from the acid, when not too dilute, and thus reducing it to the state of nitrous acid and nitric oxide. (Faraday.)

When the current of a small cup-apparatus of sixteen pairs is sent through nitric acid of 1.49 sp. gr. the negative platinum wire evolves hydrogen gas for a moment,-after which, all evolution of gas ceases, and nitrous acid is produced. The more dilute the acid, the longer does the evolution of gas continue; in a mixture of one measure of acid and one of water, it lasts for half a minute; in a mixture of one measure of acid and two of water, it goes on uninterruptedly. The thicker the wire, the sooner does the evolution of gas terminate. By the following methods, platinum wire may be brought into a peculiar condition, in which, if it be placed in an acid of such a strength that the evolution of gas would cease after a while, it will evolve no gas even from the beginning:-1. By placing the two polar wires in immediate contact, dipping them thus united into the acid, and then suddenly separating them. (In this case, however, the acid must be diluted with somewhat less than one measure of water.)-2. By igniting the negative wire, and immersing it after the positive wire.3. By communication: If the negative wire, after it has ceased to cause evolution of gas in the acid, be joined outside the liquid with another platinum wire, then the latter wire immersed and the former withdrawn, the second wire will evolve no gas either at first or afterwards; moreover, the same property may be communicated from this to a third wireand so on.-A wire which has lost the power of liberating hydrogen gas recovers it by exposure to the air. The time of exposure must be longer as the strength of the acid is greater. In the case of acid diluted with an equal bulk of water, momentary exposure to the air is sufficient: in this case, it is likewise sufficient to interrupt the current for a moment. Sometimes, again, the same effect is produced by agitating the wire in the acid.

In these experiments, the platinum wire perhaps favours the combination of hydrogen with the oxygen of the nitric acid, in the same manner as it acts in detonating gas.-With a stronger electric current, the evolution of gas at the cathode takes place uninterruptedly, because the water is decomposed too rapidly to allow the preceding effects to take place. (Schönbein.)

Iron used as an anode in dilute nitric acid behaves in the same way as in phosphoric or sulphuric acid, retaining its passive condition with even greater facility. (Schönbein.)

When platinum wire is used as the cathode and active iron wire as the anode, in a small cup-apparatus, the latter being first immersed in nitric acid of 1.35 sp. gr., and then the circuit closed, the iron continues to dissolve and evolves no oxygen gas. If, on the contrary, the circuit be closed in such a manner that the iron wire is last immersed, the iron becomes passive and evolves oxygen gas just as platinum does, while nitrous acid is formed at the cathode. If the acid be diluted with from 20 to 400 measures of water, then under the same circumstances two measures of hydrogen gas are evolved at the surface of the platinum for every measure of oxygen evolved at that of the iron. If, however, the two wires be made touch within the liquid, the evolution of oxygen ceases and does not recommence when they are separated. In a mixture of one measure of acid and 10 of water, the evolution of oxygen on the iron likewise ceases on taking the platinum wire out of the acid for a few seconds, and then dipping it in again. It is only when the acid is very dilute, that evolution of oxygen recommences some time after the closing of the cir

cuit. When the negative platinum wire which liberates hydrogen gas from dilute nitric acid, is replaced by an iron wire, no further evolution of hydrogen takes place. Iron which has been rendered passive by nitric acid in any way whatever remains passive when used as an anode, even in the most dilute acid, provided that the circuit be finally closed by the iron; (if, on the contrary, it is immersed in the acid before all the other parts of the circuit are connected, it dissolves continuously.) The existence of this passive condition in nitric acid, of a degree of dilution at which it easily dissolves peroxide of iron, is unfavourable to the supposition that the peculiar condition of passive iron arises from the formation of a film of oxide. [By the action of the current the iron is immediately surrounded with concentrated acid.] While the current continues, no gradual solution of the passive iron (such as is asserted by Faraday) can be perceived, even after several hours; but as soon as the current is interrupted, the wire begins to dissolve: hence the passive state is directly produced by the current [which causes concentrated acid to accumulate round the iron].-When dilute nitric acid is disposed in a layer above solution of potash in any vessel, and the positive iron wire is dipped through the acid into the potash, and then the platinum cathode immersed, the portion of iron wire which dips into the potash becomes oxidated, as well as that which is immersed in the acid. (Schönbein.)

When iron is used as the anode of a battery of 20 pairs in nitric acid of specific gravity from 1.47 to 1.5, it liberates oxygen but gradually dissolves, even when completely immersed in the acid, and connected by means of a platinum wire with the positive pole of the battery. If the circuit be then unclosed, the iron becomes covered with a black coating of insoluble oxide. Passive iron used as the anode in a mixture of nitric and sulphuric acid liberates oxygen gas. (Andrews.)

The gas evolved from nitric acid, either concentrated or moderately diluted, at the iron anode of a strong battery, is not oxygen but nitric oxide; that evolved in a similar manner from acid 30 times diluted, is nitrous oxide. From this it follows that the iron is oxidized by the nitric acid, although it retains its metallic lustre. (Buff.)

Tin used as an anode in strong nitric acid remains permanently passive, and stops the passage of the current more effectually than iron or bismuth. (Andrews.)-According to Buff, on the contrary, tin exhibits the same behaviour as that which he observed in the case of iron.

Zinc used as an anode dissolves in strong nitric acid, but much more slowly than when the current ceases. (Andrews.)

Bismuth when it forms the anode is attacked by nitric acid as strongly as when not placed in the circuit. (Schönbein.)-When the current proceeds from two pairs of amalgamated zinc and platinum, and the platinum capsule containing nitric acid of sp. gr. 1.4 is connected with the negative pole, the solution of the bismuth immediately ceases, and on breaking the circuit, the metal is found to have been rendered passive. If, on the contrary, the current is excited by a battery of 20 pairs, the bismuth dissolves continually though slowly, and seldom appears passive after the circuit is broken. (Andrews.)

When copper wire forms the anode in a mixture of nitric and sulphuric acid, the galvanometer after the first instant shows but a very feeble current-the decomposition of the liquid ceases entirely-the copper wire liberates no gas-is not attacked-and when the circuit is broken it is no longer soluble in the nitro-sulphuric acid (in which likewise active copper dissolves but slowly); at the same time, it

appears bright on the surface. It appears then that the copper becomes covered with an insulating film. (Grove, Phil. Mag. J. 15, 392; abstr. Pogg. 49, 600.)

Fused Arsenic acid containing water neither conducts nor suffers decomposition. (Faraday.) Aqueous solution of arsenic is rapidly decomposed, with abundant deposition of arsenic at the cathode. Aqueous Arsenious acid is decomposed in the same manner, but more slowly. (Bischof, Kastner. Archiv. 6, 438.) [Was not arseniuretted hydrogen evolved at the same time?]

Aqueous solutions of Hydrogen-acids.

These acids evolve hydrogen at the negative pole and the radical at the positive pole: the latter often combines with the metallic electrode, but is set free when graphite is used as the anode.

Hydriodic acid, whether concentrated or dilute, yields the same quantity of hydrogen gas at the negative pole; and this quantity corresponds to that collected in the voltameter interposed in the same circuit. But the concentrated acid gives only iodine at the positive pole; the dilute acid gives less iodine, but oxygen gas with it. In the former case, hydriodic acid alone is decomposed; in the latter, less of the acid but water besides. (Faraday.) The more dilute the acid and the stronger the current, the greater is the quantity of oxygen gas obtained. (Matteucci.)

Aqueous Hydrobromic acid is resolved into hydrogen at the negative pole, and bromine, which dissolves, at the positive pole.

Concentrated Hydrochloric acid, or a mixture of it with at most 8 measures of water, evolves hydrogen at the negative and chlorine at the positive pole; a small portion of the chlorine combines with the platinum of the anode. A mixture of 1 measure of strong hydrochloric acid with 9 measures of water evolves a little oxygen together with the chlorine; and from a mixture of 1 measure of strong acid with 100 measures of water, 17 measures of oxygen gas are evolved for every 64 measures of hydrogen; consequently, 30 measures of chlorine must be set set free at the same time. In this case, the quantity of hydrogen is constant, whatever may be the degree of dilution of the acid. Hydrochloric acid seems to be more inclined to decomposition than water, since it is only when the acid is very much diluted that water is decomposed at the same time. (Faraday.)-Greater intensity of the electric current likewise increases the quantity of oxygen gas. (Matteucci.)-When the anode consists of gold, silver, copper, iron, &c., a chloride of the metal is formed-and, unless it be chloride of silver, dissolves in the surrounding liquid.

Passive iron forming the anode in aqueous solutions of hydracids, such as hydrochloric acid, is always dissolved, in whatever manner the circuit may be closed. (Schönbein.)

Anhydrous Hydrofluoric acid yields hydrogen gas at the negative pole, whilst the positive platinum wire is corroded by the formation of a brown substance, probably fluoride of platinum. (H. Davy.)—The hydrated acid is not decomposed, only the water with which it is united suffering decomposition. (Faraday.)

Hydrated Hydrocyanic acid gives passage to the current with difficulty, more readily however when a small quantity of sulphuric acid is added to it. In both cases, the normal quantity of hydrogen gas is

obtained at the negative pole, whilst cyanogen dissolves in the liquid at the positive pole-as was formerly shown by Gay-Lussac. (Faraday.)

Hydrated Sulphocyanic acid and Ferrocyanic acid behave like hydrocyanic acid. (Faraday.)-Sulphocyanic acid accumulates undecomposed at the positive pole. (Porret.)

According to Faraday's and Matteucci's hypothesis, the hydracids are directly decomposed; but according to Connell, it is only the water mixed with them that suffers direct decomposition, the oxygen separated at the anode combining with the hydrogen of the acid which it there meets with, and setting the radical free. But the experiments on which he founds his opinion are by no means satisfactory.

Metallic Sulphurets, Iodides, Bromides, Chlorides, Cyanides, Sulphocyanides and Ferrocyanides.

Fused Liver of Sulphur yields potassium at the cathode. (Jaquin, Gilb. 28, 338.) The yellow solution of Sulphuret of Potassium (p. 375) yields a quantity of sulphur at the anode and hydrogen gas at the cathode. (Faraday.)-Fused Sulphuret of Silver is decomposed, to a slight extent, into sulphur at the positive and silver at the negative pole.

Fused Iodide of Potassium, or Iodide of Lead, is resolved into iodine at the anode and metal at the cathode.-Aqueous solution of iodide of potassium yields iodine at the anode, potash and hydrogen gas at the cathode, water being decomposed by the potassium there separated. (Faraday.)-Fused Chloride of Lead yields lead at the cathode, chlorine at the anode: when the latter consists of graphite, chlorine is set free; but when it consists of platinum, part of the chlorine combines with the platinum, and chloride of platinum becomes mixed with the chloride of lead. (Faraday.)-Fused Protochloride of Tin is resolved into metallic tin and bichloride of tin, the latter escaping in vapour. (Faraday.)-Fused Chloride of Silver is resolved into silver and chlorine: when the electrodes are formed of silver, the anode loses as much silver as the cathode gains, and no chlorine is evolved. (Faraday.)-Fused Protochloride of Mercury conducts, and appears to be decomposed; disturbing causes are however present. (Faraday.)-Fused Terchloride of Antimony conducts badly and is but little decomposed, perhaps only on account of the presence of a small quantity of water. (Faraday.)

Aqueous solution of Sal-ammoniac is resolved into chlorine at the positive and hydrogen gas and ammonia at the negative pole. [This accords with the ammonium theory of Berzelius, according to which salammoniac is not N H3, H Cl, but N H, Cl. The result may, however, be explained on the former hypothesis; supposing namely that the current decomposes hydrochloric acid into hydrogen at the negative and chlorine at the positive pole, and that the ammonia which has been deprived of its hydrochloric acid, is set free at the negative pole.] Solution of sal-ammoniac with silver electrodes gives both hydrogen and oxygen gases, the latter however not in sufficient quantity; the positive wire becomes covered with chloride of silver, and the liquid in the neighbourhood of the negative pole is found to contain free ammonia. When iron wires are employed, gas is evolved only at the negative wire; it gradually however diminishes in quantity as the wire becomes covered with crystalline reduced iron. The positive wire is corroded and oxide of iron separates. The liquid on the negative side is found to contain

ammoniuret of protoxide of iron besides sal-ammoniac. (Hisinger & Berzelius.)

If a cup formed of sal-ammoniac be wetted, then placed upon a piece of platinum foil connected with the positive pole of the battery, and filled with mercury into which the platinum wire of the negative pole is made to dip, chlorine is developed at the positive platinum plate, and the mercury swells up in the state of ammoniacal amalgam (Ĥg + N H4) to 5 times its original volume, its vegetations at the same time penetrating into the sal-ammoniac. When the circuit is broken, the amalgam immediately runs together in the form of liquid mercury. (H. Davy, Gilb. 33, 247.)

Solution of Common Salt gives chlorine at the positive, hydrogen gas and soda at the negative pole. The salt is probably first resolved into chlorine and sodium, and the sodium oxidized by the water; for when the_cathode consists of mercury, sodium-amalgam is obtained. (Higgins & Draper, N. Edin. Phil. J. 14, 314.)-Common salt, in which silver wires are immersed, evolves gas only at the negative wire at first, but afterwards at the positive wire also: the latter becomes covered with chloride of silver; the liquid surrounding it contains in solution chlorine and a compound of silver not precipitable by chlorine; the liquid on the negative side contains free soda.-When lead wires are used, the negative wire alone evolves gas, and subsequently becomes covered with fine crystals of lead; the positive wire acquires a coating of chloride of lead. A positive wire formed of iron or zinc likewise evolves no gas, but dissolves in the state of metallic chloride, from which a film of oxide is precipitated at the surface of contact of the chloride with the alkaline liquid on the negative side. (Hisinger & Berzelius.)

A globule of mercury immersed in solution of Chloride of Barium and connected with the negative pole of a feeble battery, forms solid vegetations of barium-amalgam. (Herschel.)-In solution of Chloride of Calcium the negative wire alone yields gas, and becomes covered, first with a white crust, then also with needles of lime (hydrate) which first diminishes the evolution of gas, and then stops it altogether. The positive wire dissolves as chloride of iron, and the solution deposits protoxide of iron at the surface of contact of the latter with the calcareous liquid. (Hisinger & Berzelius.)

From a solution of Protochloride of Iron, black magnetic oxide of iron is deposited at the negative pole. (H. Davy, according to Becquerel, it is deposited in small grains.)-A concentrated solution of Chloride of Copper or Chloride of Gold in water gives, when subjected to a feeble electric current, merely metal at the negative pole without any hydrogen gas: the gas is however evolved from a dilute solution when acted upon by a strong current. Hence it appears that water is less decomposible than these metallic chlorides,-and the order of decomposibility is: iodide of potassium, hydriodic acid, hydrochloric acid, metallic chlorides, water acidulated with sulphuric acid. (Matteucci.)

Iron acting as anode in solutions of metallic sulphurets, iodides, bromides, chlorides, and fluorides, dissolves, even when the circuit is not closed before the immersion of the iron. (Schönbein.)

Aqueous solutions of metallic Fluorides are decomposed with separation of fluorine at the anode. (Faraday.) When the electrodes are immersed in two cups of fluor-spar, filled with water and connected by fibres of asbestus, lime-water is found after two days in the negative cup, and hydrofluoric acid in the positive cup. (H. Davy.)