2 54 OXIDATION AND REDUCTION ELEMENTS, chap.

acid between palladium electrodes, which, as is well known, haye the power of absorbing the separated gases, particularly hydrogen, and noticed a slight evolution of gas. The gas element thus produced was found to be reversible, for by discharging it fairly quickly, it showed the same electro- motive force as the polarising element had to overcome during charging.

Since the combustion of a gram-equivalent of hydrogen is accompanied by the evolution of the quantity of heat ( fF) 34,200 cal., Thomson's rule requires for this element an electromotive force of i}|§^8 = 1*480 volts, which is higher by 0*418 than that found by Smale. The change of electro- motive force of this element with temperature was found to be at 20°—

(IP

= —000142 volt per degree,

dT

from which, by the Helmholtz relationship (see p. 208), we find—

23,070P- W ^ p _ W ^ rpdP 23,070 23,070 dT

= 293(-000]42) = -0-416 volt;

whilst by experiment the difiference was —

p - ^^ = 1062 - 1-480 = -0-418 volt,

which shows a very good agreement with the value calcu- lated. From this it can be gathered that the element is reversible.

Bose {10), too, found that the hydrogen-oxygen cell works reversibly, although he found a somewhat higher electromotive force (about 111 volts) than Smale did.

Helmholtz's Investigation on the Influence of Pressure. — ^When platinum electrodes are used it is found that higher electromotive forces are required to produce an

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