sition of atoms in the liquids of the body situated between the electrodes; and since in particular organs, salts may be thus produced different from those previously existing, the result of the action may be irritation. The first of these modes of action is most conspicuous in electricity of great intensity, like that of the machine,-the second in galvanic electricity: and hence, perhaps, partly arises the difference between the sensations produced by these two kinds of electricity. Even a single pair of metallic plates connected with each other, and with different parts of the body, must produce transposition of atoms and change of compositionand this is the principal cause of the phenomena observed by Galvani and Humboldt.

Oxygen-Salts of Heavy Metallic Oxides.

The compounds of oxygen-acids with heavy metallic oxides usually give acid and oxygen gas at the anode, and reduced metal at the cathode, -no evolution of hydrogen taking place, excepting when the electric current is too strong, and its action is partly exerted on the water. In the case of some metallic salts (those of manganese, lead, and silver), the oxygen liberated at the positive pole combines with the metallic oxide there situated, and forms with it a peroxide which is precipitated. If the electrodes are formed of the same metal as that contained in the solution, the acid and oxygen which collect at the anode recover by dissolving it the metal which they have lost, and reproduce the original salt; thus the liquid remains unaltered, and the anode merely loses a quantity of metal equal to that which is reduced at the cathode.

Salts of Manganese deposit peroxide at the positive pole. (Balard.) Sulphate of Zinc dissolved in water yields zinc at the negative pole, in the form of a powder possessing the metallic lustre.

Fused Borate of Lead yields lead at the cathode, oxygen and boracic acid at the anode. (H. Davy.)

Aqueous solution of Nitrate or Acetate of Lead yields lead at the negative pole, and brown peroxide of lead at the positive pole. (Faraday.)If a voltameter be introduced into the circuit, it is found that the volume of oxygen gas evolved at the anode from solution of acetate of lead amounts to only of the detonating gas collected in the voltameter, because the greater part of the oxygen is expended in forming the peroxide of lead, the quantity of which is to that of the lead separated at the cathode as 5: 3]-When the current is passed through acetate of lead which has been dehydrated and then fused, peroxide of lead is likewise produced at the anode, accompanied by a slight evolution of oxygen: for every 9 parts (1 atom) of water decomposed in the voltameter there is obtained at the cathode 103-4 parts (1 atom) of lead, and the weight of this lead is to that of the peroxide produced as 3:5 [1] (Matteucci, Ann. Chim. Phys. 71, 90.)

From a solution of Protosulphate of Iron, acted upon by a battery of 100 pairs, metallic iron is deposited in small granules on the positive platinum wire. (Becquerel.)

In solution of Sulphate of Copper, the negative platinum wire becomes covered with copper, while acid collects and oxygen gas is evolved at the positive wire. When copper wires are used, the positive wire loses as much metal as the negative wire gains, and the solution retains its original composition.

When active iron forms the anode in solution of sulphate of copper, it

dissolves without any copper being deposited on it;-but when the circuit is broken, the deposition takes place immediately. A passive iron wire acting as anode, and made to complete the circuit by its immersion, neither attracts copper nor dissolves, but evolves oxygen gas. (Schönbein.)

Solution of Nitrate of Silver deposits silver on the negative platinum wire, and crystallized peroxide of silver on the positive wire. (Ritter, Ruhland, Schw. 15, 414.)-In fused nitrate of silver, the anode evolves a large quantity of oxygen gas, while the cathode becomes covered with one atom of silver for every atom of water decomposed in the voltameter. The aqueous solution behaves in the same manner, evolving oxygen at the anode; but it likewise deposits peroxide. (Matteucci.)-Iron wire acting as anode does not oxidize, but liberates oxygen gas,-and when the current is strong, becomes covered with peroxide of silver. (Poggendorff, Pogg. 54, 357.)-Similarly, passive iron acting as anode becomes coated with peroxide of silver. (Schönbein.)

Nobili's Rings. When one of the electrodes consists of a plate of polished metal, the other of a fine platinum wire, the extremity of which approaches the plate within about half a line, there is often produced on that part of the plate which is immediately under the wire, a coloured ring or point surrounded with several, often four, concentric coloured rings. Thus a silver plate forming the anode produces coloured rings in solutions of phosphoric acid, carbonate of potash, sulphate of soda, salammoniac, chloride of potassium or sodium (in which metallic chlorides, rings are likewise formed on copper and brass, but not on zinc or platinum), tartar emetic, sulphate of zinc, chloride of cobalt, sulphate, nitrate or acetate of copper, chloride of platinum, &c. Solutions of sulphate of manganese, nitrate of bismuth, and acetate of lead, likewise form negative rings on plates of gold or platinum. A silver plate likewise has rings formed upon it when it serves as the cathode in solutions of tartar emetic, salts of copper, or chloride of platinum. (Nobili.)

[These phenomena belong to the theory of Newton's Coloured Rings,— and, according to Faraday, Warington, and Schönbein, arise from products of decomposition deposited on the metallic plate in films of various thickness. These products of decomposition are sometimes formed only from the liquid; thus, salts of manganese deposit peroxide of manganese, -salts of lead peroxide of lead,-and salts of bismuth probably deposit peroxide of bismuth; and these deposits form coloured rings on plates of gold and platinum.-Sometimes the deposits arise from the combination of a substance separated from the liquid with the metal of the plate; thus, metallic chlorides form on silver or copper insoluble chloride of silver or dichloride of copper; and the oxygen acids or their salts probably form peroxide of silver on silver plates. When the silver acts as cathode, the rings are formed by films of the precipitated metal of different degrees of thickness.]

The rings produced by acetate of lead exhibit, on chemical analysis, the reactions of peroxide of lead. Iodine in a state of sublimation produces the same colours on plates of silver or copper; they are likewise exhibited by any metal which when heated in the air becomes covered with a coat of oxide continually increasing in thickness. (Warington, Phil. Mag. J. 16, 52.)

The more oxidable metals, when used as anodes in nitrate or acetate of lead, do not allow of the formation of peroxide of lead; iron alone forms an exception, because it soon passes into the passive state: an iron

wire under these circumstances quickly and repeatedly becomes red, blue, yellow, red, blue, yellow, &c. The colour, however, becomes deeper every time, and at last so deep that the different tints cannot be distinguished. (Schönbein.)

To form a Nobili's monochromatic deposit on platinum, the following solutions may be used; Chloride of manganese in 8 parts of water, sulphate of manganese and potash in 12, acetate of manganese in 15, succinate of manganese in 16, or hippurate of manganese in 12 parts of water. A well-polished platinum capsule is filled with one of these solutions, and connected with the positive pole of a battery of 4 pairs of plates (each plate 36 square inches in surface), immersed in dilute sulphuric acid (if a stronger battery is used the solution of the salt must be more dilute); a disk of platinum of an inch in diameter is then dipped horizontally into the liquid to form the cathode. With sulphate of manganese and potash, or with succinate or acetate of manganese, one uniform tint is invariably produced, first golden yellow, then purple, then green; the current must therefore be stopped as soon as the desired tint is completely developed. With hippurate of manganese the capsule is first tinted golden yellow, then of a splendid bluish purple red. Chloride of manganese yields very beautiful broad alternating rings of purple green, golden yellow, and blue, surrounded by a broad belt of golden yellow. (R. Böttger, Pogg. 50, 45.)

Nobili's rings may also be formed by a simple galvanic circuit. When a plate of silver immersed in a solution of sulphate of copper is touched with the pointed extremity of a piece of zinc, there are formed round the point of contact a number of blue and green rings, which, when the zinc is removed, run through several changes of colour, finally becoming dark blue and lightish green. In the colourless copper solution formed by leaving copper filings in contact with a saturated solution of sal-ammoniac in a closed vessel containing air,-silver or platinum becomes covered with copper, if it be touched below the surface of the liquid with a pointed piece of zinc;-but when the zinc is removed, the coating of copper gradually disappears, the action commencing from the outside (where the coating is thinnest) and proceeding towards the middle. (Fechner, Schw. 55, 442.) If however the platinum thus covered with copper be immediately dipped into water, so as to remove the ammoniacal liquid by which it would be redissolved, the coppering remains permanent; and if the zinc be left for a longer time, about two minutes, in contact with the platinum in the ammoniacal solution of copper, until it evolves gas and precipitates copper of a black colour, the platinum loses its thin coating of copper and becomes tinted with various shades of yellow, green, red, and especially black, which latter colour it then retains after drying. (R. Böttger, J. pr. Chem. 8, 476.)

(For the decomposition of organic compounds by the electric current, vid. Oxalic acid, Tartaric acid, Acetic acid, Wood-spirit, Alcohol, Ether, and the Organic Bases.)

Decomposition of several Liquids in Contact with one another.

When the electric current has to traverse a number of liquids [or to effect a transportation of atoms in them] which are either in immediate contact or separated from one another by a porous body, it at first deposits

VOL. I.

2 H

at the anode the anions of that liquid only which is in contact with the anode; but after the current has acted for a longer time, the anions of the liquid situated at the negative pole, or in the middle, are likewise transferred to the anode. Similar effects are produced at the cathode.

[At the surface of contact of the different liquids, an atom of anion of the liquid situated next to the cathode is transferred by atomic transposition to an atom of cation of the liquid lying next to the anode, and thus the anion-atoms of the former liquid continually advance nearer and nearer to the anode, and the cation-atoms of the latter to the cathode; but they are not liberated till they actually reach the electrodes. If, for example, water is situated at the positive, and hydrochloric acid at the negative pole, an atom of water is decomposed at the former with separation of oxygen, and an atom of hydrochloric acid at the latter with separation of hydrogen. By atomic transposition, an atom of hydrogen from the water, and an atom of chlorine from the hydrochloric acid, arrive at the surface of separation, and there combine to form hydrochloric acid. (App. 33.) The first atom of chlorine which passes the boundary is followed by a second, and so on; and thus, at last, the compound of chlorine and hydrogen arrives at the positive pole-and not till then is the chlorine set free at that pole. If sulphate of copper be placed at the negative, and water at the positive pole, copper is set free at the former and oxygen at the latter; while at the boundary, the oxygen of the oxide of copper combines with the hydrogen of the water, which at the same time takes up the sulphuric acid previously liberated.-If chloride of sodium is situated at the negative, and sulphate of copper at the positive pole, sodium is separated at the former (but by contact with the water it forms soda and liberates hydrogen)-oxygen gas and sulphuric acid at the positive pole: chlorine and copper combine at the border.]

For making experiments relating to this matter, the following apparatus may be used: App. 2, in which the two dishes, cups, or tubes, a, b, are connected by moistened fibres of asbestus, and have the polar wires immersed in them;-App. 3, with a diaphragm of bladder; the U-tube, App. 4, which either contains one liquid in each arm (sometimes the two being in immediate contact, sometimes separated at h by clay, cotton, &c.), or a heavier liquid at h, and a separate liquid in each arm;-App. 5, to which similar remarks apply;-App. 6, in which the liquids are simply laid one upon the other;-App. 7, in which the tube a is closed at bottom by a piece of bladder or clay;-App. 8, in which also the tubes a b are stopped with clay;-App. 9, of which the same may be said as of App. 2. In the delineations of all these arrangements, a galvanic battery must be supposed to be introduced in place of the galvanometer.

Two Liquids in two Divisions.

When water is separated from sulphuric acid by a membrane, the current is less obstructed if the positive electricity be conducted into the water, and the negative into the acid, than in the contrary case. (Matteucci.)

The following table exhibits a general view of the decompositions observed. The first column shows the apparatus in which the experiment was made; the second, third, and fourth, the liquids placed in the positive and negative divisions respectively; the fifth, sixth, and seventh columns show what substances were separated at the positive and negative polar wires, which usually consisted of platinum;-in the last column the name

of the observer is given.-c denotes the middle vessel occurring in several arrangements, and the liquid contained in it.

Liquids

HO

I

KO, S O3

SO3

0000000OLO

KO; H
KO; H

3

3

KO; H H. Davy 4

но.

2

HO

Ca O, S 03

P11100

SO, O Mg O H

KO; H Becquerel

Ca O; H His. & Berz.

8

1. The hydrogen directly separated at the negative pole from the water of the oil of vitriol combines, according to Faraday's view, with the oxygen of the sulphuric acid, and precipitates of sulphur.

2. At first, hydrochloric or hydriodic acid collects in the positive division, free chlorine or iodine showing itself only when the current has continued for a very long time. Hence the hydrogen-acids do not appear to be directly decomposed. (Connell.) [This is a conclusion which, according to what has been already observed (p. 456), we are not obliged to admit.

3. In these cases, also, hydrochloric or hydriodic acid collects in the positive cup, but no free chlorine or iodine. (Connell.)

4. When the current has been continued for a certain time, all the potash is found in the negative, and all the acid in the positive cup. Other salts of the alkalis likewise behave with water in the same manner as sulphate of potash. If, therefore, the positive cup contains water coloured with litmus, and the negative cup water coloured with turmeric, both liquids become reddened, because acid is transferred to the positive and alkali to the negative pole, from the salts contained in these colouring matters. The greater the length of the asbestus fibres, or the distance between the positive and negative wire, the more slowly does the transference of acid and alkali take place. Thus, with a battery of 100 pairs, only five minutes elapse before the appearance of acid in the positive cup, when the distance between the polar wires is only an inch; but fourteen hours are required when the distance amounts to 8 inches, the positive