of effecting that peculiar chemical change, which other rays much less copiously endowed with heating power, were all the while producing. 159. It may be objected to this, that no proof is afforded in the above-related experiment, that any part of the paper actually attained a temperature of 180° or more; that in consequence no discoloration due to the action of heat (quoad heat) was produced; and that the discoloration which did take place was sui generis, and originated with the light and not the heat of that part of the spectrum to which it corresponded. A slip of the same paper (1168) was therefore exposed dry to the spectrum, in such a way as to leave its back accessible; and an iron, heated below redness, was then approached to it so as just not to discolour the paper. Under such circumstances, it might be expected that the additional heat thrown on the paper in the region of the thermic rays would turn the scale in their favour at their points of greatest intensity, and give ocular proof of their action by a decided discharge of colour at those points. But no such result was obtained, nor could I succeed in rendering visible any of the heat-spots a, ß, y,d, even when the heated iron was brought so near as to produce a commencement of discoloration over the whole of that region of the paper where they ought to have shown themselves. 160. On the other hand, a remarkable, but by no means an unexpected influence, was exercised by the heat so thrown on that part of the paper where the less refrangible rays fell, and where the discoloration was in progress under their agency. For it was observed that, under these circumstances, the discoloration in question went on with much greater rapidity, so much so indeed, that the same amount of it, which without extraneous heat would have required twenty minutes or half an hour's exposure to the spectrum to produce, was now produced in two or three minutes. Obscure terrestrial heat, therefore, is shown to be capable of assisting and being assisted in operating this peculiar change, by those rays of the spectrum, whether luminous or thermic, which occupy its red, yellow, and green regions; while on the other hand it receives no such assistance from the purely thermic rays beyond the spectrum, acting under presisely similar circumstances, and in an equal state of condensation. 161. When heat was similarly applied by radiation from behind, and from a non-luminous source, over the more refrangible region of a spectrum thrown on paper simply washed with tincture of guaiacum and not previously blued either by chlorine or by light, the blue colour induced in the more refrangible rays was still produced, and of the same tint in the same points as if no heat had acted. This effect, the contrary to what the previous experiment would have led to expect, shows how little any reasonings on these points enable us at present to anticipate experience. 162. The discharge of colour from blued guaiacum by mere heat, has been shown above (Art. 156.) to take place at a much lower temperature in the presence of moisture than when dry; and a similar destruction of colour, under similar circumstances, takes place with many other vegetable preparations. Paper, for instance, coloured with the juice of the Viola tricolor (Art. 90.), is speedily whitened in the dark, while wet, by the heat of boiling water, though dry heat does not affect it. And under the action of the spectrum it is discoloured (though much more slowly) by the same, or nearly the same rays which are effective in the case of guaiacum. The colour of paper tinged with the juice of the common red stock is not affected when dry by any heat short of what suffices to scorch the paper, but when wet (as when exposed to steam) it is speedily discharged. There are few, if any vegetable colours indeed which long resist the combined effects of heat and moisture, even when light is excluded, still less when admitted.* Of the Colours of Flowers in general under the action of the Spectrum. 163. In operating on the colours of flowers, I have usually proceeded as follows: the petals of the fresh flowers, or rather such parts of them as possessed a uniform tint, were crushed to a pulp in a marble mortar, either alone, or with addition of alcohol, and the juice expressed by squeezing the pulp in a clean linen or cotton cloth. It was then spread on paper with a flat brush, and dried in the air without artificial heat, or at most with the gentle warmth which rises in the ascending current of air from an Arnott stove. If alcohol be not added, the application on paper must be performed immediately, since exposure to the air of the juices of most flowers (in some cases even for but a few minutes) irrecoverably changes or destroys their colour. If alcohol be present this change does not usually take place, or is much retarded; for which reason, as well as on account of certain facilities afforded by its admixture in procuring an even tint (to be presently stated), this addition was commonly, but not always made. 164. Most flowers give out their colouring matter readily enough, either to alcohol or water. Some, however, as the Eschnolzias and Calceolarias, refuse to do so, and require the addition of alkalies, others of acids, &c. When alcohol is added, it should, however, be observed that the tint is often, apparently, much enfeebled, or even discharged altogether, and that the tincture, when spread on paper, does not reappear of its due intensity till after complete drying. The temporary destruction of the colour of the blue heartsease by alcohol has been noticed in my former paper (Art. 90.), nor is that by any means a singular instance. In some, but in very few cases, On the effects of light, air, and moisture at common temperatures, as discolouring agents on several dyeing materials, I may refer to M. Chevreul's elaborate memoir (Acad. R. des Sciences, tom. xvi.) M. Chevreul's experi. ments, however, relate to the action of light simply as it comes from the sun without prismatic separation, and have therefore little or nothing in common with the objects of this paper. it is destroyed, so as neither to reappear on drying, nor to be capable of revival by any means tried. And in all cases long keeping deteriorates the colours and alters the qualities of the alcoholic tinctures themselves, so that they should always be used as fresh as possible. 165. If papers tinged with vegetable colours are intended to be preserved, they must be kept perfectly dry and in darkness. A close tin vessel, the air of which is dried by quicklime (carefully enclosed in double paper bags, well pasted at the edges to prevent the dust escaping), is useful for this purpose. Moisture (as already mentioned, especially assisted by heat) destroys them for the most part rapidly, though some (as the colour of the Senecio splendens) resist obstinately. Their destructibility by this agency, however, seems to bear no distinct relation to their photographic properties. 166. This is also the place to observe that the colour of a flower is by no means always, or usually, that which its expressed juice imparts to white paper. In many cases the tints so imparted have no resemblance to the original hue. Thus, to give only a few instances, the red damask rose of that intense variety of colour, commonly called by florists the Black Rose, gives a dark slate blue, as do also the clove carnation and the black holyoak; a fine dark brown variety of Sparaxis gave a dull olive green; and a beautiful rose-coloured tulip, a dirty blueish green; but perhaps the most striking case of this kind is that of a common sort of red poppy (Papaver Rheum ?), whose expressed juice imparts to paper a rich and most beautiful blue colour, whose elegant properties as a photographic material will be further alluded to hereafter*. 167. This change of colour is probably owing to different causes in different flowers. In some it undoubtedly arises from the escape of carbonic acid, but this as a general cause for the change from red to blue, has, I am aware, been controverted. In some (as is the case with the yellow Ranunculi) it seems to arise from the chemical alteration depending on absorption of oxygen; and in others, especially where the expressed juice coagulates on standing, to a loss of vitality or disorganization of the molecules. The fresh petal of a single flower, merly crushed by rubbing on dry paper, and instantly dried, leaves a stain much more nearly approximating to the original hue. This, for example, is the only way in which the fine blue colour of the common field Veronica can be imparted to paper. Its expressed juice, however, quickly prepared, when laid on with a brush, affords only a dirty neutral grey, and so of many others. But in this way no even tint can be had, which is a first requisite to the experiments now in question, as well as to their application to photographia. 168. To secure this desirable evenness of tint, the following mani A semi-cultivated variety was used, having dark purple spots at the bases of the petals. The common red poppy of the chalk (Papaver hybridum) gives a purple colour much less sensitive and beautiful. + Nicholson's Journal. pulation will generally be found successful. The paper should be moistened at the back by sponging and blotting off. It should then be pinned on a board, the moist side downwards, so that two of its edges (suppose the right-hand and lower ones) shall project a little beyond those of the board. The board being then inclined twenty or thirty degrees to the horizon, the alcoholic tincture (mixed with a very little water, if the petals themselves be not very juicy) is to be applied with a brush in strokes from left to right, taking care not to go over the edges which rest on the board, but to pass clearly over those which project, and observing also to carry the tint from below upwards by quick sweeping strokes, leaving no dry spaces between them, but keeping up a continuity of wet surface. When all is wet, cross them by another set of strokes, from above downwards, so managing the brush as to leave no floating liquid on the paper. It must then be dried as quickly as possible over a stove, or in a current of warm air, avoiding, however, such heat as may injure the tint. The presence of alcohol prevents the solution of the gummy principle, which, when present, gives a smeary surface; but the evenness of tint given by this process results chiefly from that singular intestine movement which always takes place when alcohol is in the act of separation from water by evaporation-a movement which disperses knots and blots in the film of liquid with great energy, and spreads them over the surrounding surface. 169. The action of the spectrum, or of white light, on the colours of flowers and leaves, is extremely various, both as regards its total intensity and the distribution of the active rays over the spectrum. But certain peculiarities in this species of action obtain almost universally. 1st. The action is positive, that is to say, light destroys colour; either totally, or leaving a residual tint, on which it has no further, or a very much slower action. And thus is effected a sort of chromatic analysis, in which two distinct elements of colour are separated, by destroying the one and leaving the other outstanding. The older the paper, or the tincture with which it is stained, the greater is the amount of this residual tint. 2nd. The action of the spectrum is confined, or nearly so, to the region of it occupied by the luminous rays, as contra-distinguished both from the so-called chemical rays, beyond the violet, which act with the chief energy or argentine compounds, but are here for the most part ineffective, on the one hand, and on the other, from the thermic rays beyond the red, which appear to be totally so. Indeed, I have hitherto observed no instance of the extension of this description of photographic action on vegetable colours beyond, or even quite up to, the extreme red. 170. Besides these, 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 comple mentary tint may be referred. For example, yellows tending towards orange are destroyed with more energy by the blue rays; by blues, the red, orange, and yellow rays; purples and pinks by yellow and green rays. 171. These are certainly remarkable and characteristic peculiarities, and must indeed be regarded as separating the luminous rays by a pretty broad line of chemical distinction from the non-luminous; though whether they act as such, or in virtue of some peculiar chemical quality of the heat which accompanies them as heat, is a point which the experiments on guaiacum, above described, seem to leave rather equivocal. In the latter alternative, elements must henceforward recognize differences not simply of intensity, but of quality in heat from different sources; of quality, that is to say, not merely as regards degree of refrangibility or transcalescence, but as regards the strictly chemical changes it is capable of efiecting in ingredients subjected to its influence. 172. As above stated, these peculiarities, at least the first two, obtain almost universally. Exceptions, however, though very rare, do occur, as will be more particularly mentioned hereafter. The third rule is much less general, and is to be interpreted with considerable latitude; but among its exceptions I have been unable to detect any common principle capable of being distinctly enunciated. 173. Lastly, it requires to be expressly mentioned, that the habitudes of the colours, both of the flowers and leaves of plants, with relation either to white light or to the prismatic rays, vary materially with the advance of the season, and perhaps also with the hour of the day at which they are gathered. Generally speaking, so far as I have been able to observe, the earlier flowers of any given species reared in the open air (provided they are well ripened, i. e. the colour fully developed) are more sensitive than those produced even from the same plant, at a late period in its flowering, and have their colours more completely discharged by light. As the end of the flowering period comes on, not only the destruction of the colour by light is slower, but residual tints are left which resist obstinately. Ă very remarkable case of this kind was noticed in Chryseïs californica, the earliest flowers of which exhibited in the photograph of their spectrum a well-insulated round spot, eaten away by red rays almost at its extremity, which spot I never was able to reproduce with later flowers from the same root. Those gathered at the end of its flowering also left a residual yellow of extreme obstinacy, which was by no meens the case with the earlier flowers. 174. It would be waste of time to enumerate all the vegetable tints which I have subjected to experiment, comprising most of the ordinary hardy green and wild flowers of the country. To the rarer and more splendid species which adorn the stoves and greenhouses of Probably, therefore, useful in dyeing. The species is that most commonly cultivated in gardens, with bright yellow petals having orange-coloured bases, |