765.  In the dilute acid (761.), I conclude that a part of the water is electro-chemically decomposed, giving origin to the oxygen, which appears mingled with the chlorine at the anode.  The oxygen may be viewed as a secondary result; but I incline to believe that it is not so; for, if it were, it might be expected in largest proportion from the stronger acid, whereas the reverse is the fact.  This consideration, with others, also leads me to conclude that muriatic acid is more easily decomposed by the electric current than water; since, even when diluted with eight or nine times its quantity of the latter fluid, it alone gives way, the water remaining unaffected.

766. Chlorides.—­On using solutions of chlorides in water,—­for instance, the chlorides of sodium or calcium,—­there was evolution of chlorine only at the positive electrode, and of hydrogen, with the oxide of the base, as soda or lime, at the negative electrode.  The process of decomposition may be viewed as proceeding in two or three ways, all terminating in the same results.  Perhaps the simplest is to consider the chloride as the substance electrolyzed, its chlorine being determined to and evolved at the anode, and its metal passing to the cathode, where, finding no more chlorine, it acts upon the water, producing hydrogen and an oxide as secondary results.  As the discussion would detain me from more important matter, and is not of immediate consequence, I shall defer it for the present.  It is, however, of great consequence to state, that, on using the volta-electrometer, the hydrogen in both cases was definite; and if the results do not prove the definite decomposition of chlorides, (which shall be proved elsewhere,—­789. 794. 814.,) they are not in the slightest degree opposed to such a conclusion, and do support the general law.

767. Hydriodic acid.—­A solution of hydriodic acid was affected exactly in the same manner as muriatic acid.  When strong, hydrogen was evolved at the negative electrode, in definite proportion to the quantity of electricity which had passed, i.e. in the same proportion as was evolved by the same current from water; and iodine without any oxygen was evolved at the positive electrode.  But when diluted, small quantities of oxygen appeared with the iodine at the anode, the proportion of hydrogen at the cathode remaining undisturbed.

768.  I believe the decomposition of the hydriodic acid in this case to be direct, for the reasons already given respecting muriatic acid (763. 764.).

769. Iodides.—­A solution of iodide of potassium being subjected to the voltaic current, iodine appeared at the positive electrode (without any oxygen), and hydrogen with free alkali at the negative electrode.  The same observations as to the mode of decomposition are applicable here as were made in relation to the chlorides when in solution (766.).