wise shining and reflective surface of the wetted paper; but this is speedily exchanged for a perfect whiteness, marking by a clear and sharp outline the lateral extent of the calorific rays, and by due gradations of intensity, in a longitudinal direction, their law or scale of distribution both within and without the luminous spectrum."

Supposing, then, that such smoked paper is employed, the thermograph of the spectrum in its most complete state, or rather in that succession of states in which each part comes most characteristically into view, is as is represented in the drawing of this thermic spectrum given in the Frontispiece.

Sir John Herschel details some of the results obtained upon different coloured papers, each of which has its peculiar scale of action. The results, however, obtained upon smoked paper, which are the most satisfactory, are thus explained by this able philosopher.

"The most singular and striking phenomenon exhibited in the thermic spectrum thus visibly impressed, is its want of continuity. It obviously consists of several distinct patches, of which a, B, are the most continuous and intense, but are less distinctly separated, and of which, when the sun is very strong and clear, it is even difficult to trace the separation. The spot y, on the other hand, is round and well insulated; it begins to appear on the paper soon after the ovals, a, ẞ, are fully formed, and when B has assumed a sharply rounded outline. The first symptom of its appearance is the dulling of the wet and shining surface of the paper, which is speedily followed by the appearance of a small round white speck; this continues to increase rapidly in size and whiteness, and at length assumes a definite and perfectly circular outline, within which the paper is entirely white. By degrees the oval and this spot join and run together, forming a white streak deeply indented at the point of junction. It is not till this happens that similar symptoms begin to betray the existence of a still more remote heat spot, d. Indeed, it generally requires another wash of alcohol before

ABSORPTION OF HEAT RAYS.

55

this can be fully brought into evidence. It is, however, perfectly unequivocal, though very much feebler, and rather worse defined than y, with which also it is somewhat better connected than y with B.

(70.) "Of the existence of a much more remote spot," says Sir John Herschel, "I have hardly a doubt, but it is very difficult to obtain a sight of it." This is the spot, situated at about -58. The very accurate measures taken of this spot on several occasions, leave no doubt of its existence, and of the correctness of the remote position assigned to it in the drawing. The great analogy between this arrangement of the heat spots along the axis of the spectrum, and the absorptive action of nitrous gas on the luminous rays, seem, as Sir John Herschel remarks, to point to a gaseous absorption. "The gaseous media through which the rays have reached their point of action, are the atmospheres of the sun and the earth. The effect of the former is beyond our control, unless we could carry our experiments to such a point of delicacy as to operate separately on rays emanating from the centre and borders of the sun's disc. That of the earth's, although it cannot be eliminated any more than in the case of the sun's, may yet be varied to a considerable extent by experiments made at great elevations and under a vertical sun, and compared with others where the sun is more oblique, the situation lower, and the atmospheric pressure of a temporarily high amount. Should it be found that this cause is in reality concerned in the production of the spots, we should see reason to believe that a large portion of solar heat never reaches the earth's surface, and that what is incident on the summits of lofty mountains differs not only in quantity, but also in quality, from what the plains receive."

(71.) Now, these facts render it clear, at least to me, that the calorific rays have an existence independent of the luminous rays of the solar spectrum, and that Heat occupies a larger space than Light in the spectrum. Melloni has shown, by some admirable experiments, that bodies are

not alike transparent to Heat and Light. Black mica, obsidian, and black glass in thin laminæ, although nearly opaque to Light, yet allow a large quantity of radiant heat to pass them, and are called by Melloni, diathermic bodies*; whilst glasses of a green colour, in combination with a layer of water, or a very clear plate of alum, are, on the contrary, called adiathermic†, from their being perfectly opaque for heat, notwithstanding Light permeates them freely. These points appear quite sufficient to establish the distinctions of the two solar radiations, Heat and Light; although it may eventually, by the advance of experimental science, be shown that they are but different states of one power, we are justified in adopting the popular notion, and regarding them as distinct principles having a common origin.

(72.) When such a prismatic spectrum as we have been examining, is thrown upon a paper covered with chloride of silver, or any other of those chemical preparations which are susceptible of change under the influence of the sunbeam, it will be found that there is no correspondence between either the illuminating or the heating power of the spectrum and the chemical phenomena which result. The three spectra represented in the Frontispiece, will sufficiently explain the differences in the general results. Light, Heat, and Chemical Power or Actinism, as the agent supposed to produce these phenomena is called, exhibit three very dissimilar scales of action. They neither agree in their maxima nor minima in the spectrum, their refrangibility or their physical characters. To the especial consideration of the chemical phenomena produced by radiant force, the remainder of this volume is devoted. The discussion of the question now suggested, is reserved until the examination of a large number of striking phenomena has been properly instituted.

* From dia, through, and Sepuòs, warm.

† From a priv. and dialɛpμiròs, transcalescent.

CHAPTER III.

ACTION OF THE SOLAR RAYS ON PREPARATIONS OF SILVER.

(73.) ALTHOUGH the preparations to which our attention must be directed, are those employed as agents for the production of photographic pictures, my object is not to deal with the manipulatory details of the art of photography, any further than they become necessary to the elucidation of the subject before us. Almost every change in the method of applying the argentiferous preparations to the surfaces employed, gives rise to some peculiarity in the action of the prismatic rays upon them. We, therefore, by examining the effect of different modes of preparing the sensitive surface, discover the cause producing increased delicacy in the combination; and to this extent the practice of photography belongs to this Volume. It is thought desirable to examine the action of the spectrum upon the various salts of silver, principally with a view to determine, with as much exactness as possible, the rationale of the chemical changes which take place, and the chemically active state of the rays of the spectrum, viewed in relation to their refrangibility.

(74.) Oxide of silver, prepared by precipitating it in the dark, from a solution of nitrate of silver, by pure potash is, when applied to paper in a pasty state and dried, of a very dark green colour; if it be exposed for a few hours to good sunshine, it passes into a more decided olive colour than characterises it when first prepared by precipitation from the nitrate of silver, and consequently any covered portions vary in colour from those on which the sun's rays fell. Prolonged exposure to the sun's rays, for a week or more, renders this olive colour very much

lighter, and the covered parts are found to be many times darker, than those on which the solar rays have acted directly. In some instances, where the oxide of silver has been spread on paper, I have noticed a very decided whitening process, after a few days' exposure; this evidently arises from the revival of metallic silver. In some experiments I have succeeded in the height of summer, in obtaining so complete a metallic coating that, by passing the paper through a roller, a coating of white silver was obtained.

(75.) The oxide of silver, dissolved in ammonia, is a valuable photographic fluid. One application to paper of a strong solution, forms a tolerably sensitive surface. The ammonia nitrate of silver, which is much used by photographers, may be regarded as essentially the oxide dissolved in ammonia. The most simple and the best mode of proceeding is that introduced by Dr. Alfred Taylor:— Drop ammonia carefully into a solution of nitrate of silver; a copious precipitate falls; still add ammonia until this is entirely redissolved. The spectrum produced on this preparation, is not essentially different from that obtained on the nitrate of silver.

(76.) NITRATE OF SILVER.-This salt in a state of purity, whether solid or in solution in pure distilled water, does not appear to be sensibly affected by Light; but the presence of the smallest portion of organic matter occasions it to blacken, under weak luminous influence. This property induced Sir John Herschel, in bis early photographic experiments, to combine organic matter with the solution of the nitrate of silver, previously to its being applied to paper, and afterwards to introduce into the pores of the paper, salts of silver in combination with the organic acids; but without any remarkable results. The organic combinations have, however, since that time been found of exceeding value in quickening the change of many salts of silver under exposure to sunshine. We have already seen that Count Rumford found the nitrate of