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true, and are sufficient for explaining their appearances." To account for many of the phenomena of Light, philosophers have conjectured, that the unknown something to which they are due has a wave motion, – that the ether pervading all space being set into vibration, or tremor, affects the eye with the sensation of Light. Since this hypothesis explains the largest number of luminous phenomena, it is generally received. However, it must not be forgotten, that we arrive at this hypothesis by reasoning from analogy. If we cause a stretched string to vibrate, its pulsations are communicated to the surrounding air, and the waves thus produced beat upon the auditory membrane, and produce Sound. We know the fact of the existence of the air—the fact of the vibrating cord; and if we place some peculiar arrangements of mobile bodies between the cord and the ear, we prove that the air partakes of the undulations of the string. Upon a fancied analogy, hypothesis creates the ether, and then sets it vibrating to produce an effect on the eye of a similar order to that which the air produces on the ear – that is, undulations in one case give rise to sound, in the other to Light. This is not the place to discuss the entire question of the undulatory hypothesis of Light; but, since attempts have been made to apply it to the phenomena exhibited in breaking up strong chemical affinities by the operation of a radiant force, it appeared necessary to place it clearly before the reader. A most eminent European thinker has written, “ Nothwithstanding all arbitrary suppositions, the phenomena of Light will always constitute a category sui generis, necessarily irreducible to any other : a light will be for ever heterogeneous to a motion or a sound.”
The fact that chloride of silver darkens upon exposure to sunshine, or to daylight, is what we have to examine. We may take this simple phenomenon of change as representing all that are embraced in this Volume, the differences being only in degree. Since this white salt of silver will not darken in the absence of Light, it was reasonable that
the change should be referred to the luminous element; hence, those pictures produced in the camera obscura by the influence of the solar rays, have been called Photographic drawings. When, however, we proceed with our examination and clearly understand all the conditions under which chloride of silver changes colour in the sunshine, we cannot fail to observe the several peculiarities following:
1st. Those rays which give the most Light-the yellow and the orange rays—will not produce change of colour in the chloride of silver.
2nd. Those rays which have the least illuminating power—the blue and violet-produce the greatest change, and in an exceedingly short space of time.
3rd. The rays which pass through certain yellow glasses have no effect on chloride of silver.
4th. The rays which pass through very dark blue glasses rapidly change the colour.
The yellow glasses obstruct scarcely any Light; the blue glasses may be so dark as to admit of the permeation of an exceedingly small quantity.
5th. Where there is no sensation of Light under ordinary circumstances, beyond the violet rays of the spectrum, the chemical change is speedily produced.
Reasoning upon these facts, and some others of a still more striking character, mentioned under their proper divisions, it appeared to M. Berard that “solar Light consisted of three substances” to which severally belonged “the colorific, calorific, and chemical phenomena." This hypothesis did not however receive any support from the philosophers of his time, and the weight of several eminent names was brought in support of the opposite view. In the Historical Section I have directed attention to an experiment by the eminent Dr. Young, and I have printed in the Appendix a letter from the Rev. J. B. Reade, who strongly supports Dr. Young's position.
An attentive consideration of Dr. Young's experiment, as there described, proves no more than this, – that, as in the ordinary refracted spectrum the chemical action is found at its inaximum about the region of the violet rays; so in the interference spectrum, the chemical change is confined to the violet rings.
We must certainly come to the conclusion that the rays which produce the chemical changes under consideration are subject to the laws of refraction and interference like Light. But if they were Light rays, I cannot conceive why, in the yellow, and therefore most luminous rings, no chemical change occurred.
Again, M. E. Becquerel and Professor Stokes have proved that the chemically impressed spectrum—over those spaces which are more especially chemically active-exhibits inactive lines which exactly correspond with the dark lines of that same portion of the spectrum when rendered luminous. This, however, proves no more than that the cause which occasions the absorption of Light along certain lines, does, at the same time, occasion the absorption of the principle to which the chemical agency is due. This view, as will be seen in the sequel, received also the support of M. Arago, who, although most favourably predisposed to urge the theory of undulation, wherever it was possible to do so, did not fail to perceive that the phenomena of Light and chemical action were heterogeneous.
That there may exist some one all-pervading principle - an ether – which may, under different conditions of motion, give rise to effects of a dissimilar character, is a probability which is not denied; it is, however, contended that the facts observed do not support such a conjecture in connection with the chemical changes produced by the solar rays.
The undulatory theory supposes Heat—I refer entirely here to the conditions of the prismatic spectrum—to be the result of a set of vibrations of a certain length and quickness, and the ether thus vibrating is only bent, by the prism, slightly out of its path. Light is the result of
the same ether pulsating to a quicker time, consequently in shorter waves, the refraction being much greater. Chemical action is produced by a system of vibrations smaller and infinitely more rapid; while here the bending of this set of waves — the chemically active ray — is to a much greater angle than either of the others. This is the hypothesis: now take a fact. By means of two prisms, two spectra are formed, each of which produces upon chloride of silver a chemical change from the green ray to some distance beyond the visible violet. Each spectrum is now so arranged, that the inactive yellow and orange rays of one are thrown upon the most active blue and violet rays of the other. The result is, that chemical action is entirely stopped. This may be said to be due to interference; but I must confess I cannot understand upon what principle the action of rays undulating 535 millions of millions of times in a second, and producing Light, can interfere with rays vibrating 727 millions of millions of times in the same period, producing, as experiment proves, chemical change. To support the view, that Light regarded as an undulation produces chemical change, since the chemical cause must reside in- must be — the particular ray and nothing else, it is necessary to prove, that when we obliterate a coloured ray of Light, all chemical action should cease over the space which belongs to such especial ray: and also that when the luminous coloured ray is not obstructed its chemical power should still exist undiminished. Reference to the chapter devoted to the action of absorbent media will show that the blue rays, regarded as the rays to which the maximum chemical effect belongs, may be entirely obliterated without the chemical effect ceasing; and that under other conditions the blue ray may appear clear and intense in the spectrum thrown on the chloride of silver, and yet produce no chemical effect.
After many years of close experimental examination, and an equally long and careful study of the hypotheses
applied in explanation of the phenomena of Light in the first place, and subsequently to the chemical phenomena associated with Light, I cannot bring my mind to adopt the view, which refers the Photographic phenomena to the agent producing the luminous and calorific phenomena of the solar rays. As it respects Light, I am quite ready to bow to the numerous high authorities who support the undulatory hypothesis. Not so, however, with regard to the chemical radiations. Indeed, it must not be forgotten that, with the exception of Dr. Young, who reposes on a solitary experiment, and one or two men of inferior note, no one, who has examined for himself the chemical phenomena, has ever put forward the view in question.*
Careful study of this Work will show the amount of support which the following views receive from experiment:
Light, Heat, and chemical power come to us associated in the sunbeam.
No two of the phenomena produced by these agencies are similar.
They do not obey the same laws of refraction, although they appear to be capable of undergoing the conditions of polarisation, &c.
Material bodies act differently upon them.
A diaphanous body for Light may be perfectly opake for the chemical power, and a medium nearly opake for Light may be quite transparent to the chemical principle. Heat not being the subject under examination, does not require any particular mention; the power that we possess, however, of separating Light and Heat will be described in the Appendix. Regarding, therefore, the chemical principle as something distinct from either Light or Heat, it became necessary to establish some term by which it should be recognised.
In the first edition of these “Researches” the following
* See Appendix, No. II.