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than the number of positive or negative unit charges associated with the chemical atom.

In view of this, it is convenient to extend the usual atomic symbolism of chemistry, by writing for unit (atomic) positive and negative charges of electricity; and, for shortness, to distinguish ions—i.e. charged atoms or groups—from uncharged ones, by the addition of · for each positive charge, and ' for each negative. Thus, a silver ion is written Ag', a calcium ion Ca“, the negative ion of sulphuric acid SO,"; and we may make use of equations such as

CI+ = C" to indicate the assumption of a negative charge by a chlorine atom, and its conversion into a chlorion ; or

NaNO3 = Na: + NO: to indicate the dissociation of sodium nitrate into a sodion and a nitrion.

To trace further the mechanism, Grotthuss put forward a suggestion, that the neutral molecules of dissolved substance, drawn into chains by the action of the electric force, exchanged partners in such a way as to leave the end links of the chain free. Thus

(ch) (CH) (CH) (CIH) four molecules of hydrochloric acid, orientated by the electric force, might, by a small change, become

Ci (HCI) (HCI) (HCI) H i.e. three molecules, together with a free hydrogen atom at one end (that of the cathode), and a free chlorine atom at the other. This crude picture of the facts was brought more into harmony with physical ideas in general by Clausius, one of the founders of the atomic theory. Clausius assumed that salt molecules in solution occasionally dissociate into a positively and a negatively charged portion. These ions, when formed, will, according to the laws of electrostatics, move; the cations in the direction of the electric force (from anode to cathode), the anions in the opposite sense. · An ion may move as far as one of the electrodes and there give up its charge, and appear as a deposit of ordinary matter; but, if generated far in the interior of the solution, will more probably collide with other molecules. When this occurs the ion may travel on again free, or its collision may break up the molecule, and so form fresh ions; or it may, by colliding with an ion of the opposite kind, recombine. The decomposition and recomposition of molecules thus continually taking place, leave at any moment a certain fraction of the ions free; the electric force imposesapart from their irregular heat motions—a uniform drift on all the ions that are free; and so, although no one ion need move very far, the current is conveyed by a steady procession of them towards the electrodes.

It is not necessary to suppose that the electric force causes dissociation of the salt molecules; rather, since there are cases of electrolysis which will start on the application of any electric force, however small, it is more natural to assume that the dissociation is spontaneous, i.e. it is due to ordinary chemical action, and exists in a solution apart from the application of electric force at all.

This leaves open the question how much of the salt is at any moment dissociated. It was at first tacitly assumed that only an infinitesimal amount existed in the ionised condition, so that the properties of the salt would, on the whole, be the same in the dissolved condition as in the solid. It was Arrhenius ? who first put forward reasons for supposing that an electrolyte might be largely, and in some cases almost completely, dissociated in solution. This view, though violently in opposition to the current chemical doctrine of the time, has continually gained support from experiment since, and may be looked upon as thoroughly established. We shall not attempt to give the arguments in its favour here, but shall adopt it as a working hypothesis, and allow the evidence in favour of it to accumulate, as the hypothesis is applied to various phenomena in turn. The theory of atomic electric charges; of the convection of

· Zeitschr. phys. Chem., 1. 631 (1887).

such charges as constituting a current; and the consequent theory of electrolytic dissociation, seemed, a few years ago, at variance with electrical science in general. In the most familiar case of electric conduction along wires, the current seemed continuous, and it was hardly imagined that it could be of a convective character. The theories of electric action that had been developed always regarded that action as being continuous in space, so that the notion of an atom of electricity was quite foreign to them, and the explanation of electrolysis that suggests itself was looked upon with some doubt in consequence. This want of harmony has now been resolved, but not by any serious modification of views on electrolysis : it is the rest of electricity that has been converted to the atomic theory. Recent discoveries on the discharge of electricity through gases are the cause of the change. It is found in dealing with electric currents through gases that a convective explanation is the only tenable cne. There can be no doubt that positively and negatively charged particles-ions-exist in gases; that these move under the action of electric force; and that their motion constitutes a current. Further, that such ions can be produced within a gas in various ways: by the action of ultra-violet light, of Röntgen rays, etc. It has even been found possible to estimate the number of particles in a cubic centimetre of ionised air, and the mass and electric charge of each. It appears that the electric charge conveyed by particles in gases is identical with that conveyed by ions in electrolytes; but as regards mass there is an important distinction. The ratio charge in the case of

mass

hydrogen is 96600 coulombs

ps, or roughly. 10%. But in several

I gram instances that have been measured, thi coi responding ratio for the charged particles of a gas is about 1o8. The particles for which this is true are, however, always negative ones. The explanation of this number might be either that the charge conveyed was much greater, or the mass much less than for hydrogen ions; J. J. Thomson showed that the latter reason is the true one, and that, consequently, negative particles in a gas have a mass of about one-thousandth that of hydrogen atonis. Positively charged particles as small as this are not known, and the hypothesis thus arises that there is only one kind of electricity—the kind we conventionally call negative; that this exists in the form of unit charges, atoms of electricity, or, as they are now called, electrons; and that a negatively charged body is one containing an excess of electrons, a positively charged one, a defect; while in a neutral substance the electrons are so in equilibrium with the rest of the matter that no external electric field is produced. This is, in the barest outline, the electron theorr which is coming to be the basis of all explanations of electrical phenomena.

The charge on a single electron has been estimated at about l'1 X 10-19 coulombs. If this is correct, the number involved in the transport of one faraday of electricity must be 96600l'X 10-19 = 9 X 10%. In the case of an univalent ion such as chlorine, one electron is associated with each atom, and therefore this is the number of atoms of chlorine in a gram equivalent (35'4 grams). The number is so large that even in the most dilute solutions with which one has to deal. there must be a very large number of ions per cubic centimetre. Thus it has been estimated that in pure water 10-10 gram equivalents are dissociated per cubic centimetre; there is consequently 10 em ent of hydrogen ions; i.e. aboui 10-10 X 9 X

r ninety million million actual charged ato

the same number of hydroxyl. In all the

ctice, therefore, the current is carried b

imber of ions, and a statistical m

fied. finite, though small, mass, 1110 rge with matter must calls S Thus when we write

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disadvantage of requiring nearly two volts to work it, and so absorbing an appreciable fraction of the electric energy it measures.

If a water voltameter be provided with a capillary tube, through which the gases have to escape, the pressure in the tube will be roughly proportional to the rate at which the gas is flowing, and may therefore be taken as a measure of the rate of flow of the electricity, i.e. of the current. Such an instrument, called an ampere-manometer, has occasionally been used instead of an ordinary ampere-meter.

The electrolytic process is used for the preparation of oxygen and hydrogen commercially. The vessels holding the electrolytes are of iron, as also the electrodes; 15 per cent. soda solution is employed, and as the water is electrolyzed away it must be replaced from time to time, distilled water being used to prevent accumulation of chlorides. The gases are collected separately in domes, under a pressure of about 60 mm. of water-greater pressure causes a risk of mixing. In order to reduce the resistance of the cell, it is packed in a wooden box with sand, so that the heat developed by the current keeps the temperature up to about 70°C. The voltage required is then only about 2.8 volts for each cell. The cells are constructed to take 600 amperes, and yield 220 litres of hydrogen and 110 of oxygen per hour, the purity of the gas being about 97 per cent.

2. Silver Voltameter. — This is undoubtedly the most accurate of all. The weight of silver deposited from a solution by a measured current in a measured time has been determined several times, the most important determinations being those of F. and W. Kohlrausch 3 and Lord Rayleigh.4 The former found o‘0011183 gm. per coulomb, the latter o‘0011180. The greatest difficulty in such experiments is the measurement of the current in electromagnetic measure, i.e. in accordance with the definition of the ampere. Many subsequent experimenters

"Ostwald, Zeitschr. fhys. Chem., 35. 36 (1900).
? Zeitschr. f. Elektroch., 7. 857 (1901).
3 Wied., 27. 1. (1886).
+ Phil. Trans., 175. 458 (84).

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