This experiment was repeated in a somewhat different manner, the bra.s.s wires from a pile being brought under a tube filled with water and closed at the top. Gas bubbles were produced by the wire in connection with the negative pole of the pile, and the water was observed to diminish gradually. At the positive wire, on the contrary, no gas came off, but the metal lost its metallic l.u.s.tre, became dark, and finally crumbled away. The gas which had collected in the tube proved to be hydrogen; while on examining the black ma.s.s it was found that the const.i.tuents of bra.s.s, viz., copper and zinc, had become oxidized.

[Ill.u.s.tration: FIG. 89.--An electrolytic cell. The parts are: A, cell; B, electrolyte; C, positive electrode or cathode; D, negative electrode or cathode.[9]]

=Electrolysis.=--_Electric a.n.a.lysis_ or more briefly _electrolysis_ was the term applied by Faraday to the process of decomposing a liquid by the pa.s.sage of a current of electricity through it.

The vessel containing the liquid is known as an _electrolytic cell._ In fig. 89, A is the cell, which may be of gla.s.s or of any other suitable material, and B is the liquid which is to be electrolyzed. Current enters by the _positive electrode_ C, also known as the _anode_, traverses the liquid, and leaves by the _negative electrode_, or _cathode_, D.

[Ill.u.s.tration: FIG. 90.--Modern apparatus for decomposing water by electrolysis. Platinum electrodes P and P' are placed at the bottom of two upright tubes O and H, and are connected to the terminals T and T' by platinum wires, which are fused through the gla.s.s of the tubes. These tubes have gla.s.s stop c.o.c.ks S and S' at their upper ends, and at their lower ends are connected by a short gla.s.s tube, from the center of which rises the large central tube which expands with a bulb at its upper end, which is open at the top. The three tubes can be filled with acidulated water from the central tube, the previously contained air being allowed to escape through the stop c.o.c.ks, which are afterwards closed. If it be so filled, and the terminal T be attached to the positive and T' to the negative pole of a suitable battery, bubbles of gas will be observed to rise from the plates P and P', and finding their way to the top of the respective tubes, will displace the liquid, which will be driven into the open central tube. The gas rising from the anode P is oxygen (O), and that rising from the cathode P' is hydrogen (H). If the tubes are graduated, the latter will be found to occupy about twice the volume of the former.

The proportion is theoretically 2 to 1; however, on account of the different solubilities of the two gases in water, oxygen being the more soluble of the two, is deficient in quant.i.ty.]

The pa.s.sage of current through the water splits up its molecules into their const.i.tuent atoms of oxygen and hydrogen, the former being given off in bubbles at the anode, and the latter at the cathode.

When current is pa.s.sed through a solution of copper sulphate between platinum electrodes, the liquid is decomposed, atoms of copper being deposited at the cathode, bubbles of oxygen being given off at the anode, and sulphuric acid being formed in the liquid, which latter becomes more and more acid as the copper is withdrawn.

[Ill.u.s.tration: FIG. 91.--Grotthuss' theory of electrolysis. Grotthuss in (1806), announced his theory that the molecules in an electrolyte have their individual electro-positive and electro-negative atoms charged positively and negatively respectively. In an ordinary liquid, for instance in water, the molecules are arranged indifferently, like row 1, with their positive and negative ends pointing in all directions. When the charged plates A and B connected to the + and - poles of a battery are inserted in the water, the molecules under the action of the laws of electrostatic action turn as shown in row 2, so that all the hydrogen or shaded ends (+) are turned towards the (-) plate B and all the oxygen or unshaded ends (-) towards the (+) plate A. All along the row the electrical forces are supposed to tear the molecules asunder, depositing H on B and O on A. The atoms in the middle of the liquid, however, recombine, for the hydrogen atoms in their journey towards B meet the oxygen atoms travelling in the opposite direction, and we get the state of affairs represented in row 3. The next step is to rotate once more the atoms into the positions shown in row 2, and so on. In this way the theory accounts for the products only appearing at the electrodes and not in the body of the liquid.]

If, however, the anode be of copper instead of platinum, no sulphuric acid will be formed, neither will oxygen be given off at the anode. As copper is deposited at the cathode, an equal quant.i.ty will be dissolved at the anode, so that the original const.i.tution of the liquid is maintained.

The atoms separated from each other by the electric current were called _ions_ by Faraday; those going to the anode being _anions_, and those going to the cathode being _kathions_.

Anions are generally regarded as _electro-negative_, because they move as if attracted to the positive electrode, while kathions are regarded as _electro-positive_.

In order to explain the transfer of electricity and the transfer of matter through the electrolyte, Grotthuss put forward the hypothesis that when two metal plates at different potentials are placed in a cell, the effect produced in the liquid is that the molecules of the liquid arrange themselves in innumerable chains, as shown in fig. 91, in which every molecule has its atoms pointing in a certain direction, the electro-positive atom being attracted towards the cathode and the electro-negative towards the anode. An interchange then takes place all along the line, the free atoms appearing at the electrodes, and every atom discharging a minute charge of electricity upon the electrode at which it is liberated.

=Electro-chemical Series.=--This is an arrangement of the metals in a series in such a manner that the most electro-positive is at one end and the most electro-negative at the other.

The order of the metals varies with the electrolyte in which the metals are tested.

The following table shows such series for the most common metals, in three different solutions:

_Sulphuric acid._ _Hydrochloric acid._ _Caustic potash._

Zinc Zinc Zinc Cadmium Cadmium Tin Tin Tin Cadmium Lead Lead Antimony Iron Iron Lead Nickel Copper Bis.m.u.th Bis.m.u.th Bis.m.u.th Iron Antimony Nickel Copper Copper Silver Nickel Silver Antimony Silver Gold Platinum

Faraday stated several laws of electrolysis, as follows:

1. _The quant.i.ty of an ion liberated in a given time is proportional to the quant.i.ty of electricity that has pa.s.sed through the voltameter[10] in that time._

2. _The quant.i.ty of an ion liberated in a voltameter is proportional to the electro-chemical equivalent of the ion._

3. _The quant.i.ty of an ion liberated is equal to the electro-chemical equivalent of the ion multiplied by the total quant.i.ty of electricity that has pa.s.sed._

=Electric Osmose.=--Porret observed that if a strong current be led into certain liquids as if to electrolyze them, a porous part.i.tion being placed between the electrodes, the current mechanically carries part of the liquid through the porous diaphragm, so that the liquid is forced to a higher level on one side than on the other. This phenomenon is known as _electric osmose_.

=Electric Distillation.=--Closely connected with the preceding phenomenon is that of the _electric distillation of liquids_. It was noticed by Beccaria that _an electrified liquid evaporates more rapidly than one not electrified_.

Gernez has recently shown that in a bent closed tube, containing two portions of liquid, one of which is made highly + and the other highly -, the liquid pa.s.ses over from + to -. This apparent distillation is not due to difference of temperature, nor does it depend on the extent of surface exposed, but is effected by a slow creeping of the liquid along the interior surface of the gla.s.s tubes. Bad conductors, such as turpentine, do not thus pa.s.s over.

[Ill.u.s.tration: FIG. 92.--Effect of the electric current on a frog's legs; discovered in 1678 by Galvani.]

=Muscular Contractions.=--It was discovered in 1678 that when a portion of muscle of a frog's leg, hanging by a thread of nerve bound with a silver wire, was held over a copper support so that both nerve and wire touched the copper, the muscle immediately contracted.

More than a century later Galvani's attention was drawn to the subject by his observation of spasmodic contractions in the legs of freshly killed frogs under the influence of the "return shock" experienced every time a neighboring electric machine was discharged.

The limbs of the frog, prepared as directed by Galvani, are shown in fig. 92. After the animal has been killed the hind limbs are detached and skinned; the crural nerves and their attachments to the lumbar vertebrae remaining. For some hours after death the limbs retain their contractile power. The frog's limbs thus prepared form an excessively delicate galvanoscope.

=Electroplating.=--This is the process of depositing a layer or coating of a rarer metal upon the surface of a baser, or of a metal upon any conducting surface, by electrolysis.

The electric current used may be obtained from a battery or other source.

The battery has its positive plate connected to a rod extending across a trough or tank containing the plating bath.

Suspended from the rod are anodes of gold, silver, or copper or whatever metal from which a deposit is desired. The other plates of the battery or the negative elements, are connected with another rod across the trough, to which are suspended the articles to be plated.

=Electrotyping.=--This is the process by which, type, wood cuts, etc., are reproduced in copper by the process of electroplating. A mould is first made of the set type in wax; this mould is next coated with black lead to give it a metallic surface, as the wax is an insulator; the mould is then subjected to the process of electro deposition, resulting in the formation of a film of copper on the prepared surface.

The copper sh.e.l.l is removed from the mould by applying hot water; the sh.e.l.l is then backed up with electrotype metal to render it strong enough for use.

Almost all the ill.u.s.trations in this book, for example, are printed from electrotype copies, and not from the original wood blocks, which would not wear so well.

CHAPTER IX

MAGNETISM

=Magnetism.=--The ancients applied the word "magnet," _magnes lapes_, to certain hard black stones which possess the property of attracting small pieces of iron, and as discovered later, to have the still more remarkable property of pointing north and south when hung up by a string. At this time the magnet received the name of _lodestone_ or "leading stone." It is commonly, though incorrectly, spelled loadstone.

[Ill.u.s.tration: FIG. 93.--Simple compa.s.s. It consists of a magnetic needle resting on a steel pivot, protected by a bra.s.s case covered with gla.s.s, and a graduated circle marked with the letters N, E, S, W, to indicate the cardinal points; _a b_ is a lever which arrests the needle by pushing it against the gla.s.s when the b.u.t.ton _d_ is pressed.]

=Ques. Describe two kinds of magnetism.=

Ans. Magnets have two opposite kinds of magnetism or magnetic poles, which attract or repel each other in much the same way as would two opposite kinds of electrification.

=Ques. What is the nature of each kind of magnetism?=

Ans. One has a tendency to move toward the north and the other toward the south.

[Ill.u.s.tration: FIGS. 94 to 96.--Simple _bar magnet_ and _horse shoe magnet_ with _keeper_. These are known as _permanent magnets_ in distinction from _electromagnets_. The horse shoe magnet will attract more than the bar magnet because both poles act together. A piece of soft iron, or keeper is placed across the ends of a horse shoe magnet to a.s.sist in preventing the loss of magnetism.]

[Ill.u.s.tration: FIGS. 97 and 98.--Horizontal magnetic needle, and magnetic "dip" needle. A magnetic needle consists of a small bar magnet, supported upon a pivot or suspended so that it is free to turn in a horizontal or vertical plane. The form of magnetic needle ill.u.s.trated in fig. 97 is arranged to show the magnetic meridian; the needle moves upon a perpendicular axis or pivot _ab_. In fig. 98, the needle _sn_ turns upon a horizontal axis _ab_. This needle indicates the dip or inclination, that is, the angle which it makes with the horizontal plane, due to the fact that in most places the lines of force are not horizontal. In the northern hemisphere the N pole of the needle is depressed, in the southern hemisphere the S pole is similarly affected. When used, the dip needle must be set so that the plane in which the needle swings contains the magnetic meridian, as indicated by the horizontal needle.]

=Ques. Where is the magnetism the strongest?=

Ans. In two regions called the _poles_.

=Ques. Describe the distribution of magnetism in a long shaped magnet.=