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242 sq. centims. inner coating, connected with the terminals of the secondary coil.

This discharge also lasts a second, and is similar to the preceding, except that larger circles are made on the disk by the dissipation of the carbon, and that there are fewer flashes, viz. 71. The total number of spark-holes in these flashes is 123. Thus there are fewer flashes than in the experiments with the platinum points, but the total number of spark-holes is the same in each case. Hence there is, on an average, 1.34 spark to each flash with the points, and 1.73 spark-hole to each flash with the balls.

Experiments have also been made with rotating disks formed of " sensitized”

paper; and interesting results have been obtained

VI. On Polarization by Diffusion of Light.

By J.-L. SORET*.

On the Reflecting-power of Flames. I

PUBLISHED, some months since, a first Notet, on the

occasion of a memoir by M. G.-A. Hirn, in which he put forth the hypothesis that the incandescent solid particles which, according to Davy's theory, produce the brightness of flames, become transparent at the high temperature to which they are raised, and no longer possess any sensible reflecting-power. One of the arguments which he advances in support of his hypothesis consists in the fact that no phenomena of polarization are observed in the light of a flame exposed to the rays of the sun.

I indicated the results I had obtained by causing a pencil of solar light to fall on lampblack, either when deposited on another body, or at the moment of its formation—that is to say, when it is in the state of smoke or of a smoky flame. In this latter case the trace of the pencil of solar rays is perfectly visible: the part of the flamewhich receives the rays appears bluish white, contrasting with the reddish tint of the adjacent parts. If the trace be observed with an analyzer, the light diffused in a direction at right angles to the incident pencil is seen to be completely polarized in the plane of vision, so that the white trace ceases to be visible when the analyzer is turned so as to intercept the rays which are polarized in the plane containing the eye and the pencil of solar light.

* Translated from a separate impression, communicated by the Author, from the Archives des Sciences of the Bibliothèque Unirerselle, July 1874, pp. 1-26. An abstract of this memoir has been communicated to the Paris Academy.

·† “On some Phenomena of Polarization by Diffusion of Light” (Archives, Nov. 18, 1873, vol. xlviii. p. 231), Phil. Mag. S. 4. vol. xlvii. p. 205. I take this opportunity to point out two important misprints in that Note (Archives, pp. 235, 238): Phil. Mag. t.c. p. 208, line 12-13, for horizontal read vertical; p. 209, line 10 froin bottom, for 500 read 90°

When the flame is not smoky and when complete combustion augments its brilliance, I had found, like M. Hirn, that the polarization-phenomena are not sensible; but I expressed some doubts of the necessity of concluding from this the absence of the reflecting-power of the particles at a high temperature ; the dazzling of the eye and the less quantity of these particles in a brilliant flame, in comparison with a smoky. Aame, appeared to me sufficient to account for the facts.

I have since endeavoured to control this way of viewing the subject by concentrating the sunlight much more than 1 had previously done, so as to give more brightness to its trace.

The sunlight, reflected from a silvered mirror, falls upon a good achromatic lens of 72 millims. aperture and 1.5 metre focal length. Lastly, when greater concentration is required, a second lens, much more convergent, is added, near the focus of the first. The flame is then placed at the point where the image of the sun is found.

Working thus with different flames proceeding from carburetted substances, the trace of the sun's rays can be perceived, in most cases, very distinctly, and the usual phenomena of polarization ascertained. When the flame is not too bright, and does not fatigue the eye, the observation is readily made, by aid of a Nicol, with the naked eye; but if the flame is dazzling, there is a great advantage in looking through one or several plates of blue (cobalt) glass. The portions of the flame which do not receive the light of the sun appear then of a purple tint, while the trace of the pencil is clearly distinguished by its blue colour. If observed through the Nicol in the proper position, the blue trace disappears, and the whole flame appears purple.

I have verified these facts in the following cases :—the flame of a wax candle; flanie of ordinary gas from a Bengel burner with a glass chimney, or from a butterfly burner ; flame of illuminating gas strongly carburetted, butterfly burner; flame of a petroleum lamp, and of a moderator lamp with oil.

I finally tried the very brilliant flame obtained by burning illuminating gas strongly carburetted, with the addition of oxygen. With the process of concentration of the sunlight above described, the trace and its polarization are still distinctly observed as long as the oxygen is not too abundant, the brightness being, however, already incomparably more vivid than that of an ordinary flame. By employing more energetic means of concentration that

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is to say, by causing the sunlight, reflected by the large mirror of a siderostat, to pass through an objective of 8 inches aperture, and then through a lens of short focus, I could perceive the trace on this flame of carburetted gas fed by a larger proportion of oxygen than in the preceding case; but when the oxygen became too abundant, the trace was no longer visible which, beside difficulties of observation, may be explained, 1st, by the fact that, the flame having become quite wbite and even bluish, there is no longer any difference of tint between the part which receives the solar rays and the parts which do not receive them, the trace can only manifest itself by a difference of intensity more difficult to perceive; 2nd, by the carbon particles being immediately consumed at the moment of their formation, and consequently the reflecting matter becoming relatively much rarer.

In short, these experiments show that carbon retains its reflecting-power at very elevated temperatures, which temperatures it would nevertheless be difficult to state precisely.

Further, these facts appear to me to have some interest because they confirm, at least for ordinary fames, the theory of Davy, which has recently been strongly contested ; in fact, a pencil of solar light is reflected by diffusion and polarized in precisely the same manner, whether it falls on a very brilliant Hame, or whether it illuminates non-incandescent smoke, in which the presence of carbon particles is incontestable. The Cause of the Illumination of Transparent Bodies and

of Diffusion. In my former publications I have maintained the opinion that the illumination of transparent bodies traversed by a pencil of rays must be attributed to a defect of homogeneity in the medium-a defect consisting most frequently in the dissemination of foreign particles of great tenuity, but may also result from differences of refrangibility in the component parts of the medium, or, in the case of a solid body, from minute voids or fissures. In other terms, illumination is for me only a particular case of diffusion of light.

My learned friend M. Lallemand attributes this phenomenon to the molecules themselves of the transparent body; he regards the illumination as a lateral propagation of the incident luminous motion, caused by the condensation of the æther around each molecule. Thus, for him, a pencil of light traversing a body whieh is transparent, not fluorescent, and absolutely homogeneous, must in general give rise to a trace visible laterally, and the phenomenon must depend essentially on the nature itself of the medium in which it is produced.

M. Lallemand, however, in a recent communication relative to what he calls the illumination of opaque bodies with a dead surface * (that is to say, on diffusion), seems to me to have taken an important step towards my view. In other interesting ob. servations, he has arrived, for a smoked surface, at a result identical with that which I made known in my previous Note, viz. that the light diffused by lampblack is subject, with respect to its polarization, to precisely the same laws as the light enitted by the trace of a pencil of rays traversing a transparent body t.

Now the lampblack covering a slip of glass, for example, is only an agglomeration of very minute particles in juxtaposition. It seems evident that these particles must continue to diffuse the light according to the same laws, but with less intensity, when, instead of being sufficiently abundant to be contiguous and heaped up on one another, they are more scattered and form only a light deposit on the glass, which partially retains its transparency. This is, in fact, confirmed by experiment. No more can this property be refused to the same particles in suspension in a gas (that is, in the state of smoke or flame), or in suspension in a liquid (for instance, water containing a little Indian ink). It must hence be concluded, therefore, that, given a medium destitute by itself of all power of illumination, it will be sufficient to spread in it very thin particles in order to see produced the phenomenon of the lateral propagation of light polarized according to the laws just mentioned. This is an important point which I sought to demonstrate in my previous researches , and of which I shall presently give fresh proofs.

But if, as I think, we are agreed upon this material fact, we again diverge as regards its interpretation, and even upon the cause of the phenomenon. M. Lallemand considers that, in the smoked surface, it is each molecule of carbon, or rather the atmosphere of' æther condensed around each molecule, that determines the propagation of the light in all directions. For myself, I do

Comptes Rendus de l'Académie des Sciences, May 4, 1874, p. 1272. † It is necessary to remark only that with a smoked surface the polarization is not complete-a fact, besides, often met with in the illumination of transparent bodies. I insisted on this point in my previous Note, and shall return to it further on.

I must here mention that MM. de la Prevostaye and Desains in their memoir“ On the Diffusion of Heat” (Annales de Chimie, 1852, vol. xxxiv. p. 215 et seq.), published some results accordant with these laws, but without enunciating them in a complete manner. Sir D. Brewster, in his memoir entitled “On the Polarization of Light by Rough and White Surfaces (Philosophical Magazine, 1863, vol. xxv. p. 344), did not the case of black bodies.

| See Archives des Sciences, 1870, vol. xxxvii. pp. 150 et seqq.

not go so far, but continue to attribute it to the general fact of the reflection produced at the surface of separation between two unequally refracting media. Each particle of carbon, although very minute, is composed of a great number of molecules; it forms a minute body reflecting light, only, as its dimensions are very sinall, there is no annihilation by interference of the rays emitted in directions different from those determined by the ordinary laws of reflection : there is not specular reflection, but diffusion in all directions.

Let us look at some consequences of these two interpretations. Suppose a liquid having no power of illumination; then introduce some very fine particles of a solid body also having no power of illumination. If M. Lallemand's theory is accurate, the liquid, on ceasing to be homogeneous, will not receive the property of being illuminated, since neither of its constituents possesses it; while, according to my view, the trace of a luminous pencil must be marked in the liquid aud present the usual phenomena of polarization, except in the quite exceptional case of the solid and liquid having the same index of refraction. The experiment is hardly realizable absolutely; but an approximation is possible. Water purified to the highest degree attainable, and containing only very few particles in suspension, has very little illumination-power; and calcspar has no sensible capacity of illumination, the trace of a pencil manifesting itself in the interior only by the slight red fluorescence of this body. Now, on suspending finely pulverized spar in water, and tben filtering the liquor to separate the particles of too great a volume, I have found the power of illumination become much superior to that of the water alone*.

On the other hand, if it is the molecules of the body then. selves that produce the diffusion, it seems to me that the intensity of its manifestation should not depend on the refrangibility of the medium in which we operate. A smoked surface, for instance, whether it be in the air or in a more or less refracting liquid, shou

emit sensibly the same quantity of light. Now this is not what takes place; for it is easy to prove that the diffusing-power of lampblack is less intense in a liquid than in the

* I cannot, however, give this experiment as entirely conclusive. The trace becomes incontestably more evident, and the polarization more pronounced ; but the polarization is not complete, and there remains a residue of neutral light analogous to that which would be produced by an action of fluorescence. The colour of this residue is greenish, while the spar has a red fluorescence; besides, the proportion of solid particles is so trifling that it would be difficult to attribute a sensible fluorescence to them. Further, the trace presents very pronounced differences of colour, according to the angle under which it is viewed- a phenomenon manifested in other cases, and to which I purpose, one day, to return.

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