender rays, while at the same time a perfectly circular spot of equal whiteness was seen under the red rays and a little way below them, which had the appearance of being an image of the sun. After more washing with the juice, the two bleached parts were united by a long white neck, which speedily vanished, and was succeeded by a dark crimson image, whose greatest intensity of colour was under the yellow rays. At some distance below the red rays two crimson spots were strongly marked, especially the

: uppermost, both surrounded by a paler halo.

(359.) “ The juice of the petals of pale blue Plumbago auriculata, in distilled water imparted its tint to writing paper, which after exposure to the action of diffused Light acquired a pale yellowish green hue. The part under the lavender and violet rays of the spectrum, repeatedly washed with the juice, assumed a pale brown colour. The indigo rays seemed to have no effect, although, from their lowest edge to the distance of half the length of the spectrum below the red rays, a lavender

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