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 to indicate the assumption of a negative charge by a chlorine atom, and its conversion into a chlorion; or 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 (CIH) (CIH) (CIH) (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 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 hydrogen is 96600 coulombs , mass or roughly. 10. But in several I gram instances that have been measured, the corresponding ratio for the charged particles of a gas is about 10. 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 atoms. 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 theory which is coming to be the basis of all explanations of electrical phenomena. -19 The charge on a single electron has been estimated at about I'I X 10-19 coulombs. If this is correct, the number involved in the transport of one faraday of electricity must be 96600 I'I X 10 = 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 10-10 equivalent of hydrogen ions; i.e. about 10-10 X 9 X 1023 = 9 × 1013, or ninety million million actual charged atoms of hydrogen, and the same number of hydroxyl. In all the cases that occur in practice, therefore, the current is carried by an enormously great number of ions, and a statistical method of treatment is fully justified. Since electrons possess a definite, though small, mass, the association of an e ric charge with matter must cause a certain difference in its mass. Thus when we write the equation the masses involved are of chlorine 35'4, of electrons o'o01, SO that the mass of chlorions formed will be 35'401. Similarly, to give a positive charge to sodium means to take away an 1 J. J. Thomson, Phil. Mag., 5. 346-355 (1903). T. P. C. C electron from each atom, so that the mass of sodions formed must be less than that of the sodium used. There is unfortunately no means of testing this conclusion by actual weighing. But the conclusion may serve to emphasise the distinction between sodium and sodions, between chlorine and chlorions, and so on. The chief objection raised against Arrhenius' theory was that it supposed thoroughly stable bodies like sodium chloride to break up, on mere solution in water, into such highly active and unstable ones as sodium and chlorine. This objection is at least diminished, if not removed, when we remember that the dissociation is not into sodium and chlorine, but into other substances, viz. sodions and chlorions, of which we know nothing but their behaviour in solution (and perhaps a little of the effects they produce in vapour form). The electron theory naturally leads to a revision of the current notions on chemical atoms; and it has been suggested that there is only one kind of ultimate particle-the electronand that atoms of ordinary matter are really constellations made up of these. In this way it would be as intelligible that one, two, or more electrons should be detached from a given atom, as added to it. We might, therefore, form this picture of the phenomena: the constellations known as a sodium and a chlorine atom attract one another, and unite to form a molecule of sodium chloride in which each of the two groups is somewhat, but not greatly, modified. It is difficult to break up the molecule into the atoms out of which it was formed, but comparatively easy to split it in a slightly different manner, so as to leave one extra electron with the chlorine and one less with the sodium: just as in a long chain carbon compound it may be difficult to break the chain at a certain link, and yet quite easy to break it at the next link. It is, of course, pure convention to call the electricity that is carried by chlorine negative, that carried by hydrogen or sodium positive. The nomenclature was fixed long before study of the discharge through gases had led to the recognition of any qualitative distinction between the two. If one had to start afresh, it is probable that, in the light of that study, the terms positive and negative would be used the other way |