206 SOCIETY OF PHYSICS AND NATURAL HISTORY OF GENEVA. should hope this to be the case. The experiment, even if it should not have the romantic character indicated, will probably teach some curious details of the life of these parasites. Almost everywhere the commonest rules of cleanliness are disregarded in the rearing of pigs. Yet pigs are naturally clean animals, avoiding, like dogs and cats, all contact with ordure. Though they burrow in the earth, and in summer wallow in the mud, they abhor the heaps of excrements mixed with straw in and upon which they are frequently kept. A due regard to cleanliness will prevent trichinæ in the pig. In wild boars, of which many are eaten in the country round the Hartz mountains, trichina has never been found. Neither has it been met with in sheep, oxen, or horses. Beef is the safest of all descriptions of meat, as no parasites have ever been discovered in it. They have also never been found in the blood, brain, or heart of those animals in whose striated muscles they love to reside.-British Medical Journal. [Lately, the common ground-worm has been found to be infested by trichinæ, one of the probable sources of the infection of swine.] The interest excited by this case has induced a more careful investigation into the consequences résulting from the imprudent use of hog's flesh, and fatal cases have been recently reported in this country. It had, indeed, been long known among men of science that the trichina was occasionally found incysted in the muscles of man in the United States as well as in other countries, but no case of death resulting from the presence of the worms is known to have been observed till recently. In February, 1864, "an instance of the poisoning of a whole family and the death of one member caused by eating ham" infested with the trichina was observed by Dr. Schnetter in the city of New York.* Dr. L. Krombein has since recorded some cases of a fatal nature noticed by him in the western part of the State. Having been summoned to attend a man and his wife resident in the village of Checktonaga, he found them afflicted apparently with "acute muscular rheumatism of a somewhat peculiar character," and was sustained in his opinion by the concurrent belief of an associate, Dr. Dingler. He subsequently surmised that the symptoms might indicate trichiniasis; and the patients having soon afterwards died, a microscopical examination by Dr. Krombein, assisted by Dr. Homberger, demonstrated the presence of trichinæ both in the incysted and free state." "The specimen of human muscle taken from one of these cases after death, and also the sausages eaten of, were examined by Dr. J. R. Lothrop and Professor George Hadley under the microscope, and the trichina found in both in great numbers. In the muscle the parasite was free, in the sausage incysted." Other members of the family, attacked by the same parasite, were only less unhappy in escaping a fatal end. [The foregoing accounts, though they indicate an alarming cause of disease, point out a ready means by which the evil may be averted, particularly in the great pork marts of this country, namely, inspection by the microscope. It will probably be found that the disease is exceedingly rare, but the assurance which the inspection would give of this fact would be of sufficient importance to warrant its adoption.-J. H.] * American Medical Times, February 20, 1864. EXPERIMENTAL AND THEORETICAL RESEARCHES ON THE FIGURES OF EQUILIBRIUM OF A LIQUID MASS WITHDRAWN FROM THE ACTION OF GRAVITY, &c. BY J. PLATEAU, PROFESSOR AT THE UNIVERSITY OF Ghent, ETJ. From the Memoirs of the Royal Academy of Brussels. INTRODUCTION BY THE SECRETARY OF THE SMITHSONIAN INSTITUTION. [THE interesting investigations of which we commence in this article to give an account consist of a series of parts originally published in the Transactions of the Brussels Academy. A translation of the first three parts was published in Taylor's Scientific Memoirs; the remainder has been translated for this Institution, and the whole will be published in this and the next volume of the Smithsonian Annual Reports. The author has devised an ingenious method by which a liquid may be withdrawn, as it were, from the influence of gravity, and left free to assume the figure or external form which is produced by the interaction of its own molecules. The experiments described in the first and second parts of the series have excited much interest, and have frequently been presented in popular lectures as precise illustrations of the mode of formation of Saturn's ring, and almost conclusive proofs of the truth of the hypothesis of La Place as to the genesis of the solar system. It should, however, be observed that the force in operation in the phenomena of the heavenly bodies and that in the experiments of our author are very different, and can only give rise to accidental similarities, and not to identical results. Gravity, which is operative in the first case, is the most feeble of all known attractions, while its sphere of action is indefinitely great. On the other hand, molecular attraction, which is operative in the second case, is exceedingly energetic, while its sphere of action only extends to the nearest contiguous particles, and becomes imperceptible at sensible distances. The great power exhibited by the earth on heavy bodies, near its surface, arises from the combined effect of an immense number of attracting atoms. We know that the attraction of the whole earth gives to a body near its surface a velocity of 32 feet in a second, and by comparing the masses and distances from the centre of the earth, and a globe of the same density and a foot in diameter, we can easily calculate the velocity the latter would give a small body near its surface. The velocity thus determined is less than that of an inch in a year. From this result we may infer that small liquid masses, possessed of a slight degree of vicidity, would never assume the form of a globule under the mere force of gravitation. On the other hand, the great power of molecular attraction is shown by the energy with which water is drawn into wood and other porous substances. But the difference of the two forces is still more strikingly exhibited by the difference of their spheres of attraction. In the case of gravitation every atom, for example, of the earth attracts every other atom of the whole mass, each conspiring with all the others to produce the spherical form. While under the influence of molecular attraction the atoms of a liquid globule only acts upon the other atoms which are immediately around them; and hence the atoms in the interior of a globule are, as it were, in a neutral condition, attracted equally in every direction. The only atoms, therefore, which are active in producing the globular forms and in giving rise to the phenomena described in this memoir, are those at the surface of the liquid, since these are only attracted on one side, and are, therefore, free to exert their energy towards the mass, and their tendency to bring this into the smallest compass, namely, that of a sphere. According to this view a globule of water may be considered an assemblage of atoms, without attraction, compressed into the spherical form by a contractile film, within which the atoms are enclosed. The amount of contractile force of such a film will depend on the energy of the attraction between the contiguous atoms and the degrees of curvature. To illustrate this, let us suppose a slip of India-rubber to be stretched horizontally between two supports. If to the middle of this we attach a small weight, the slip will sag downwards, and the point to which the weight is attached will descend until there is an equilibrium between the weight and the contractile force. If an additional weight be attached, the descent will be increased until a new equilibrium is attained, and so on, the contractile force will increase with the degree of bending. A similar force is exerted at the free surface of all liquids. If this surface is horizontal, the attraction will be equal in every direction in the horizontal plane; but if at any point we press the surface so as to bend it out of this plane, the contractile force will be called forth, tending to bring the point back into its former position. It is this surface contractile force which causes a small globule of water or mercury, when flattened, to spring back into the spherical form when the compressing force is removed. The more the globule is compressed, or the greater the curvature at the circumference, the greater will be the resistance. Hence, also, the smaller the bubble the greater will be the contractile power of its surfaces, and the more energetically will it assume the spherical form. This is converse of the action of gravity, the tendency of which to produce the globular form will be the greater in proportion to the greater size, and consequently less curvature of the surface. These remarks will enable the reader to comprehend more definitely the nature of the phenomena exhibited in the following paper. J. H.] FIRST SERIES. 1. Liquids, being gifted with an extreme molecular mobility, yield with facility to the action of forces which tend to modify their exterior form. But amongst these forces there is one which predominates so much over the rest that it almost entirely masks their action. This force is gravity; this it is which causes liquids to assume the form of vessels which contain them; and it is this, also, which makes smooth and horizontal the portion of their surface which remains free. We can scarcely recognize, along the contour of this free surface, a slight curve which reveals the action of the combined forces of the attraction of the liquid for itself, and of its adherence for the solid matter of the vessel. It is only by observing very small liquid masses, upon which the relative action of gravity is thus weakened, that we can see the influence of other forces upon the figure of these masses manifested in a very forcible manner. Thus the small drops of liquid, placed upon surfaces which they cannot moisten, assume a spherical form more or less perfect. Leaving these minute quantities, if we wish to observe liquid masses which have freely taken a certain form, we must quit the earth, or rather consider the terrestrial globe itself and the other planets as having been primitively fluid, and having adapted their exterior form to the combined action of gravitation and centrifugal force. Theory then indicates that these masses ought to take the form of spheroids more or less flattened in the direction of their axis of rotation, and observation confirms these deductions of theory. Observation shows us, also, around Saturn, a body of annular form, and theory finds, in the combined actions of gravity and centrifugal force, means of satisfying the equilibrium of that singular form. If, however, we could, by some means, withdraw from the action of gravity one of the liquid masses upon which we have to operate, at the same time leaving it free to be 'acted upon by other forces which might tend to modify its form, and if our process allowed of giving to this mass sufficiently large dimensions, would it not be very curious to see it take a determinate figure, and to see this figure vary in a thousand ways with the forces on which it depends? Now I have succeeded, by an extremely simple means, in submitting to the above conditions a considerable liquid mass. 2. Fat oils are, it is known, less dense than water, and more dense than alcohol. Accordingly, we may make a mixture of water and alcohol having a density precisely equal to that of a given oil-of olive oil, for example. Now, if any quantity of olive oil is introduced into the mixture thus formed, it is evident that the action of gravity upon this mass of oil will be completely annihilated; for, in virtue of the equality of density, the oil will only hold the place of an equal mass of the ambient liquid. On the other hand, the fat oils do not mix with a liquor composed of alcohol and water. The mass of oil must therefore remain suspended and isolated in the midst of the surrounding liquid, and it will be perfectly free to take the exterior form which the forces that may act upon it will give to it. This being supposed, if the molecular attractions of the oil for itself, those of the alcoholic mixture for itself, and those of this mixture for the oil were identical, there would be no reason that the mass of oil left in the midst of the ambient liquid should take spontaneously one form more than another, since, relatively to all the forces acting upon it, it would be exactly in the same position as an equal mass of alcoholic mixture whose place it would occupy. But it is evident that this identity between the different attractive forces does not exist, and that the attraction of the oil for itself greatly exceeds the two others. The mass of oil, therefore, ought to obey this excess of its own attractive forces. We thus come to this conclusion, that our mass of oil may be perfectly assimilated to a liquid mass without weight, suspended freely in space, and submitted to its own proper molecular attractions. Now, it is clear that such a mass must take the spherical form. Well, experiment confirms all this in a complete manner. The mass of oil, whatever its volume, remains, in fact, suspended in the midst of the alcoholic liquid, and takes the form of a perfect sphere. 3. In order to obtain this singular result with facility, it is necessary to take certain precautions, which I will describe. The first concern the formation of the alcoholic mixture. The density of this mixture necessarily varies with the kind of oil which is used. For the olive oil which I employed, and for the purity of which I cannot vouch, the proper mixture marked twenty-two degrees on the areometer of Beaumé. If, therefore, any one wishes to use olive oil, he may always consider the above value as a first approximation, and, by successive attempts, will bring the liquor at length to the exact point which it ought to reach. To accomplish this, a test tube is filled with the liquor, into which a little oil is afterwards poured by means of a long-necked funnel, which reaches about half way down the test tube. The oil, on reaching the liquor, forms a globule, to which a diameter of about two centimetres must be given, and which a little shake will detach from the mouth of the funnel if it does not detach itself. Then, accordingly as this globule falls to the bottom. of the liquor or rises to its surface, we conclude that the quantity of alcohol of the mixture is too great or too small; we therefore add to this a little water or alcohol, taking care to stir it well, and recommence the experiment of the test tube. The same operations are repeated until the globule of oil remains suspended in the liquor, without appearing to have a tendency either to fall or rise. The mixture may then be considered as approaching very nearly the desired point. I say very nearly, for the globule of oil of the test tube, being of small dimensions, has more difficulty in moving in the liquor than spheres of a large diameter, and it may seem to be in equilibrium of density with the surrounding liquid, whilst for a larger volume of oil this equilibrium does not exist. 4. When the alcoholic mixture, which I presuppose to be contained in a large glass flask of the ordinary form, has attained this point of approximation, the next thing is to introduce the mass of oil. For this purpose the long-necked funnel which has been mentioned above must be again used, and this must reach to a certain depth in the liquor contained in the flask. Letting the funnel rest on the neck of the latter, we pour the oil slowly. Then, if the alcoholic mixture is by chance exactly in the requisite proportions, the oil forms, at the extremity of the neck of the funnel, a sphere, the volume of which increases gradually in proportion as we add this last liquid. When the sphere has attained the volume we desire, the neck of the funnel is withdrawn with caution; the sphere which adheres to it rises with it toward the surface of the liquor, and the oil which it still contains is added to the preceding. Lastly, when the sphere has nearly reached the surface of the alcoholic mixture, a little shake detaches it from the funnel. Ordinarily, however, the mixture has not so exactly the desired density. We then see, in general, several successive spheres of oil formed, which, detaching themselves one after another from the mouth of the funnel, fall slowly to the bottom of the flask, or rise to the surface of the alcoholic liquor. In this case all these spheres should, in the first place, be united into one, which is easily done by the following means. We introduce into one of them the end of an iron wire. The adherence which the oil contracts with this metal then allows the sphere in question to be easily conducted in the ambient liquid, and to be led to join with a second sphere. By continuing this treatment, we soon succeed in uniting all. Then, according as the whole sphere shall remain at the bottom or on the surface of the liquor, add cautiously to the latter a certain quantity of water or of alcohol; and, after having corked the flask, we next turn it several times slowly, and so as not to disunite the sphere of oil, until the mixture is well effected, which will take place when we no longer perceive any striæ in the liquor on looking through it at a window. Lastly, the same operation is to be repeated until the sphere of oil is perfectly in equilibrium in the surrounding liquor. 5. If the experiment has been made, as I have supposed, in a flask of the ordinary form, that is to say cylindrical, the mass of oil does not, however, appear exactly spherical; it is widened in the horizontal direction. But this is only an optical illusion, attributable to the form of the flask. The latter, with the liquor which it contains, acts in the manner of a cylindrical lens whose axis * See table of measures at the end of this volume. In order thus to compel two spheres to unite, it does not suffice to put them in contact with one another. They might touch for a long time without mingling into one; one would say that they are enveloped in a resisting pellicule which opposes their union. It is also necessary, therefore, to introduce the extremity of the metallic wire into the second sphere, as if we wished to break the partition which separates the two masses. The union is then effected immediately. I shall revert to these phenomena hereafter. |