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This result was afterwards compared with those which have been deduced by calculation from the different hypotheses relative to the aether.
According to the supposition that the aether is entirely free and independent of the motion of bodies, the displacement ought to be null.
According to the hypothesis which considers the aether united to the molecules of matter in such a way as to participate in its motions, calculation gives for the double displacement the value 0-92. Experiment gave a number only half as great, or 0’46.
According to the hypothesis by which the aether is partially carried along, the hypothesis of Fresnel, calculation gives 0-40, that is to say, a number very near to that which was found by experiment; and the difference between the two values would very probably be still less if it had been possible to introduce into the calculation of the velocity of the water a correction which had to be neglected from the want of sufficiently precise data, and which refers to the unequal velocity of the different threads of fluid; by estimating the value of that correction in the most probable manner, it is seen that it tends to augment a little the theoretical value and to approach the value of the observed result.
An experiment similar to that which I have just described had been made previously with air in motion, and I have demonstrated that the motion of the air does not produce any sensible displacement in the fringes. In the circumstances in which that experiment was made, and with a velocity of 25 metres a second, which was that of the motion of the air, it is found that according to the hypothesis by which the aether is considered to be carried along with the bodies, the double displacement ought to be 0'82.
According to the hypothesis of Fresnel, the same displacement ought to be only 0-000465, that is to say, entirely imperceptible. Thus the apparent immobility of the fringe in the experiment made with air in motion is completely in accordance with the theory of Fresnel. It was after having demonstrated this negative fact, and while seeking for an explanation by the different hypotheses relating to the aether in such a way as to satisfy at the same time the phae-nomena of aberration and the experiment of M. Arago, that it appeared to me to be necessary to admit with Fresnel that the motion of a body occasions an alteration in the velocity of light, and that this alteration of velocity is greater or less for different mediums, according to the energy with which those mediums refract light, so that it is considerable in bodies which are strongly refractive and very feeble in those which refract but little, as the air. It follows from this, that if the fringes are not displaced when light traverses air in motion, there should, on the contrary, be a sensible displacement when the experiment is made with water, the index of refraction of which is very much greater than that of air.
An experiment of M. Babinet, mentioned in the ninth volume of the Comptes Rendus, seems to be opposed to the hypothesis of an alteration of velocity in conformity with the law of Fresnel, But
on considering the circumstances of that experiment, I have remarked a cause of compensation ■which must render the effect of the motion imperceptible. This cause consists in the reflexion which the light undergoes in that experiment; in fact it may be demonstrated, that when two rays have a certain difference of course, that difference is changed by the effect of the reflexion upon a mirror in motion. On calculating separately the two effects in the experiment of M. Babinet, it is found that they have values sensibly equal with contrary signs.
This explanation renders still more probable the hypothesis of an alteration of velocity, and an experiment made with water in motion appears to me completely appropriate to decide the question with certainty.
The success of the experiment seems to me to render the adoption of Fresnel’s hypothesis necessary, or at least the law which he found for the expression of the alteration of the velocity of light by the effect of motion of a body; for although that law being found true may be a very strong proof in favour of the hypothesis of which it is only a consequence, perhaps the conception of Fresnel may appear so extraordinary, and in some respects so difficult, to admit, that other proofs and a profound examination on the part of geometricians will still be necessary before adopting it as an expression of the real facts of the case.—Comptes Rendus, Sept. 29, 1851.
ON THE FORMATION OF ANHYDROUS CRYSTALLIZED ALUM.
BY THE PRINCE OF SALM-HORSTMAR.
Alumina, obtained by precipitating ammonia-alum by ammonia and heating the precipitate to redness, was fused with four times its weight of bisulphate of potash; on treatment of the fused mass with water, six-sided tables which did not doubly refract light were left, and on analysis were found to consist of anhydrous alum.—Journ. fur Prakt. Chem. vol. lii. p. 319.
ON THE COMPOSITION OF THE GASES EVOLVED ON THE PRODUCTION OF COKE FROM COAL. BY M. EBELMEN,
The question might arise, whether in the formation of coke from coal in a furnace, the air which enters the furnace gives up its oxygen to the matters which are evolved in the gaseous state, or to the solid carbon; and again, whether the oxygen forms carbonic oxide or carbonic acid. Ebelmen examined the composition of the gases of the coke-ovens at Seraing, and found that more than two-thirds of the hydrogen of the coal is burned, the remainder existing in the evolved gaseous mixture. The quantity of carbonic acid is three times that of the carbonic oxide.—Comptes Rendus, vol. xxxii. p. 92.
Phil Mag. S. 4. No. 14. Suppl. Vol. 2. 2 Q