platinum not at all), the oxide disappears quickly at the point of contact, and that which is at a greater distance from this point flows quickly towards it, and disappears in like manner. (Herschel.) When Experiments with mercury under solution of nitrate of copper. the positive wire touches the mercury, a current is produced, proceeding from the side on which the negative wire is placed. This current still continues with slight force after both wires have been completely removed: it even becomes gradually stronger, and drives the film which has been formed during electrolization, towards the place at which the positive wire was situated, so that oxide accumulates there while the opposite surface remains bright: at length, the current which proceeds from the negative side becomes very violent, and this spontaneous motion often continues for a long time. If the negative wire be made to touch the mercury for an instant at two opposite points in succession, while the positive wire touches the solution only, and both wires be then removed from the liquid, these two points form centres from which spontaneous currents issue simultaneously. If the positive wire be left in contact with the mercury (under nitrate of copper?) till a film of oxide has been formed, and the current then made to pass through the liquid only, the film of oxide is driven towards the positive wire by a violent current proceeding from the negative wire. If the circuit be broken after a time, the motion continues still longer. If the mercury be agitated during the motion, so as to scatter the coating formed near the positive wire, spiral currents are produced, moving towards every particle of oxide diffused over the surface, the liquid being thrown into a state of tremulous motion, till the film of oxide becomes reunited, and a more uniform motion is produced. (Herschel.) When the mercury is impure, very anomalous appearances are often produced. Fusible metal melted at the bottom of a boiling solution of sugar, exhibits motion with predominant radiation from the positive wire, when the polar wires are made to dip into the solution; when the sugar solution contains phosphoric acid, the mixture exhibits negative rotation like pure mercury. (Herschel.) Thirty globules of mercury, from 10 to 100 grains in weight, placed in a flat glass dish, at the bottom of a solution of one part of sulphate of potash in 2000 parts of water, in which are immersed the polar wires of a battery of 1000 pairs of plates, lengthen themselves out to a greater extent in proportion as they are nearer to the line joining the polar wires -and approach, some to the positive wire, some to the positive pole of the neighbouring globules. No hydrogen gas is evolved, but oxide of mercury moves with violence from the positive to the negative poles of the several globules. The addition of hydrochloric acid to the solution stops the motion. (H. Davy.) Experiments with mercury under salts of ammonia, potash, baryta, strontia, carbonate of potash, sulphate of soda, or nitre. When the polar wires dip into the solution only, the mercury moves with perceptible elongation towards the positive wire. If the positive wire dips into the mercury, that liquid shrinks together slightly at first, then becomes covered with oxide, and spreads itself out. If, on the contrary, the negative wire dips into the mercury, a sudden flattening of the mercury is produced, and a visible current is formed over its surface from the positive towards the negative wire, then right and left back again. towards the positive wire. If the negative wire be now withdrawn from the mercury, the motion becomes stronger for a time, but ceases when the alkali-metal which has been separated from the solution becomes oxidized. The currents are weaker in solutions of the salts than in those of the alkalis, and weakest of all in pure water.-Solutions of salammoniac, chloride of potassium, or chloride of sodium, behave in a similar manner, but with certain differences. (Pfaff.) Experiments with mercury under oil of vitriol or concentrated hydrochloric acid. When the wires dip only into the acid, the mercury moves towards the negative wire, and a perceptible current goes from the negative to the positive wire, and then on both sides back again to the positive wire. When the positive wire touches the mercury, the motion ceases, and the mercury spreads itself out. If the negative wire be dipped into the mercury, that liquid contracts, and exhibits a slower and straighter motion. (Pfaff, Schw. 48, 190; this memoir contains many other remarkable particulars:-comp. Runge, Pogg. 8, 106.) These motions of mercury should be compared with those previously described (pp. 381...384). Development of Heat in the Galvanic Decomposition of Liquids. Development of Heat in the Exciting Cell of the simple Galvanic Circuit. [When an atom of zinc is burnt and enters into combination with an atom of oxygen, about twice as much heat is set free as in the combustion of an atom of hydrogen (p. 292). Now since, according to Faraday's experiments, an atom of metallic oxide does not require for its decomposition a greater quantity of positive and negative electricity than an atom of water, we are led to suppose that in the combustion of an atom of metal, the quantity of negative electricity which combines with the positive electricity of an atom of oxygen is not greater than the corresponding quantity in the combustion of an atom of hydrogen, and that the quantity of positive electricity given up by an atom of oxygen is the same in both cases. That however a much greater quantity of heat is obtained in the combustion of an atom of zinc, may arise either from this -that the heat obtained does not proceed solely from the combination of the two electric fluids, but likewise from heat existing in the zinc in a state of intimate combination, and set free during combustion—or more probably still, perhaps, that of the heat developed by the union of the two electricities, a greater quantity is retained in a state of intimate combination by water than by oxide of zinc. With this is connected the fact, that heat is evolved during the solution of zinc in dilute acids. Now, according to the theory already laid down, this development of heat cannot arise from the combination of the two electricities; for the solution of zinc in acids is attended merely with a transference of negative electricity from the zinc to the hydrogen (p. 342, c). Part of the heat thus evolved doubtless arises from the combination of the sulphuric acid with the oxide of zinc produced; but, on the other hand, a large quantity of heat must be rendered latent from two causes;-first, because the sulphuric acid which combines with the oxide of zinc is separated from its state of intimate union with the water,-and secondly, because the hydrogen gas which escapes renders latent a certain quantity of heat of fluidity. Hence the development of heat must be attributed mainly to the escape of the combined heat either of the zinc or of the water.-A similar evolution of heat likewise takes place when zine is placed in contact with copper, &c. Hence, in simple galvanic circles, a rise of temperature generally takes place, both in the metals and in the liquid. The evolution of heat must be the same for a given quantity of zinc oxidated, whether the oxidation takes place by ordinary or by electro-chemical action. In the latter case, however, it must be observed that the hydrogen receives its negative electricity, not from the zinc but from the copper, the latent electricity of which-i. e. its caloric is decomposed, and thereby diminished in quantity,-and that the liberated positive electricity goes from the copper through the connecting wire to meet the negative electricity of the zinc, and combines with it in the zinc to form heat. The greater therefore the quantity of heat generated in the zinc, the smaller will be the evolution of heat in the cell. The quantity of heat evolved in the cell would therefore increase as the conducting power of the wire diminished, if the oxidation of the zinc could then go on with equal rapidity; but since the oxidation of common zinc is retarded under such circumstances, and that of amalgamated zinc almost wholly arrested, the contrary result is obtained.] If an amalgamated zinc plate and a silver plate platinized by Smee's method (p. 419), are immersed in 2 lb. of dilute sulphuric acid, and produce a current in a thick conducting wire of sufficient force to decompose 9 grains of water in an hour, the temperature of the 2 lb. (= 18432 grains) of liquid rises 47° Fah. in an hour. If from this we deduct the heat developed by the combination of the sulphuric acid with the oxide of zinc produced, there remains 2.1° Fah. for the rise of temperature due to the current. (Joule.)-[18-432 × 2·138707 2. Consequently, one grain of water would be heated 38707.2° Fah. or 21504 C. At the same time, since the current acting for an hour decomposes 9 grains of water, 8 grains of oxygen are transferred to the zinc 21504 = 2688; while, therefore, 1 part of oxygen has been transferred from the hydrogen to the zinc, a quantity of heat has been evolved in the trough sufficient to raise the temperature of 1 part of water by 2688 C or 4838° Fah. Since the two plates were united by a thick wire, which allowed the negative electricity to pass without hindrance from the zinc to the copper, the heat evolved in the wire may be reckoned as nothing. The difference in the quantities of heat evolved in the combination of 1 part of oxygen with zine and with hydrogen respectively is (52903000) 2290. The experiment just described gave 2688: this excess arises from the circumstance mentioned by Joule himself, that a small quantity of zinc was oxidated by ordinary chemical action. In a second experiment, in which the two plates were placed only half an inch instead of an inch apart, the rise of temperature in the liquid was 4.4°,-or, after deducting the heat developed by the combination of oxide of zinc with sulphuric acid, 1.85° Fah. [The greater proximity of the plates probably diminished the ordinary chemical action. Calculation as above gives for every 1 part of oxygen which combines with zinc, a rise of temperature in 1 part of water amounting to 2368 Co., which agrees very nearly with the quantity of heat developed by the oxidation of the zinc.] = [Joule explains the development of heat on totally different principles, but appears not to be aware that in his calculations he has taken the quantity of the current for its intensity-a mistake of frequent occurrence.] Grove's battery gives similar results. When the circuit is closed by a long thin wire, so that the quantity of the current is diminished, the rise of temperature in the liquid likewise becomes less [partly because the chemical action is slackened, partly because the combination of the electric fluids within the wire develops a quantity of heat which would otherwise have been evolved in the troughs]. In Daniell's apparatus, on the other hand (zinc, dilute sulphuric acid, sulphate of copper, and metallic copper, p. 421), a considerable reduction of temperature takes place. (Joule, Phil. Mag. J. 19, 260.) [This deserves attention, inasmuch as heat is evolved during the precipitation of a solution of sulphate of copper by zinc.] Development of Heat in the Troughs of a Trough Battery. When a battery consisting of four porcelain troughs filled with dilute nitric acid-each trough containing 10 cells, having a pair of zinc and copper plates immersed in each, and therefore making 40 pairs in allis left for some time with its poles connected, a rise of temperature takes place in the liquid contained in the cells. In one experiment of this kind, the temperature of the liquid was at first 16.6° C.; in the middle of the trough, containing the positive pole, it rose to 55.5°; in the next to 547; in the third to 53.6; and in the trough which contained the negative pole, only to 43.2°. (Murray, N. Ed. Phil. J. 12, 57.) Development of Heat in the Decomposing Cell. [Watery liquids are the only substances, the decomposition of which has hitherto been observed to be accompanied by elevation of temperature. If we suppose (according to p. 494) that of the heat produced in the combination of oxygen and hydrogen, a great part remains united with the water (2,290 parts out of 5,290), then this quantity of heat must be liberated during the electrolysis of the water. For, at the positive poles, the full quantity of heat combines with one atom of oxygen, and at the negative pole, one atom of hydrogen takes up the full quantity of negative electricity, the heat, which was held in a state of combination by the atom of water decomposed, remains in the liquid in the free state.] Two gold cups containing water are connected by asbestus, and into one of the cups, in which the positive wire of a battery of 100 pairs is immersed, a drop of a solution of sulphate of potash is let fall: the potash then passes rapidly into the negative cup, and the water is raised in two minutes to the boiling point. When the current acts on a solution of nitrate of ammonia, all the water evaporates in three or four minutes, producing a hissing noise and a white cloud, and the remaining nitrate of ammonia takes fire. When one of the cups contains strong solution of potash, and the other oil of vitriol, only a slight elevation of temperature is produced. (H. Davy.) When dilute sulphuric acid is decomposed by Grove's battery, it is converted into oil of vitriol, and becomes so hot, that wood placed under the vessel which contains it is charred. (Grove.) In the electrolysis of a watery liquid, a greater rise of temperature takes place at the positive than at the negative pole,-because less gas is evolved at the former, and therefore less heat is rendered latent. The evolution of heat is greater when the liquid is divided into a number of separate parts by porous bodies, such as membranes, bundles of thread, &c. The liquid in a bundle of cotton fibres is more strongly heated than that contained in a glass tube of equal width and length; for the cells in which the liquid is enclosed retard the transmission of the electricity. If the polar wires are inserted into the extremities of a cutting of a waterplant, the water near the wires rises to the boiling point. (De la Rive.) [Experiments of Prescott Joule (Phil. Mag. J. Î9, 260).] TECHNICAL APPLICATIONS OF GALVANISM. 1. Galvanic Precipitation of a thin Layer of one metal on the surface of another. Gilding. This process may be performed upon silver, brass, or copper, but not upon iron. The apparatus consists of a bladder containing dilute acid, in which zinc is immersed, and a jar within which the bladder is placed. The jar contains the solution of gold, together with the metal to be gilt, which is connected by a wire with the zinc. (The gold solution and the metal to be gilt may also be placed within the bladder,-the dilute acid and a cylindrical zinc plate surrounding the bladder being placed in the outer vessel.)-The more dilute the acid, the feebler is the current, and the better does the gilding go on;-e. g., six drops of acid to a glass of water. Sulphuric acid is used for silver, nitric acid with copper or brass. -The gold solution, which is made as neutral as possible, contains 5 milligrammes of gold in a cubic centimetre, and therefore one gramme of gold in a litre (about 2 pounds). A weaker solution gives a darker, and a solution containing copper mixed with the gold a redder gilding.-The metal to be gilt must be either polished or merely cleaned. In the former case, the metal takes the gilding more readily, and the gilt surface has a much greater lustre, and merely requires rubbing with fine linen or with leather to give it a very high degree of polish; in the latter case, the gilding is taken slowly, has a duller surface, and requires to be rubbed with the burnishing steel. Ignited silver takes a finer gilding than that which has not been ignited. The zinc is attached to a thick copper wire, and this to a silver or platinum wire, which touches at one point the metal to be gilt: this point must however be changed from time to time, otherwise no gold will be deposited upon it. Before the gilding process is commenced, the metal is dipped into dilute acid to free it from all impurities,-silver in sulphuric, copper and brass in nitric acid. If the zinc, contained in a bladder filled with the same acid, be at the same time immersed in the liquid, the gas-bubbles evolved on the surface of the silver or copper will serve to cleanse it still more effectually. After this the gilding is commenced. The bladder with the zinc being first placed in the gold solution, the circuit is closed by immersing the object previously metallically connected with the zinc. The metal to be gilt, especially if it be silver, must not be left for a moment in the gold solution without galvanic connection, otherwise it will either not be gilt at all or the gilding will be very bad. If therefore the inside of a vessel is to be gilt, the bladder with the acid and zinc being suspended within it, the gold solution must be poured into the vessel down the sides of the bladder, so that galvanic connection may be immediately formed. The galvanic current must be so weak that scarcely any gas VOL. I. 2 K |