Page images
PDF
EPUB

If two decomposing cells with platinum partitions contain two liquids of unequal decomposibility-e. g., water and dilute sulphuric acid-the current passes through them more easily when the positive electricity of the latter is conducted into the liquid of least conducting power, i. e., the water, and the negative electricity into the dilute acid, than in the contrary case. (Matteucci.)

The current of a single pair of zinc and copper in water or dilute sulphuric acid, is completely stopped by three or four decomposing cells with copper partitions, when the decomposing cells are likewise filled with water or dilute sulphuric acid. But if a quantity of nitric acid be added to the latter, sufficient to produce a slight action upon the copper, the current will not be greatly weakened, even by a considerable number of decomposing cells: the copper will be most attacked at that part from which the positive electricity passes from it to the liquid. (Buff.)

The current of one pair of zinc and platinum in dilute sulphuric acid is not perceptibly weakened by three decomposing cells containing dilute sulphuric acid, when the partitions consist of ordinary zinc plates; but when these plates are amalgamated, a considerable diminution of power ensues. Partitions of copper give free passage to the current at first, but stop it completely after a few minutes, probably because they become polarized. On turning one of these copper plates round, the current is reproduced for a short time. (Faraday.)

If the decomposing cells contain dilute sulphuric acid, the current is most impeded when the electrodes and interposed plates consist of platinum; then follows lead, then copper, then tin, then cadmium. Îf a zinc plate enclosed between two moist conductors, be placed between a pair of plates of a voltaic battery, the current is much less weakened than it would be by a copper plate introduced in a similar manner. (De la Rive.) Decomposing cells containing dilute sulphuric acid offer the greatest resistance to the current when the electrodes consist of platinum; then follows copper, then zinc. If, on the contrary, the cells contain ammonia, copper and zinc produce the same amount of resistance.

Volume and Surface of the Liquid.

If the current of a battery of 100 pairs be passed through decomposing cells by means of platinum plates, each of which presents a square inch of surface to the liquid on either side, the strength of the current is the same, whether the partitions are placed at the distance of a foot or of only four lines from each other; but when the thickness of the interposed liquid exceeds a foot, while its section is only one square inch, the current diminishes. Consequently, the loss of the current in its passage through the liquid is almost as nothing [or in other words, the length of the portion of liquid throughout which atomic transposition must be effected, is almost without influence], and the principal loss is sustained in the passage of the electric fluids between the plates and the liquid [or in the decomposition of the liquid, which must take place if the current is to pass]. (De la Rive.)-Faraday likewise found no difference.

The current passes more easily when the partition is nearer to the anode than when it is nearer to the cathode. (Matteucci.)

Even when the interposed platinum plate is nearly a foot long and the polar wires are only three inches distant from it, gas is evolved over the whole surface of the platinum plate. When the decomposing cells

are connected by wires instead of plates, the evolution of gas becomes less, in consequence of the diminished surface. (De la Rive.)

Particular Cases.

When a series of rapidly alternating currents is made to pass, by means of the commutator, from a Daniell's constant battery through two cells filled with dilute sulphuric acid, the following effects are observed, varying according to the nature of the metal of which the electrodes and interposed plate are formed:

With platinum, the alternating current is much less weakened than a current which passes constantly in the same direction (and when Clarke's magneto-electric apparatus is used-if 40 alternating currents follow one another rapidly in a second, and the platinum plates present a large surface, a slight diminution takes place just at first; but when the surfaces, in consequence of the formation of pulverulent platinum, have become more susceptible of oxidation and reduction, the current suffers no further diminution). With copper, the alternating current exhibits nearly the same quantity, and with tin and cadmium exactly the same, as in the case of metallic connection without decomposing cells; whereas the continuous current, under the same circumstances, is more or less weakened. Lead, which checks the continuous current almost as much as platinum, and at the same time becomes covered, on the side by which the positive electricity passes into the liquid, with a white crust [sulphate of lead?], which impedes the passage of the current, and does not appear to be reduced by hydrogen, offers nearly twice as much resistance to the passage of the alternating current, because, by the action of such a current it becomes coated with this white crust on both sides. A similar white crust [nitrate of lead?] is produced in nitric acid diluted with an equal quantity of water.-In solution of sal-ammoniac, platinum weakens the alternating current less than in dilute sulphuric acid; chloride of platinum is alternately formed and decomposed by the hydrogen. (De la Rive.)

If a cracked glass tube, closed at bottom and filled with solution of nitrate of silver, be immersed in a glass vessel filled with water, and one of the polar wires dipped into each vessel, electric action takes place through the crack, and decomposition ensues, though but slowly-oxygen gas being evolved at the positive wire and on the side of the crack towards the negative wire (peroxide of silver being at the same time deposited on the positive wire), and silver being reduced on the negative wire and on the side of the crack towards the positive wire (no evolution of hydrogen takes place). The crack acts like a solid conductor, through which, on account of its narrowness, the decomposition of the liquid or the transference of the liberated elements cannot take place. (Grotthuss, Schw. 28, 315.)

Union of Opposed Batteries.

[To the theory of interposed plates likewise belong the cases in which two galvanic batteries are connected together with their plates in inverse order, so that their currents act in opposition to one another. In this arrangement, the exciting cells of each battery act as decomposing cells to the other. The current of the one battery has to overcome not only the resistance which the liquid in the cells of the other battery offers to

its decomposition, but likewise the resistance arising from the affinity of the electro-positive metal of the second battery for the anions of the liquid. Unless therefore the intensity of one of the currents far exceeds that of the other, little or no electric force appears to result from such combinations. In every case, it is not the quantity but the intensity of the individual currents that determines the result.]

When a battery of four pairs of zinc and copper in salt water, which by itself produces a deflection of 30°, is oppositely connected with a battery of four zinc and copper pairs in water, which gives a deflection of only 6°, the deflection is reduced to nothing. (Marianini.) [The salt water increases the quantity, but not the intensity of the current; hence the result.]

If five zinc and copper pairs A in cells are oppositely connected with five other pairs B, no current shows itself, even though the plates in A are immersed in spring water six times as deep as those in B. If hydrochloric acid be then added to the water in A, no current is at first apparent; but after a time, a current is produced and gradually increases till it produces a deflection of 0-45°, its direction indicating that the pairs in B which are immersed in only a small quantity of water, are more than a match for the pairs in A, although the five pairs A in water acidulated with hydrochloric acid, produce by themselves a deflection of 447°, and the five pairs B in spring water, a deflection of only 14. (Fechner.)

If two batteries of equal numbers of plates, A zinc and copper, B zinc and tin, are oppositely connected-spring water being used in both, A is found to have the advantage: the current even becomes stronger the higher the cells in B are filled with water, a still further increase taking place when hydrochloric acid is added to the water in the cells B. (Fechner.) [The tension of zinc and copper exceeds that of zinc and tin; the cells B are therefore decomposing cells.]

Two series of cups A and B, each consisting of five equal pairs of zinc and copper, are oppositely connected. If the cups in A and B are filled with spring water, a slight deflection is at first produced, bu. soon ceases. On withdrawing a pair from one series, the needle deviates 20°, but soon returns to 0° [in consequence of polarization?] if the circuit be kept closed. The number of pairs in A and B being still five, but the water in A being mixed with of oil of vitriol, A shows a preponderance of 180°,-but this is soon reduced to nothing if the circuit remain closed. If a pair be then withdrawn from apparatus B, and again immersed, a deflection of 50° [from cessation of polarization?] is produced in favour of apparatus A (sometimes no deflection takes place). Even the mere raising and sinking of a pair of plates in B is sufficient to produce this effect, though in a less degree; but the current always ceases after a time, when the circuit is kept closed. If the cells of В contain spring water, and those of A contain water mixed with oil of vitriol, a deviation of 90° is produced at the moment of closing the circuit, but it is soon reduced to nothing. With 1 or 2 parts of oil of vitriol to 100 of water in A, no deflection is produced. With 24 oil of vitriol in A, a slight deflection is produced in favour of B. If, however, in this experiment the water of the cells B be renewed, a deflection of 90° in favour of A is produced on closing the circuit, but it sinks to 0 after a minute. Likewise when from 3 to 20 parts of oil of vitriol are mixed with 100 parts of water in A, there is sometimes no current, sometimes a feeble one which soon ceases, the direction being sometimes in favour of the acid cells, sometimes of the water cells.-If the five cells of B contain spring

water, those of A 100 parts of water with 10 of hydrochloric acid, no current is apparent. When the hydrochloric acid amounts to 15 parts, the deflection is first 0°, then 40° in favour of B, then 0°, and then again a deflection takes place in favour of the water cells: these alternations are several times repeated. With 20 or 25 parts of hydrochloric acid, there is a deflection in favour of B, continuing for some time; with 30 parts of acid, a still stronger deflection in the same direction; but in a second experiment made in the same manner, no current was apparent.-If the cells of B contain spring water, and those of A a mixture of 100 parts of water with from 5 to 20 parts of nitric acid, no current is apparent; with 25 parts of nitric acid, a long continued current of 50 in favour of the water cells; with 30 parts of acid, the same current but weaker.-In most of these experiments, the plates of B may be replaced by platinum wires as soon as the currents have become equal; the current, which may perhaps appear at the beginning, soon ceases. It appears, then, that the deflection is sometimes in favour of the acid cells, sometimes of the water cells a circumstance which probably arises from slight alterations in the surface of the plates-since, when the number of pairs is the same, the currents produced by acid and water have the same intensity. (Schönbein.)

When the liquid is dilute sulphuric acid, six pairs of zinc and copper are about equal in power to nine pairs of zinc and iron. (Poggendorff.)

Imperfect Partitions.

When a partition divides a watery liquid imperfectly, less gas is evolved on its two surfaces than upon the two electrodes, because a portion of the current, instead of first passing into the partition, goes round it through the liquid from one electrode to the other. When the two ends of the galvanometer are immersed in the liquid on which the electric current is acting, a current is produced in the liquid itself. This current is strongest when the ends of the galvanometer are situated in the straight line between the electrodes, and stronger when they are nearer to one electrode or the other, than when they are more in the middle of the liquid. But the current in the galvanometer likewise shows itself when the two ends are immersed at a considerable distance from the straight line between the two electrodes,—almost throughout the whole liquid, if it be two feet in diameter-and at a greater distance from the straight line between the two electrodes, in proportion as the liquid is a less perfect conductor. (De la Rive.)

[The transposition of atoms in the liquid proceeds partly from one electrode to the other round the imperfect partition, whether it be a plate or a wire; and partly from the two electrodes to the partition itself, whose latent electricity it decomposes, causing various products of decomposition to make their appearance on its surface. When the ends of the galvanometer are immersed, the latent electricity in the wire is decomposed, and produces a current. The less easily the liquid conducts, the greater appears to be the distance between the lines in the direction of which the transposition of atoms may take place.]

Movements of Mercury in the circuit of the Voltaic Battery.

When mercury is put into a U-tube half an inch wide, in quantity sufficient nearly to fill it-water poured on the surface of the mercury in

both arms-and the golden polar wires of a voltaic battery dipped into the water, the surface of the mercury nearest the negative pole becomes covered with oxide, and remains tranquil; but the surface connected with the positive pole evolves hydrogen gas and produces a disturbance in the water, so that light powders, such as sawdust, or small laminæ of mica, move up and down in it, at first to the height of an inch, afterwards to that of a line,-forming a circular zone when the positive arm is placed upright, and when it is placed in a slanting position, collecting on the opposite side and rotating on their axes in the midst of the water.-If the positive wire be made to touch the mercury, the powder immediately moves towards the point of contact, and attaches itself to the wire; if the wire be separated from the mercury, the powder flies towards the sides of the tube and resumes its former motion.-If, while the two polar wires are immersed in the water, a third wire not connected with the battery be dipped into the water of the positive arm, it pushes the particles of the powder aside, and modifies their motion, which ceases entirely as soon as the third wire is brought into contact with the mercury. (Gerboin, Ann. Chim. 41, 196; also Gilb. 11, 340.)

The same experiment with a U-tube, a quarter of an inch diameter, the columns of water and mercury being two inches high:-The mercury in the negative arm becomes covered with oxide and remains tranquil: that in the positive arm rises, whilst the water insinuates itself by sudden starts continually deeper and deeper between the mercury and the sides of the tube, and then mounts up again. Powder introduced into the water of the positive arm moves in vortices, the motion increasing as the water sinks to a greater depth between the mercury and the surface of the glass. When the circuit is interrupted, the mercury sinks to its former level and again comes in contact with the sides of the tube. If the tube be smeared internally with fat or lycopodium, the water does not sink between the mercury and the glass, and no motion is apparent. If the positive wire be placed at the edge of the tube at a distance of only a quarter of a line from the mercury, that liquid rises up on it and leaves it again, and thus gives rise to continued oscillations in both arins. (Erman.)

If a globule of mercury two or three lines in diameter, be covered with a small quantity of water, and the two polar wires dipped into the latter, in such a manner that the positive wire may be within a short distance of the mercury, that liquid extends itself out in a line till it touches the positive wire-then starts quickly back and becomes rounded on the surface then extends itself in the direction at right angles to that of the former extension-again becomes round-then again extended-and at length comes in contact with the wire, &c. &c. These motions take place so rapidly that nothing is seen but a shining star, and go on as long as the current continues. When the negative, instead of the positive wire is brought near the mercury, no motion ensues, because the mercury becomes covered with oxide. (Hellwig & Ermau, Pogg. 32, 289.)

If mercury be covered with a thin film of water, and the lower surface of a round plate of iron placed in contact with the water, so as to adhere to it, the plate being suspended from one arm of a balance, while the other arm is loaded with a counterpoise just sufficient to cause the water to rise in a cylinder under the plate-and the mercury be then connected with the negative pole of a battery of 100 pairs, and the iron plate with the positive pole,-the water which has been elevated in the cylinder spreads itself with a jerk over the whole surface of the mercury, and pulls the plate down with it; but the plate immediately rises again to

« PreviousContinue »