NIEPCE'S COLOURED PICTURES.

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action induced a fine brown surface results. This surface, if it is exposed to coloured radiations, suffers a change in direct relation to the illuminating power of the ray; and hence there results a molecular change, producing a colour of an order corresponding to the colour of the Light.

(589.) M.Niepce, from observing when chloride of sodium (common culinary salt) was used in excess in the bath, that the plate became more susceptible of receiving a yellow colour than any other; and, remembering that this salt has, in a remarkable degree, the property of imparting a yellow colour to flame, was led to believe some curious relation existed between one phenomenon and the other. These he appears to have investigated with much care, and in the main point, the relation between the colour of the flame and the most decided colour obtained upon the plate, appears to be established.

(590.) The bath is prepared with water holding chlorine in solution, and to this is added the salt, which is essential to give prominence to any particular colour. The salts of strontian are well known for their property of imparting a red, or reddish purple, colour to flame. If a plate of silver connected with the battery is passed into a trough containing chlorine and chloride of strontian, and when thus prepared we place upon it a coloured design, we shall perceive, after an exposure for a few minutes to sunshine, that the colours of the image are reproduced upon the plate, but the reds far more decidedly than the others.

(591.) To obtain in brilliancy the other rays of the solar spectrum, other salts are employed.

YELLOWS are obtained by chlorides of uranium, of sodium, or potassium, and by the hypochlorite of soda. Very fine yellows have been obtained, says Niepce, with a bath composed of water slightly acidulated with muriatic acid and a salt of copper.

ORANGE is obtainable by using the chloride of calcium. GREEN is obtained by using boracic acid, the chloride of nickel, and all the salts of copper.

BLUE and INDIGO are secured when the double chloride of copper and ammonia are in the bath.

RED and VIOLET result from the use in the bath of the chloride of strontian and sulphate of copper. The substances which burn with a white flame, such as the chlorides of lime, lead, and antimony, yield no colour by solar action on the plate.

(592.) All the colours of a picture have been produced by preparing a bath composed of the dento-chloride of copper; and this salt, when thrown into the flame of burning alcohol, produces a variegated flame. Niepce says:"If we put a salt of copper in liquid chlorine, we obtain a very sensitive surface by a simple immersion in the bath; but the colourific result is rarely good. I prefer a mixture of equal parts of chloride of copper and of chloride of iron, with three or four pints of water; the chloride of iron has the property of receiving on the prepared plates of silver colourific impressions of a similar order to those obtained when copper is employed, and of yielding many colours; but they are much more feeble, and the yellow always predominates. This agrees with the yellow colour produced in flame by this salt."

(593.) If a bath is formed by uniting with the chlorine water all the substances which give a dominant colour, lively results are obtained. The greatest difficulty arises from our inability to determine the proportions in which the salts should be mixed, one colour frequently obliterating another. Liquid chlorine is necessary; dry chlorine will not produce the same results. Such is an analysis of the researches of M. Niepce de St. Victor, which are in every respect valuable. To the coloured pictures which he has produced by this method he has given the name of HELIOCHROMES.

CHAPTER XIV.

GENERAL SUMMARY, AND DEDUCTIONS FROM THE FACTS

DISCOVERED:

(594.) AT the time (1814) when this work was first published, I felt a strong conviction that, as far as the chemical agency of the solar beam was concerned, the undulatory theory of Light furnished us with no interpretation of the phenomena. The period of ten years which has passed has been, with a few short interruptions only, devoted to the consideration of the question; and I yet feel myself, after many thousands of experiments, the most careful observation of each phenomenon, and the closest possible examination of every section of the subject, unable to surrender my opinion that the theory of vibrations does not explain the action of the solar rays in breaking up the strongest chemical affinities. My views are materially modified, as will be evident, when the reader compares the sequel with what I have formerly written. It is not my intention to discuss the theories of Light with reference to the luminous phenomena of the sunbeam any further than they bear upon the chemical changes to which this volume is devoted.

(595.) In my second chapter I have attempted to give as simply as possible, an "Analysis of the Sunbeam," the results of the inquiries of the most eminent natural philosophers, and my own examination. This analysis leads to the conclusion that neither the Newtonian system of seven primary rays, nor that of Brewster, which reduces these to three, can now be received as correct. At the same time we are not in a position to decide the question,

whether the primary rays of Light are three or ten. We know the Newtonian spectrum-that is, the spectrum as known to Newton and described by him-consisted of seven chromatic bands:-Red, Orange, Yellow, Green, Blue, Indigo and Violet. To these we now add the Extreme Red below the Red, and the Lavender beyond the Violet. Then we have the sets of rays, which are still of higher refrangibility, appearing in Solution of Leaf-Green as Red, in Green Fluor Spar as Purple, in Sulphate of Quinine and Decoction of Horse Chesnut Bark as Blue, and in the Yellow Uranium Glass as Green. These last-mentioned phenomena of colour are referred by Mr. Stokes, — to whose investigations we are deeply indebted for many discoveries in this important branch of physical optics,- to an alteration in the refrangibility of the rays. This view is in perfect accordance with the undulatory theory of Light, in which Mr. Stokes has expressed his firm belief. "To one who regards Light as a subtle and mysterious agent, of which the laws indeed are in a great measure known to us, but respecting the nature of which we are utterly ignorant, the phenomenon (epipolic dispersion) might seem merely to make another striking addition to the modes of decomposition with which we are already acquainted. But in the mind of one who regards the theory of undulation as being for Light what the theory of universal gravitation is for the heavenly bodies, it was calculated to excite a much more lively interest." To one, however, who is not wedded to any hypothesis, who has ever desired to wait for an interpretation, rather than render up his mind to imagination, the view entertained fails to carry conviction. Without rejecting the hypothesis, or substituting another for it, I receive the beautiful series of facts and, examine them in all their bearings on the other phenomena wit which more particularly this volume professes to deal.*

"Some enlightened students imagine that the science acquires a satisfactory rationality by being attached to the fundamental laws of universal

TEMPERATURE OF PRISMATIC RAYS.

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(596.) The temperature of these rays has been made the next subject of consideration, and it has been shown. that the radiations producing HEAT do not coincide with those producing LIGHT. Their points of maximum effects differ widely, and a remarkable extension of Heat rays has been traced over a large space, where there is no probability we shall ever detect Light. The sensations of these two phenomena are dissimilar-the refrangibility of these radiations are different-the physical effects are in every respect unlike each other, therefore it appears scarcely philosophical to group them together as the results of the same cause. It is, important, however, that their points of resemblance should be noticed.

(597.) A remarkable extension of luminous radiations over a space ordinarily regarded as a dark space has been already noticed, and the question with which we have at

mechanics. The emission doctrine, if it means anything, must suppose luminous phenomena to be in analogy with those of ordinary motion; and if the doctrine of undulation means anything, it means that the phenomena of light and sound are alike in their vibratory agitation, and thus the one party likens optics to barology and the other to acoustics. * No one doubted the mechanical nature of the principal effects of gravity and sound long before the progress of rational dynamics admitted of their exact analysis. The application powerfully tended to the perfection of barology and acoustics; but this was precisely because there was nothing forced or hypothetical about it. It is otherwise with optics. Notwithstanding all arbitrary suppositions, the phenomena of light will always constitute a category sui generis necessarily irreducible to any other: a light will for ever be heterogeneous to a motion or a sound. Again, physiological considerations discredit this confusion of ideas, by the characteristics which distinguish the sense of sight from those of hearing, and of touch and pressure. If we could abolish such distinctions as these by gratuitous hypotheses, there is no saying where we might stop in our wanderings. A chemical philosopher might make a type of the senses of taste and smell, and proceed to explain colours and tones by likening them to flavours and scents. It does not require a wilder imagination to do this, than to issue as a supposition, now become classical, that sounds and colours are radically alike. Physicists must then abstain from fancifully connecting the phenomena of light and motion."-AUGUSTE COMTE.

*