CONDUCTION

conduction, electric, s ii., "nature of electrolytes"). it is probable that the electrical effects constitute the strongest arguments in favour of the theory. it is necessary to point out that the dissociated ions of such a body as potassium chloride are not in the same condition as potassium and chlorine in the free state. the ions are associated with very large electric charges, and, whatever their exact relations with those charges may be, it is certain that the energy of a system in such a state must be different from its energy when unelectrified. it is not unlikely, therefore, that even a compound as stable in the solid form as potassium chloride should be thus dissociated when dissolved. again, water, the best electrolytic solvent known, is also the body of the highest specific inductive capacity (dielectric constant), and this property, to whatever cause it may be due, will reduce the forces between electric charges in the neighbourhood, and may therefore enable two ions to separate. this view of the nature of electrolytic solutions at once explains many well-known phenomena. other physical properties of these solutions, such as density, colour, optical rotatory power, &c., like the conductivities, are _additive_, i.e. can be calculated by adding together the corresponding properties of the parts. this again suggests that these parts are independent of each other. for instance, the colour of a salt solution is the colour obtained by the superposition of the colours of the ions and the colour of any undissociated salt that may be present. all copper salts in dilute solution are blue, which is therefore the colour of the copper ion. solid copper chloride is brown or yellow, so that its concentrated solution, which contains both ions and undissociated molecules, is green, but changes to blue as water is added and the ionization becomes complete. a series of equivalent solutions all containing the same coloured ion have absorption spectra which, when photographed, show identical absorption bands of equal intensity.[5] the colour changes shown by many substances which are used as indicators (q.v.) of acids or alkalis can be explained in a similar way. thus para-nitrophenol has colourless molecules, but an intensely yellow negative ion. in neutral, and still more in acid solutions, the dissociation of the indicator is practically nothing, and the liquid is colourless. if an alkali is added, however, a highly dissociated salt of para-nitrophenol is formed, and the yellow colour is at once evident. in other cases, such as that of litmus, both the ion and the undissociated molecule are coloured, but in different ways. electrolytes possess the power of coagulating solutions of colloids such as albumen and arsenious sulphide. the mean values of the relative coagulative powers of sulphates of mono-, di-, and tri-valent metals have been shown experimentally to be approximately in the ratios 1:35:1023. the dissociation theory refers this to the action of electric charges carried by the free ions. if a certain minimum charge must be collected in order to start coagulation, it will need the conjunction of 6n monovalent, or 3n divalent, to equal the effect of 2n tri-valent ions. the ratios of the coagulative powers can thus be calculated to be 1:x:x^2, and putting x = 32 we get 1:32:1024, a satisfactory agreement with the numbers observed.[6] the question of the application of the dissociation theory to the case of fused salts remains. while it seems clear that the conduction in this case is carried on by ions similar to those of solutions, since faraday's laws apply equally to both, it does not follow necessarily that semi-permanent dissociation is the only way to explain the phenomena. the evidence in favour of dissociation in the case of solutions does not apply to fused salts, and it is possible that, in their case, a series of molecular interchanges, somewhat like grotthus's chain, may represent the mechanism of conduction. an interesting relation appears when the electrolytic conductivity of solutions is compared with their chemical activity. the readiness and speed with which electrolytes react are in sharp contrast with the difficulty experienced in the case of non-electrolytes. moreover, a study of the chemical relations of electrolytes indicates that it is always the electrolytic ions that are concerned in their reactions. the tests for a salt, potassium nitrate, for example, are the tests not for