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tres was substituted", which renders necessary, in order to form the ring well, a less velocity of rotation than with the preceding disc, (the most suitable appears to me to be a little less than two turns in a second.) Now, instead of stopping the movement of the disc at the instant when the ring has attained its greatest development, we must continue to move the handle. The film of oil will then break in a little time, as if the disc had been stopped; but, the latter continuing to revolve in the alcoholic liquor, the portions of that liquor which are in contact with it will themselves assume a rotatory movement, and the centrifugal force which results from it will drive them continually towards the ring, so that the latter will not be able to return upon itself. Now, in these circumstances, we soon see the ring lose its regularity, then divide into several isolated masses, each of which immediately takes the spherical form. Thus the ring, when it cannot preserve its figure on account of the decrease of its centrifugal force, and an obstacle prevents its reforming itself into a single sphere, resolved itself into several isolated |. As soon as the separation begins to take place, the movement of the disc must be stopped. This is not all : one or more of these spheres are then almost always seen to assume, at the instant of their formation, a movement of rotation upon themselves—a movement which constantly takes place in the same direction as that of the ring. Moreover, as the ring, at the instant of its rupture, had still a remainder of velocity, the spheres to which it has given birth tend to fly off at a tangent; but as, on the other side, the disc, turning in the alcoholic liquor, has impressed on this a movement of rotation, the spheres are especially carried along by this last movement, and revolve for some time around the disc. Those which revolve at the same time upon themselves consequently then present the curious spectacle of planets revolving at the same time on themselves and in their orbit. The movement of rotation of these masses is, however, too slow, in relation to their diameter, to cause any sensible flattening. Finally, another very curious effect is also manifested in these circumstances. Besides three or four large spheres into which the ring resolves itself, there are almost always produced one or two very small ones, which may thus be compared to satellites. The experiment which we have just described, presents, as we see, an image in miniature of the formation of the planets, according to the hypothesis of Laplace, by the rupture of the cosmical rings attributable to the condensation of the solar atmosphere. 20. When some oil is introduced into a mixture containing a little excess of alcohol, a phenomenon is observable, which is connected with that of the resolution of the ring into isolated spheres. If the oil be poured in with sufficient rapidity it forms a long cylindrical train, extending from the beak of the funnel to the bottom of the vessel, where the mass gathers. Now, this kind of tail, which connects the mass of oil with the beak of the funnel, remains as long as the oil which forms it has a sufficiently rapid movement of translation—that is to say, as long as we continue to pour; but, as soon as we cease to pour out, and the movement of translation is slackened, the train of oil is instantly resolved into several isolated spherules. 21. The formation of a ring analogous to that of Saturn naturally inspires the desire to carry further the resemblance to the system of that planet, and to seek, whether, by some modification of our experiment, it would not be possible to contrive so that a sphere of oil should remain in the middle of the ring. Now, I have succeeded in producing this effect, by means of a process which I shall proceed to describe; only that this experiment must be regarded mercly as a scientific sport, for the circumstances which give rise to the result have evidently no analogy with those which can have occasioned the configuration of the system of Saturn.

*This substitution is accomplished by detaching the upper end of the axis of the first disc from the large wire which passes through the metallic stopper (§ 8,) and screwing in its place the end of the axis of the new disc.

It is first necessary to be able to give to the disc a considerable velocity of rotation. To do this, we adapt to the upper part of the vessel a system of two pulleys—one small, and fixed on to the prolongation of the axis of the disc at the place of the handle, which is taken away; the other larger, and to the axis of which the same handle is attached. In my apparatus the diameters of the two pulleys are, respectively, 12 and 75 millimetres. In the second place, the diameter of the sphere being always nearly six centimetres, that of the disc should be only two centimetres. Lastly, the disc should not have, as in the |...}} experiments, its centre coinciding with that of the sphere. It should e placed lower, toward the inferior part of the latter. Matters being thus arranged, the handle is turned with a velocity which experience soon enables us to determine. In my apparatus this velocity ought to be about two turns and a half per second, which nearly corresponds to fifteen turns of the disc in the same time. We then see, in general, a ring rapidly formed, which extends itself, leaving in its centre a mass of oil, to which it remains united by a thin pellicle. At the instant when the ring has attained a sufficient development, (and by habit alone can this be correctly learned,) the rotation is suddenly stopped. The pellicle then breaks, the ring remains completely isolated, and the central mass forms into a sphere. We have thus, during some instants, a curious representation of the system of Saturn, except the flattening of the ring. The ring returns rapidly, afterwards, upon itself, and is again united to the central sphere. This experiment does not offer any great difficulties. It requires, however, some skill to succeed perfectly.* 22. In describing (§ 10) the experiment in which the flattening of the sphere is effected by the immediate action of the disc, I have remarked that the movement of the latter should not be continued too long, because the mass of oil then comes to lose its form. Now, if we continue, nevertheless, to turn the handle, with a view to observe the results of this disfigurement, we see manifested new and very capricious effects. The sphere being well centred with relation to the disc, if we give velocities of one turn in six, five or four seconds to the latter, we begin, after seven or eight turns, to see the mass of oil elongate itself horizontally in one direction, taking a form which resembles much an ellipsoid of three axes; and, what is more singular, this kind of ellipsoid is placed in an eccentric manner with relation to the axis of rotation. Figure 4 represents, for a velocity of a turn in

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four seconds, the mass viewed from three different sides; that is to say, from above and in the two lateral directions of the smallest and of the largest horizontal axis: the dotted parts indicate the positions of the disc and of the axis of rotation. The aspect of the mass seen from above shows that it is slightly bentinone direction; but this effect is evidently owing to the resistance of the ambient liquid.

When once the mass has taken this form, it preserves it indefinitely as long as the movement of the disc continues; it continues to revolve eccentrically round the latter, and with a velocity much less than that of this disc. This inferior velocity, I may add, evidently also proceeds from the resistance of the ambient liquid. If a greater velocity is given to the disc without, however, passing a certain limit—if for example, we give it one turn in three seconds, the phenomena are still of the same kind; only the mass is more elongated, the flexure due to the resistance of the ambient liquid is more decided, and the Fig. 5. form is more removed from an ellipsoid. Figure 5 repre- ... --sents the mass viewed on the side, and showing to the eye its greatest length. If the velocity of the disc is increased to a turn in two seconds, the phenomena become less constant and less regular. We should say that there is, for this velocity, a transition from one order of phenomena to another, and that the mass hesitates between the two. In fact, with a velocity still a little greater, namely, about one turn in a second and a half, the phenomena begin again to be regular and constant, but they are different from the first. They are exhibited in all their beauty when the velocity is increased to a turn in a second. The mass then is at first deeply hollowed around the axis, as if the ring was on the point of being developed; and it remains under this form of a circular bourrelet during sixteen to eighteen turns of the disc; we then see it elongate gradually according to a horizontal diameter, but no longer eccentrically, so that, seen from above, it presents an elliptic figure sometimes very perfect, of which the disc occupies the centre, (fig. 6.) This ellipse then lengthens more and more, rather rapidly, and begins to bend

* On communicating this very experiment to the academy, in the sitting of April, 1842, (see the Bulletins,) I stated that it was necessary to vary the velocity of rotation. I have since found that, having adopted a convenient velocity, it was best to io, it uniform.

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Fig. 6. Fig. 7.

by the resistance of the ambient liquid, (fig. 7.) Lastly, on a sudden the mass becomes strongly inflected from both sides, and its form seen from above is then as represented in fig. 8. The mass afterwards preserves this last form in a perfectly fixed manner, as long as the movement of the disc continues. 23. However capricious these phenomena may appear, chance, or accidental causes, have still no part in them. I have repeated a great number of times the experiments detailed above, and the effects have aways been identically the same for the same velocities. After having seen the stable figures which the mass takes in these circumstances, we cannot help making a comparison between these figures and the ellipsoids of three axes of MM. Jacobi and Liouville, ($15,)—ellipsoids which are also always, as the latter of these geometricians has shown, figures of stable equilibrium. Would the identity of the phenomena in the case of universal gravitation and in that of molecular attraction hold good so far? I)oubtless the singular figures which we have just described are not ellipsoids; but their aspect admits of our attributing the difference to the resistance of the ambient liquid, which on one side determines the flexures of which we have spoken, and on the other maintains a permanent inequality of angular velocity between the portions adjoining the disc and the more distant portions. Calculation alone could inform us up to what point the above comparison is well founded; the complete solution of the problem, for the case of molecular attraction, would perhaps not present difficulties so insurmountable as for that of universal attraction.

Fig. 8.

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24. In all the experiments which I have described in this memoir, I have supposed that the oil and the alcoholic mixture were rendered chemically inert with regard to each other, and I have said. (§6) that it was easy in a short space of time to obtain two such liquids. I proceed now to detail the process by means of which this object is attained. We begin by making a mixture of alcohol and distilled water, containing a certain excess of alcohol, so that when submitted to the trial of the test tube (§ 3) it lets the small sphere of oil fall to the bottom rather rapidly. After having formed the mixture in quantity more than sufficient to fill the vessel which is to serve for the experiments, we introduce into this same mixture a quantity of oil about double what is considered necessary for these experiments.” If a flask is not at hand large enough to contain the whole, we divide the masses among several separate flasks: but care must then be taken that each one may contain the same proportions of water, alcohol, and oil. After this we invert these flasks rapidly a great number of times, but without shaking them, until the oil has been divided into spherules of the size of a pin's head; the whole is then left to rest. Then if the alcohol of the mixture is in proper quantity, the spherules should sink with extreme slowness, so as to take about a quarter of an hour for the greater part to collect at the bottom of the flasks. If it is otherwise, water or alcohol is to be added, as may be required; the contents to be mixed by inverting the flasks several times, as above, then left again to settle, and the operation thus to be recommenced until the result is obtained which I have described. When this point is obtained the whole is thrown upon filters, care being taken to cover the funnels containing these last with plates of glass. This precaution is necessary in order to prevent, as much as possible, the evaporation of the alcohol, and for another reason, of which we shall speak hereafter. The alcoholic liquor passes the first through the filters, ordinarily carrying with it a certain number of very minute spherules of oil. When the greater part has thus passed, the spherules become more numerous. What still remains in the first filters, namely, the oil, and a residue of alcoholic liquor, is then thrown into a single filter placed on a new flask. This last filtration takes place much more slowly than the first, on account of the viscosity of the oil. It is considerably accelerated by renewing the filter once or twice during the operation. If the funnel has been covered with sufficient care the oil will collect into a single mass at the bottom of the flask, under a layer of alcoholic liquor. The preceding operations have thus given us the following results: On the one hand, the inert alcoholic mixture, still holding a small excess of alcohol, and containing a certain number of small spherules of oil; on the other hand, the oil equally inert, and covered with a little of this same alcoholic liquid. Now, a second filtration completely clears the first from the spherules which it holds. With respect to the oil, it is extracted from below the alcoholic layer by means of a small siphon, armed with a lateral tube, and received into a dry flask, which is to be perfectly corked. In this manner we have the two liquids separate and inactive, with regard to each other. When it is desired to use them, if we perceive that the alcoholic liquid is a little too dense, we correct it with pure alcohol; and if, on the contrary, there is too little density, we correct it with alcohol at 16 degrees. In this latter case we must not use pure water, because this, when it mixes with the prepared alcoholic liquor, produces in it a cloudiness more or less decided. The various trials I have made relatively to the above process, have led me to ascertain that the two liquids, when they have not been submitted to this preparation, are both modified by their mutual contact. The alcoholic liquid

* It is indispensable to have the two liquids thus in excess, on account of the quantities which are necessarily lost during the different operations which we shall describe, and in the preparation of the experiments.

dissolves some oil, and this in its turn probably dissolves some alcohol. It is especially from the modification which the oil undergoes that its great diminution of relative density results, (§ 6.) Now, when the oil thus modified remains exposed to the air, it passes again gradually to the state of fresh oil, and resumes its former density. It is partly to avoid this that I have recommended the funnels which enclose the filters to be kept constantly covered, and the oil to be kept in a flask perfectly corked. As for the alcoholic mixture, it is evident that this last precaution is equally necessary. 25. Before I conclude, I must forewarn those persons who may wish to repeat my experiments of two effects which sometimes occur, and which cause disturbance in the operations if the experimenter does not know the means of preventing or destroying them. When some oil is introduced into a mixture containing an excess of alcohol, it happens sometimes that the mass which has sunk to the bottom of the vessel contracts adherence with this bottom and spreads itself out more or less on its surface. There is then no means of removing it entire; but the spreading of the adhesion may be prevented by contriving that the bottom of the vessel should be occupied by a layer of a mixture more dense than the oil, (§ 9.) The second effect to which I allude is presented in the inverse case—that is to say, when the sphere of oil, instead of reaching the bottom of the vessel, rises, on the contrary, to the surface of the alcoholic liquor, either because this liquor contains too little alcohol, or on account of a lowering of temperature, or because we have not been able to use prepared oil. When this happens the mass flattens at first, more or less, at the surface of the mixture, as if this last opposed a resistance to it. Then, after some time, it makes its way through, and then presents a portion of plane surface, more or less extended, on the level with that of the alcoholic liquor. But what occasions trouble is, that then, so to speak, it has contracted an adherence with this same surface, from which it is not detached without great difficulty. It is, at first, easy to prevent the production of this effect by pouring on the surface of the liquor a small layer of pure alcohol; and this same means will serve also to destroy the effect in question, if it is already produced. In this latter case we may again invert the vessel with caution. The movement thus imparted to the ambient liquor suffices, ordinarily, to detach the mass of oil, with the exception of a small portion, which almost always remains adhering to the surface. 26. Lastly, I have already mentioned the fact that, after a certain number of experiments, the oil becomes filled with small spherules of alcoholic liquor. Now, reciprocally, the ambient alcoholic liquor is also often sprinkled with a multitude of small spherules of oil. It is scarcely necessary to remark that, when all these spherules have become too numerous, and we desire to restore the liquids to their original transparency, this is easily accomplished by filtrations similar to those of which I have spoken above, (§ 24.) 27. We have been hitherto engaged with the figures assumed by a liquid mass abstracted from the action of gravity and submitted to the attraction of its molecules, either when this mass is at rest, or when a movement of rotation upon itself is imparted to it. Notwithstanding the difference of the laws which the attractive forces follow in this case and in that of the large planetary masses, we have seen produced, on a small scale, a striking representation of the majority of the phenomena of configuration relative to the celestial bodies. In the second part of this investigation we shall submit our liquid masses to new forces, and we shall then see developed a series of phenomena quite as curious but of a different class.

Note.—Professor Faraday, who has repeated many of M. Plateau's remarkable and beautiful experiments, coloured his oil green, for the P. of rendering it more distinctly visible in the spirit, by dissolving in it a little oxide of copper. This, he states, is easily done by heating a little oil with the oxide, and then mingling that with the rest.

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