quiry, as we are only looking for the cause of the heat produced in chemical combination, not for the cause of the combination itself. I have no doubt, however, but that the clue to the proper understanding of chemical combination lies, not in any attractions and repulsions of particles, but in the preservation of the balance between the distances of these particles of which I have spoken above. And if we can extend the laws that regulate the phænomena observed among simple bodies to their combinations, we have, I think, gained much. We divest chemically combining particles of all mysterious influences, such as caloric, investing atmospheres of electro-negative and electro-positive fluids, &c., and look on them only as particles of matter influenced by external circumstances, and seeing the cause of their adherence, not in attraction, but in the closeness with which they are placed to each other, and the absence of an equal and opposite movement among other particles which should accompany their separation. (28.) The expansion among the particles of some bodies when combination takes place, such as in the formation of carbonic acid, the explosion of gunpowder, &c., would seem to show that expansion is sometimes accompanied with heat; but this expansion is effected, not between the bodies combining, as between the oxygen and carbon, but between the compound particles, as between those of the carbonic acid. And this expansion is not without its effect-it produces cold; for the heat produced by combination is not so great when the resulting compound is a gas or liquid, as when it is a solid. For instance, when oxygen combines with hydrogen and forms water, the heat produced amounts to 43 units; but when it unites with zinc, the oxide of zine being a solid, the heat amounts to 53 units. When oxygen and phosphorus combine and form a solid compound, the heat evolved is nearly twice as much as when they give rise to a gaseous one. The difference, however, does not, I think, entirely result from the distances between the compound particles of the two oxides of phosphorus being unequal, but, as I have shown (25.), that the amount of expansion between the masses and particles of which they are composed react on each, or determine each other's limits; so if the distance between the particles of the compound, water, be greater than that between the particles of oxide of zinc, may it not cause the particles of the oxygen and hydrogen to be further apart than the particles of the oxygen and zinc, and consequently be productive of less expansion or less heat in other bodies?.

(29.) My theory of the molecular constitution of matter, and assimilation of the expansions and contractions of bodies, with chemical combination, from which I make them only differ in degree, and that in the latter case the contractions take place between different bodies, leads to Berthollet's view of chemical

It

action, viz. that affinity depends on external circumstances. would be needless to quote the many instances by which this idea is verified--instances in which the power to combine between substances is altogether destroyed or heightened, according to the circumstances in which they are placed. It will be found that in proportion as the two opposite effects are provided for, so will bodies combine, just as condensation or expansion can be accomplished, as the reverse is made easy. The objection that, because chemical action changes the properties of bodies, it cannot be a mere mechanical one, does not, I think, hold good. An acid and alkali combining neutralize each other's previous effect, and an innocuous compound results; but their properties are not changed. If we take a certain quantity of steam and of ice, one will scald, the other freeze-one is solid, the other gaseous. When mixed, the compound is innocuous, and neither solid nor gaseous; yet it must be admitted that it is a mechanical mixture.

(30.) I would remark, before summing up, that not only does the approximation of diverse particles, as in chemical combination, assimilate in the effect it produces, the contraction of like particles in the same body, but that, as we noticed in (24.), the amount of expansion or contraction in certain cases depended on ONE of the elements only, so in chemical combinations one element determines the amount of heat or expansion also; for instance, oxygen uniting with several combustibles gives the same amount of heat; the same base always produces the same quantity of heat, no matter how different the body may be with which it unites; these analogies between the conduct of the particles of the same body, and that of particles of a diverse nature, adding to the proof that the theory I advance to account for the heat of chemical combination is the true one.

(31.) I have endeavoured to condense as much as possible into a reasonable limit, but I am afraid I have taken up too much space, I will therefore sum up briefly what I think I have proved.

That there is a balance between the distances of the particles of all matter, these distances being different for different bodies. That to preserve this balance, motion among the particles of one body cannot be effected without a relatively equal and opposite one in some other.

That in contractions and expansions, as the volume gained or lost has a relation to the volume already possessed by the body, an extensive movement among particles far apart compensates a small one among those which are close together.

That, therefore, when particles, though of an opposite nature, come together in chemical combination, the particles of other bodies must expand to supply the opposite effect; and when these chemically combined particles separate in decomposition, a contrary movement, or contraction, or cold is produced.

That these propositions having been proved, show that the heat produced by chemical combination, or the expansion occasioned by it among the particles of bodies, differs in nothing from that occasioned by the contraction of like particles; that in both cases the resulting heat or expansion in other bodies is merely the necessary effect, or rather accompaniment of the contraction going on in the combining or contracting ones.

And that the analogy between the approximating of particles in chemical combination, and that of those of a cooling body, extends also to other particulars.

VIII. On the Cause of the Aberration of Light.

By Professor CHALLIS.

To the Editors of the Philosophical Magazine and Journal.
GENTLEMEN,

Ν

IN page 568 of the Supplementary Number of the Philosophical Magazine for December, the following passage occurs in an extract from the Comptes Rendus of the French Academy for September 29, 1851 :-"Many hypotheses have been proposed to account for the phænomena of aberration in accordance with the doctrine of undulations. Fresnel, in the first instance, and more recently Doppler, Stokes, Challis, and many others, have published memoirs on this important subject; but it does not seem that any of the theories proposed have received the entire assent of physicists. In fact, the want of any definite ideas as to the properties of the luminous æther and its relations to ponderable matter, has rendered it necessary to form hypotheses," &c. As it might be supposed from this statement, that, in common with others, I had attempted to account for the aberration of light on certain hypotheses respecting the æther, I beg permission to say a few words for the purpose of correcting such a misapprehension.

In what I have written on aberration, I have expressly maintained that the phænomenon may be explained by known facts, without making any hypothesis whatever, and independently of any theory of light. As all that is essential in the explanation I have given on these principles admits of being condensed within a very small compass, and as by exhibiting it I shall best attain the object of this Note, I propose to reproduce it here, after making one preliminary remark. The first attempts to explain aberration took account of the course of the ray, and only one point on its course which partakes of the earth's motion, namely, the eye of the spectator. The explanation I have proposed takes account of two such points. This addition, however simple it may appear, removes all the obscurity attaching to the original explanations. The two points selected were the eye of the spectator, or rather the point of the eye through which the

axes of all rays pass, and the point where the course of the ray intersects the wire of the telescope. It would answer the purpose equally well, and would perhaps be more distinct, to select with the latter point the optical centre of the object-glass, through which the axes of all rays incident upon the object-glass from a star necessarily pass.

Let O be the position in space of the optical centre of the object-glass at the instant when light from a star passes through it, and let W be the position in space of the point of the wire on which the same portion of light impinges. As it is a known fact that light near the earth's surface travels through small spaces sensibly in straight lines, the straight line joining O and W is the course of the ray in space. As it is also a known fact that light occupies time in passing from one point of space to another, the optical centre of the object-glass is carried by the earth's motion to some position, during the transit of the light from O to W. Now the instrument to which the telescope is attached necessarily determines the direction of the ray to be the straight line joining the two points and W, through which it is known that the ray has passed. For these are points of the instrument; and that which the instrument performs is, to determine the direction of the line joining these points with reference to certain fixed directions. But the course of the ray in space is from O to W. The angle OW is aberration. The constant of aberration is the ratio of O to OW, that is, the ratio of the earth's velocity to the velocity of light. This ratio is known by the theory of the earth's motion about the sun, and by observations of eclipses of Jupiter's satellites. Thus the angle OW for a given star at a given time may be obtained by calculation and expressed numerically. The same angle may be measured instrumentally. The two values when compared are found to agree as nearly as possible, and thus aberration is accounted for in as complete and satisfactory a manner as can be desired. The explanation is a strict deduction from admitted facts; and the cause assigned for aberration, being a vera causa, admits of no dispute.

At the same time that I maintain the above to be the explanation of aberration, I am fully sensible of the value of an experiment, such as that of M. Fizeau, which appears to demonstrate that the motion of bodies alters the velocity with which light propagates itself in their interior. This fact must necessarily be of great importance with regard to the undulatory theory of light, but appears to me to be in no way inconsistent with the preceding account of aberration.

I am, Gentlemen,

Cambridge Observatory,
Dec 22, 1851.

Your obedient Servant,

J. CHALLIS.

IX. Explanation of an Optical Illusion.

By Sir DAVID BREWSTER, K.H., F.R.S., & V.P.R.S. Edin.*

A

BOUT eleven years ago I received from a correspondent well-instructed in optics, a letter informing me that he had composed an essay "On the seeming anomalies which take place in the vision of persons whose eyes are either long-sighted or short-sighted, and which, though they are most truly surprising, have hitherto, as far as I can find, escaped observation." "The leading fact," he adds, "which gave rise to the composition of this essay is as follows:

"If a silhouette or black profile of the human face (the features of which should be very little prominent) looking from RIGHT TO LEFT be placed against a window, and be viewed by a short-sighted eye through a narrow slit (about the thirtieth of an inch wide) in a sheet of black cardboard placed at some distance (about a foot or so) from the window, while the spectator himself is at about the same distance from the slit, that silhouette will be seen by that short-sighted eye looking from LEft to right." After an elaborate investigation of the progress of the rays before reaching the retina, founded upon the known structure of the eye, he deduces a fundamental proposition, "from which, by means of a train of demonstrations (which he has not given) he shows

"1. If a silhouette is fixed in a window looking from left to right, and a short-sighted spectator stands with one eye shut at some distance (two feet or so) from it, and moves a piece of black cardboard with a smooth edge, at about two-thirds of the way between him and the window, from RIGHT TO LEFT, then as the left edge of that piece of cardboard approaches the silhouette, a second or phantomic silhouette will appear in that left edge looking from RIGHT TO LEFT, so that the two silhouettes will look each other in the face; and this astonishing appearance takes place accordingly."

My correspondent deduces from his train of demonstrations other two results, in one of which the cardboard and the silhouette are made to change places, and are held at greater distances than before. In this case "the phantom silhouette appears to come out behind the real silhouette, and to look in the same direcIn the third case analogous phænomena are seen by longsighted persons, the distances of the card and the silhouette being varied.

Not having seen the demonstrations above referred to, I cannot of course state that they contain an explanation of these apparently very extraordinary phænomena; but upon carefully repeating the experiments, I soon saw that the phantom silhouettes * Communicated by the Author.