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water of twenty feet diameter, very soon stops the motion of cam“ phor revolving at the other end. This rapid diffusion even of a fixed oil over a great surface of water, prevents the camphor from extending itself, and stops the solution and rotation of the small particles. It is for this reason, likewise, that particles of sawdust, soaked in fixed oil, begin to turn the moment they touch the water, but cannot continue their motion because the film which they form at the surface of the water is not dissipated in the atmosphere. From this last observation the author deduces this consequence, that volatility and the odorant property are not qualities necessary to produce these rotations. Volatility is merely necessary for their continuation.
At the end of his memoir the author speaks of certain other motions observed in nature, which, by the mechanism of their cause, are in some respect analogous with the motion of oily bodies at the surface of water. In bodies brought near the fire, the humidity retires always to the extremities most remote from the fire itself ; because the vapour which is disengaged from the part most heated repels the rest in the opposite direction. So likewise drops of water thrown on an ignited plate of metal, remain, and are agitated in the form of a sphere; because the vapour which is produced by the contact of the plate agitates them, and does not permit them to touch the metal. By the same principle it is very easy to explain the motions of the tremella described by Adanson and Corti, which had inclined some naturalists to rank this byssus in an intermediate class between plants and animals. The tremella is a mass of very minute fibres, which being suspended in the water must be sensible to the slightest impression. It is at present known that plants when exposed to light, decompose the water in their vessels, of which they seize the hydrogen, and extrude the oxygen in the form of gas, by an operation nearly the reverse of animal respiration. If from one of the fibres of the tremella the gas issues on one side, the fibre itself must bend toward the opposite part. If, as in large trees, the
gas issues from the tremella in the greatest quantity from the side of the plant which is opposite the light, the tremella must be repelled toward the source of the light itself, to which it always tends when included in an opake vessel, into which the light enters through a hole. It is not therefore necessary for the explanation of this fact, either that we should suppose any degree of animality in the plant, or attraction at a distance between the oxygen and the light. In the same manner the gas which is abundantly produced by the light of the day, becomes accumulated between the fibres of the tremella, and floats it to the surface of the water. The author takes occasion to express his acknowledgment to Citizen Fourcroy, who afforded him the means of repeating the experiments in his laboratory, and who, by his excellent lessons, informed him of the facts newly discovered since the communication between his country and France had been unfortunately interrupted.
Additions to the preceding Memoir, in a Letter from the Author to
Čitizen Fourcroy. CITIZEN:
As I understand that an extract of my memoir upon the section of camphor at the surface of water, is intended to be printed, I request you will add the following facts and reflections :
1. Dry camphor is very perceptibly volatilised under the weight of the atmosphere, at the 50th degree of Reaumer (145 Fahrenheit). It melts at the 120th degree (302 Fahrenheit), and its volatilization is then extremely rapid. In the Torricellian vacuum it rises even at the ordinary temperature of the atmosphere. This vapour has very little elasticity. It crystallizes along the sides of the tube.
2. A column of camphor is cut asunder much more speedily at the surface of boiling than of cold water. Camphor upon boiling water sublimes in great abundance with the vapour of the water itself.
3. Camphor, when floating upon water, turns and is dissipated by the contact of the oxygen, carbon, hydrogen, and azotic gases. These two last afforded more rapid movements and a more speedy dissipation. They likewise more readily dissolve carbon, phosphorus, and sulphur.
4. When camphor is burned or heated on a float at the surface of water, if it touch the liquid it gives a considerable motion to its support, but if it do not touch the water it remains motionless. The motion is not therefore produced by the simple emission of volatilized particles from the camphor. An action likewise takes place on the part of the water.
5. This action appears to me to depend on the principle of the motion of bodies which float at the surface of water, and has been explained with remarkable perspicuity by Monge. Of two small pieces of paper, twisted up and moistened, the one with the pure water, and the other with water well saturated with camphor, the first attracts and the second repels camphor on the surface of water which does not actually hold that substance in solution. Water consequently has more attraction for solid camphor, than for the little portion which it has already dissolved to saturation. It mounts along the solid piece, where it forms a curvilinear inclined surface. The small portion which is dissolved and saturated, descends along this inclined surface, and in its descent it repels backwards, by the laws of mechanics, both the surface itself and the solid particle to which it adheres. This separation of the dissolved part accelerates the dissipation of the solid piece, by affording a current of water constantly renewed. The atmosphere absorbs that part of the camphor which is already dissolved and extended on the surface of the water; the evaporation being perhaps assisted by a small portion of the water itself.
6. If a drop of oil had no affinity with the surface of the water
it would lodge itself in a small cavity ; and though more elevated than the surface itself, it would preserve the globular form by its affinity of aggregation. But since it extends in a film over the water, the drop itself, or some of its principles, must necessarily be attracted in the same manner as fluids which rise along the sides of vessels.
7. Air, strongly impregnated with ether, or very hot exhalations of camphor, exercises on the small floating particles at the surface of the water a repulsion similar to that of oil, or of camphor dissolved without heat in the water itself. The former are elastic fluids, and the latter simple liquids. They must not be confounded together.
I have perhaps dwelt too long upon a subject which seems rather to be matter of curiosity, than immediate utility. But it is always a pleasure to me to embrace an opportunity of expressing my esteem and attachment to you.
J. B. VENTURI.
ON VOLTAIC PILES.
(Continued from page 380.) We have stated at page 377, that the discovery of the development of electricity by the simple contact of metals, was made by the Rev. Abraham Bennet, prior to the year 1789. This discovery, however, is subsequent to that of M. Sultzer, by applying two metals to the tougue (see page 363); but prior to Galvani's discovery of the convulsive motions of prepared frogs, by the application of metallic conductors, as stated also in page 363.
Mr. Bennet's experiments were made by means of an instrument called a doubler, which is a modification of the ordinary condenser of Volta. By the employment of this doubler, Mr. Bennet was enabled to ascertain that the simple contact of metals produces electric action. The results of this very ingenious electrician's experiments may be pretty fairly represented by the following table.
The following bodies being brought into contact with each other, and afterwards separated, produced positive and negative electric action, respectively, as below:Positive.
Iron wire. Iron wire, Mr. Bennet calls the electric action which bodies are naturally possessed of, with respect to that of other bodies, their adhesive electricity; and states that those bodies which are marked positive in the above table, have a negative adhesive electricity. His words are
these : “ By the experiments of single contact, it now appears that the adhesive affinity of electricity to lead ore is positive, and to zinc negative.” He adds, also, that “gold, silver, copper, brass, regulus of antimony, bismuth, tutenag, mercury, various kinds of wood, and stone, were tried by this method of single contact, and appeared to cause a positive charge. Tin was negative, and a large piece of zinc much more weakly negative than a thin plate of the same metal used in the preceding experiments.”
After Mr. Bennet's experiments became known, they were repeated by Cavallo and others, even prior to the experiments of Volta on the electricity produced by the simple contact of metals ; but those of the Italian philosopher were made in a very different manner, and gave perfectly unequivocal results. Volta, however, as well as his predecessors in these discoveries, employed his condenser in its simplest form; whilst Bennet and Cavallo had used it in its most complex form of the doubler, whose indications are confessedly not at all times to be depended on. The first set of experiments on record which showed the electric action of metals by simple contact, without the aid of a condenser, were published in Nicholson's Journal for December, 1804. The following is a copy of the author's own account of these experiments, and of the motives which induced him to proceed with them. On the Electricity exhibited by Metals, without the help of any
Condensing Instrument. By Mr. Wm. Wilson. When I set about making the compound electrical condenser described in my last letter to you, I intended to repeat the experiments of Cavallo, relating to the electricity obtained by the contact of metals related in the third volume of the fourth edition of his Treatise of Electricity ; but before the instrument was finished, I was induced (by some experiments I had made relative to the cause of excitation of electricity), to suppose that it is not the contact of the metals that is the cause of the appearance of electrical signs, but the separation of the metals from contact. And this supposition was very much strengthened, when, upon examination, I found that all who have made experiments on this subject have separated the metals from contact before they examined them as to their eleccity.
If the contact of the two metals be the cause of the electrical signs, the whole effect that one metal can have on another will be communicated at the time of contact, however few the points are that form the contact, because both of the metals being conductors of electricity, if one possessed a greater proportion of it than the other, a part will be communicated to the other at the time of contact, to form an equilibrium, and this will be done as well by a few points of contact as by a great many. But if it is the separating them from contact, that is the cause of the electrical sign, the more extensive the contact is the more powerful will the signs be when the metals are separated.
To put this to the test of experiment, I pierced a piece of thin sheet copper full of small holes, just big enough to permit to pass through them two or three particles of blings of another metal at a time, so that almost every particle must be in contact with the copper before it can pass through, and consequently the surface of contact be very great with a comparatively small quantity of metal.
I sifted through this copper sieve some filings of zinc into a tin plate laid on the cap of a gold leaf electrometer, and the gold leaves diverged near an inch with positive electricity, when about half an ounce of filings had been sifted into it. Encouraged by this striking result, I procured sieves and filings of different metals. The results of the trials with them are contained in the following table; where P stands for positive electricity, N for negative, and when it was not strong enough to affect the electrometer, that is denoted by 0.