Page:Text-book of Electrochemistry.djvu/268

 is due to the fact that mercurous chloride is appreciably more soluble at the ordinary temperature than mercurous sulphate.

Secondary Elements. — Secondary elements produced by the polarisation of two electrodes may be regarded as a special type of oxidation and reduction elements. If we connect two plates of platinum (or other metal not attacked), which are immersed in an electrolytic solution, with the poles of a galvanic battery, a separation takes place at each plate. If the electrolyte is a base, an oxygen acid, or the alkali salt of an oxygen acid, hydrogen is separated at the cathode and oxygen at the anode. If, after disconnecting the battery, the two plates be joined by a wire, we obtain a current in the opposite direction to that of the polarising current (see p. 1). We may therefore regard the two pole plates as electrodes of different metals, and the whole as a galvanic element. Such gas dements were suggested by Eitter at the beginning of the nineteenth century, and have been much studied since then.

Polarisation Current. — The strength of the polarising current falls quickly when a small electromotive force (under 1 volt) is used for the polarisation. It never, however, com- pletely disappears, because the poltuised plates become gradually depolarised by diffusion, so that new quantities of gas must be separated in order to maintain the polarisation near the polarising electromotive force. By* breaking the circuit and examining the electromotive force of polarisation at different times, it has been found that the speed with which the polarisation spontaneously disappears by diffusion of the separated gases, partly in the liquid and partly in the electrodes (particularly if these be platinum or palladium), is not only dependent on the nature of the electrodes, but also on that of the liquid. The smallest current strength required to replace the gas which is lost by diffusion is called the polarisation current.

Smale's Experiments. — ^When higher electromotive forces (1*062 volts) are used, Smale (9) found some ■comparatively simple relationships. He electrolysed sulphuric

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