and which, attracted by the surface of the water, on which it becomes rapidly extended in an extremely thin layer or lamina, it repulses mechanically all the light bodies which float upon this liquid, and consequently repels the piece of camphor itself. This attraction of surface is exercised in the same manner by water on the fixed or volatile oils, and that by preference to the oil which is reputed to emanate from the camphor, in such a manner that the movement of the latter finds itself suddenly arrested by the projection on the water of a single drop of one of these oils.

8. Serullas having discovered that potassium allied to different metals, and placed in small fragments on the water, presented movements altogether similar to those of camphor, sought for the cause of these movements. The potassium by decomposing the water, seizes on its oxygen, and disengages the hydrogen in the form of gas. Now Serullas having observed that the movements of fragments of alloy of potassium on the surface of water, or even when submerged in water, always took place in a contrary direction to the point where he found the hydrogen most freely liberated, concluded therefrom that it was the effluvium of this gas which, encountering resistance, either from the water, or on the part of the air, reacted on the fragment of alloy, and thus impressed the movement upon it. Analogy would lead us to admit that it was in the same manner, by an effluvium from its own substance, that the camphor struck or acted upon the water or the surrounding air, and that this was the cause of its movement on the surface of water; it is in fact a reproduction of the theory of B. Prévost.

9. M. Matteucit admits that it is entirely to the evaporation of the camphor, and its solution in the lamina of water which encircle it, that we ought to attribute the cause of its movement. He has made many experiments in this branch, from which he believes that he may conclude that it is to the currents of the vapours of volatile substances that their rotation is due.

10. We remark that in the experiments made by the philosophers whom we have just quoted, all the bodies susceptible of moving themselves on the surface of water are accounted as having no power to produce this movement except by the means of an emission of a vapour or a gas, according to the opinion of B. Prévost, Serullas, and Matteuci; or by means of an expansion of their substance on water, in virtue of the attraction of surface produced by this liquid, as Carrodori views it. Now, experiment has proved to us that this phenomenon of spontaneous movement may be observed in many circumstances where there is no production of vapour or gas, nor any attraction of surface on the part of the water. found that caustic soda and potash sustained on the surface of the water by pieces of cork, soaked in their solution and dried, moved with a great rapidity; parcels of soap presented the same phenomenon. Soap made from grease and soda possessed this property • Journal de Physique, T. 91, P. 172.

† Annales de Chemie et de Physique, T. 53, page 216.

I have

more particularly; its movements on the surface of water were of great duration, and far longer than those of soap made from oil and soda. I noticed even here a remarkable fact, because it tends to establish the analogy of the cause to which the movement amongst camphor and soap likewise is due. It often happens that a piece of camphor in undergoing these movements on water is suddenly divided into two pieces, which repel each other with great activity at the moment of their separation; now the pieces of soap which move on the surface of water often present the same phenomenon. Camphor makes its movements by sudden jerks or intermissions; but it is not so with soap, and I may even say here that these sudden jerks are a general character of this phenomenon of movements in all the substances which present it. The acids and the neutral salts possess too, but in a more feeble degree, the property of spontaneous movement on water when they are held in suspension. Pieces of cork also, very slightly soaked in concentrated sulphuric acid, move slowly on the water by sudden jerks; if the pieces of cork were completely soaked in the acid, they would be precipitated to the bottom of the vessel. Pieces of cork soaked in a solution of tartaric acid or oxalic acid, and afterwards dried, present the same phenomenon of feeble movement on water. The neutral salts, prepared in the same manner, also present this movement on water, but with still greater feebleness. In general it is the substances which have the greatest affinity for water which are apt to present this phenomenon of motion. Thus the chloride of calcium moves spontaneously on water, whilst the chloride of sodium or marine salt does not move in a sensible manner.

11. The theory of these movements may be established, at least in an empirical manner, by observation of what passes when we make a solution in water of a solid body which is very soluble; of a small morsel of soda, or of caustic potash, for example. By observing with a microscope what takes place during this rapid solution, we see the small fragment of allkali immediately become the centre of a very active centrifugal movement; it projects from all its parts the dissolved substance in the surrounding water, as if it repulsed its solution. Now an observation reported above proves that this repulsion exists effectively. A piece of cork, soaked in an alkaline solution and then dried, being placed on the surface of the water under the microscope, we see the alkaline solution project itself actively in the water, and the piece of cork, unequally impregnated with the alkali in its circumference, moves in the opposite direction to that in which is going on the most active projection of this same solution. This movement of the piece of cork is evidently an effect of recoil produced by the repulsion which the solid alkali that it contains exercises on its own solution. It is very probable that this repulsion is electric, and proceeds from the fact that the solid body which is dissolved possesses a similiar electricity to that possessed by the solution. But from whatever cause it proceeds, the fact of the reciprocal repulsion of soluble bodies and their aqueous solutions

is certain, and it is to this repulsion that we may attribute the movement which takes to the surface of the water all the floating bodies, which are there dissolved; this is what is observed not only in the alkalies, the acids, and the salts, but also in certain gum-resinous substances, such as opium, succotrine aloes, &c. We now come to the movement which takes place of oily or resinous bodies on water. 12. A drop of the essential oil of turpentine or of lavender, thrown on the surface of the water, spreads itself rapidly there, in a lamina so thin, that it reflects the colours of the iris; a second drop does not produce the same effect. The fixed oils present a similar phenomenon when they are not too viscous. Thus, for example, the oil of wild cabbage, which is very viscous, does not spread on the surface of water; but when it is rendered more fluid by purification, which renders it fit for being used for lighting, it spreads itself very well on the surface of the water. This phenomenon does not always take place with the first drop; the second drop preserves its globular form at the surface of the water. Having one of these gross drops of fixed oil floating in the water, I have deposited gently in its centre a drop of the essential oil of turpentine; this gradually pierced through the thickness of the drop of fixed oil, as if it was attracted by the subjacent water, and at the moment of its contact with the water it made a sort of explosion, by projecting circularly its substance on the surface of the water; chasing before it in a circular movement the fixed oil which imprisoned it, and which after this explosion, returned on itself, so as to form itself anew into a drop. The cause of this centrifugal movement is exactly the same as that which has been determined above with relation to soluble bodies. The solution, considered in its latter limits, is, in fact, only the reunion of molecule with molecule of the liquid dissolving and of the body dissolved. We then have the penetration of the dissolving liquid until this end is obtained. But if the dissolving liquid and the body to be dissolved are not miscible, then there will be no penetration of liquid, and the solution only operates at its surface. This is what takes place with oils brought into contact with water. The drop of oil, which by a centrifugal movement, extends itself on the water in a thin and irisated lamina, here presents, in fact, the phenomenon of the solution considered in its latter limits, that is to say, the reunion, side by side, of each molecule of water at the surface, with the molecule of oil. In the action of this superficial and non-penetrating solution, we observe, as in all the solutions which have the property of penetration, a movement of repulsive projection, parting centrally from the body which is being dissolved, by projecting around it its own substance. Here it is each molecule of oil which repels the neighbouring ones, in such a manner that a circular projection is the result of this assemblage of repulsions. If a second drop of fixed oil projected on the water does not produce the same phenomenon as the first, it is because it has saturated the surface of the water; but this saturation of fixed oil does not hinder the subsequent solution of a drop of essential oil. The oil, by thus

rapidly invading the surface of the water, mechanically chases before it the light bodies which float on the surface of this liquid, as we have just seen that the drop of essential oil chases around it the drop of gross fixed oil which imprisoned it. This propulsion of floating bodies is quite mechanical; it is not repulsion in the sense generally attached to that word; the true repulsion exist here only between the molecules of the oil. These observations directly conduct us to the study of the movements of camphor on water.

13. Camphor is an essential oil concreted; it ought then to offer the same phenomena as the essential oils upon its contact with water, with this difference, always: that it is not in the state of an oily liquid that it unites at the surface of the water, but much in the state of oily vapour, which is in reality the same thing. This inanner of viewing this phenomenon conciliates the opinions of Benedict Prévost and of Carrodori, whose opinions represent, each in part, a part of the truth. The oily vapour of camphor becoming united to the surface of the water and undergoing an active molecular repulsion in the action of this superficial solution, extends itself rapidly on the surface of these liquids, by chasing before it all the light and floating bodies which it there encounters. This apparent repulsion, which is in fact only a propulsion, is often extended to a distance of more than thirty millimetres around the piece of camphor. If we project on the water a drop of essential or fixed oil, this oil invades the surface of the water in preference to the vapour of camphor, and the latter instantly ceases its movement. We here have a phenomenon of elective affinity. The surface of the water becomes saturated with the fixed or essential oil, and from that time is no longer susceptible of dissolving the vapour of camphor; it is sufficient even that the air be charged with the odour of an essential oil, or of any odour whatever, to present an obstacle to the movement of camphor or water; and that because of the odorous vapour being dissolved on the surface of water in preference to the vapour of camphor. When the air is highly charged with the vapour of camphor, the movement of the latter on water is equally arrested, and that because the surface of the water saturates itself completely with this vapour. We now see why the movement of camphor is arrested when the vase which contains the water in which it moves is covered. It is necessary that the vapour of camphor dissolved by the surface of the water be able to evaporate freely and promptly, in order to give up its place to a new dissolution.

14. When it happens that a small fragment of camphor, which is moving on the surface of the water, becomes divided into two parcels, these two pieces repel each other with extreme vivacity at the moment of their separation. This effect is due to the expansion, or to the molecular repulsion of the vapour of camphor interposed between these two pieces. We can perfectly conceive, that in the expansion of the vapour of camphor at the surface of the water, it meets with a resistance on the part of this liquid which ought to

induce a reaction against the piece of camphor, and the latter ought consequently to move by the effect of recoil. Now, it is not on this cause alone that the movement of camphor placed on the surface of water depends; the most powerful cause, and the most important in consideration in the production of this phenomenon has hitherto escaped observation. We now proceed to observe the developement of physical phenomena whose existence has been far beyond our suspicion. I shall now resort to experiment.

15. I put a small quantity of water in a watch glass; it amounted to two or three millimetres in height, and I placed on this water a piece of camphor less than half a millemeter in diameter. This piece of camphor, confined in a small space, was not long in gaining the side of the water, and it there remained fixed, agitated solely by a trembling movement. 1 submitted it to the microscope, employing the feeble magnifying power of ten times its diameter, which enabled me to perceive a field of seven or eight millimetres of diametrical extent. I then added to the water a drop of muddy water, holding particles of white clay in suspension. Instantly a curious phenomenon was presented to my observation. The particles of the clay were observed to be precipitated towards the piece of camphor; the current which they formed, having arrived in the neighbourhood of the camphor, then divided into two currents, one of which was directed towards the right and the other towards the left; that is to say, towards the two opposite extremities of the piece of camphor; there they undergo a sudden repulsion, and retreat from each other with a quickness which gradually diminishes. On arriving at three or four millimetres distance from each other, they describe a curve which brings them back into the flowing current, and they are, now, again precipitated towards the camphor which attracts them, and impresses on them an accellerated movement. There is thus established two vortices, directed in inverse directions, and in each of which the terreous particles in suspension undergo a real circulation which operates in an ovoide curve, the smaller end of which is near the camphor, and the larger end at that part opposed to this curve. Thus the two actions of attraction and of repulsion which the terreous particles undergo are not exercised in right lines, but following the two parts opposed to an ovoïde curve more or less elongated. We observe that the repulsion takes place at a small distance from the camphor, in such a manner that the particles of clay are repulsed by it without having touched it. This repulsion takes place at a distance from the camphor which I value as of a millimetre, and sometimes at or of a millimetre at

most.

16. I have rendered these phenomena still more easy to be observed by replacing the terreous particles in suspension by the yellow and flocky precipitate which is formed in the solution of sulphate of iron in water. In that case, in place of adding, as in the preceding case, a drop of muddy water to that contained in the watch glass, I here add a drop of the solution of sulphate of iron, highly