Another fact which is well known, and largely put to practical use, is that fused salts are, as a rule, good electrolytic conductors of electricity. Even when the salt is as pure as it can be made, it still conducts in the molten state. Although the conductivity of fused salts has not been investigated with the same completeness as that of solutions, yet it cannot be doubted that the salt must be more or less ionised. The ionisation appears to increase with rise of temperature, for the salt becomes a better conductor. This, however, may be due, in part at least, to the smaller frictional resistance which it offers to the passage of the ions towards the electrodes. .But recent experiments have shown that the molecules of some salts, at least—those in which measurement is possible —are more complex than would be supposed from their formulae. Thus, nitre, KNO3, consists of molecules of four times that complexity, or K4N4O12 ; and it is not improbable that among these complex molecules there are some ions of K and of NO3 capable of conveying an electric charge. It may indeed be stated that those liquids which possess complex molecules have the power of ionising salts dissolved in them. Water is one of the most striking examples ; and it is to be presumed that such complex molecules are able to surround and prevent ions from at once discharging into one another by protecting them from each other.
To sum up:—Certain substances, in the state of gas, exhibit dissociation—that is, they decompose into simpler constituents, which combine again on cooling. This dissociation is favoured by a rise of temperature or by a lowered pressure, and reversed by a fall of temperature or a rise of pressure. From a determination of the density of the mixture of gases the extent of the dissociation can be calculated. Certain substances, in like manner, and such substances are generally named " salts," when dissolved in water or certain other solvents, undergo electric dissociation or ionisation ; this dissociation is often increased by a rise of temperature, and always by dilution. The constituents of such solutions, the anions and kations, can be urged in opposite directions by an electric current ; they usually " migrate " at different rates ; and when they discharge, by contact with the electrodes, they are sometimes liberated in the free state, as, for example, many metals, and bromine and iodine ; but sometimes the discharged ion is incapable of existing in the free state in contact with the solvent, and in this case they react with the water, and form new secondary compounds. The amount of this ionisation can be measured by determinations of the depression of freezing-point, or of the conductivity, of the solution.