This hypothesis, that a kind of dissociation takes place in salt solution, might have failed to gain acceptance had it not been for a very remarkable coincidence. It appears that all solutions which show this behaviour allow an electric current to pass through them, whereas all solutions of compounds such as cane-sugar, do not permit the passage of a current of electricity. The latter class of compounds is called " non-conducting ; " the former, class contains compounds which are " conductors ⁹⁹ of electricity. But metals and certain compounds, chiefly consisting of the sulphides of the metals, are also conductors of electricity, with this difference, however : while the latter are apparently unaltered by the passage of the electric current, solutions of salts undergo profound change. In some cases, oxygen appears in bubbles at the plate connected with the positive pole of the battery, while hydrogen is evolved from that connected with the negative pole ; in others, when the dissolved substance is a salt of such metals as copper, silver, or mercury, the metals themselves are deposited on the negative pole, or, as it is usually termed, the " kathode ; " while if chlorine, bromine, or iodine is one of the constituents of the salt, it is evolved at the " anode ⁹⁹ or positive pole.