﻿428 Mr. J. Dewar on the Motion of a Palladium Plate 



distribution of tensile strain produced by rolling; at least after 

 repeated use the plate lost the power of returning after having 

 passed towards the left. If the plate after saturation is connectd 

 with the positive pole of the battery, the first effect on the index 

 is to move quickly towards the left, then to return to where it 

 was, this double motion taking place before any gas makes its 

 appearance on the palladium. If the platinum electrode B is 

 placed on the opposite side of A and the saturation of the plate 

 repeated, the index goes through the same series of positions, but 

 the direction of motion is reversed. The direction of motion 

 depends, therefore, on the relative positions of the electrodes, 

 but is constant for the same position. This is easily shown by 

 allowing the index to commence its motion, say, from left to right; 

 then by moving the positive electrode to the other side of the 

 palladium plate, the motion immediately commences in the op- 

 posite direction, although the saturation was far from being com- 

 plete. The motion of the index when oxygen is thrown on the 

 hydrogenized plate depends also on the position of the electrodes. 

 The index has also a motion at right angles to the plane of 

 the scale, the resultant motion being compounded of the separate 

 flexures of the plate. Many other devices could be used to show 

 the motion, such as a plate bent into the form of a cylinder with 

 a narrow channel left between the two edges, which would shut 

 and open alternately, or vice versa, according to whether the po- 

 sitive electrode were without or within the cylinder. 



Graham has shown that the formation of the alloy of hydro- 

 genium and palladium is attended with an enormous increase in 

 the volume of the metal. He found that a wire of palladium 

 100 millims. in length became 101*5 millims. when saturated 

 with hydrogen. Now, if a uniform hydrogen atmosphere sur- 

 rounds a symmetrical piece of palladium, there is no reason why 

 it should penetrate with a greater rapidity one surface rather 

 than any other. But if the absorption is not uniform on all the 

 surfaces, from want of uniformity in the hydrogen atmosphere, 

 the surface absorbing must produce a flexure of the plate from 

 the expansion of the metal. If a thin plate of this rigid metal 

 can be so arranged as to induce absorption on one side rather 

 than the other, then, as a necessary consequence, the plate will 

 become convex on the side where the greatest relative absorption 

 is taking place ; and as the saturation approaches uniformity, 

 the convexity should disappear, the plate regaining its original 

 form if the elasticity of the metal is not changed during the 

 action. A plate of palladium, when it functions as the negative 

 pole during the electrolysis of water, is subjected during the 

 course of the action to the supposed non-uniformity of the gaseous 

 atmosphere, if the surface of the plate is parallel to a similar 



