12 M. H. Herwig on t lie Disintegration of the 



I have tried the following means, and ascertained its perfect 

 utility. 



For one electrode I took a large, thick plate of copper ; for 

 the other, in order to have a metal not oxidable in air, a knob 

 of pure silver. The latter was screwed on an iron rod, which, 

 wrapped round with a spiral spring, was passed through a thick 

 caoutchouc cork, which bore three insulated supports running 

 from the sides of the rod. If the copper plate was touched with 

 the feet of these, the silver knob could, by a screw-arrangement 

 on the iron rod, be brought as near to the plate as was desired. 

 It was then only necessary to press the head of the iron rod 

 (which at the same time contained the polar wire), in order by a 

 very brief contact between the silver knob and the plate to intro- 

 duce the luminous arc. The spiral spring immediately jerked 

 the silver knob back to its previous position. 



The arc thus produced, which was maintained very perfect 

 with a battery of 36 Grove's elements, I now carried over conti- 

 nually new points, by holding the caoutchouc cork in my hand 

 and making the three insulated supports glide over the plate. 

 Iu this manner the end of the arc was always formed by a fresh 

 point ; so that, on the part of the silver that had once gone 

 over, repeated action was not possible. With respect to the cop- 

 per itself, it was to be expected that nowhere would any material 

 disintegration take place ; for the heating was distributed more 

 uniformly over the considerable mass of the entire plate, which 

 never became so hot that the hand could not have been held for 

 a long time in contact with it. Indeed the result was this — 

 that never was the slightest trace of copper found on the silver 

 knob, and scarcely anywhere on the plate was a layer of cu- 

 preous oxide to be perceived, which otherwise very readily forms 

 on heated copper. The copper, then, remained substantially 

 intact ; and thus the conditions prescribed for the experiment 

 were fulfilled. 



The one-sided disintegration of the silver electrode in these 

 experiments manifested itself in a form a little complicated. 

 First, following the course of the arc, a stripe of bright metallic 

 silver was sharply impressed into the copper ; but this was suc- 

 ceeded by a rather thin layer above and on each side of it, and 

 more or less impressed, of black powder, which, cautiously re- 

 moved from the plate and analyzed, proved to be a silver-com- 

 pound, probably an oxide. This pow T der, it seems, forms at the 

 periphery of the arc, and consequently where the air has access. 

 It is therefore deposited on the plate sidewards of the proper 

 course of the arc, and of course, moreover, where at each moment 

 the arc itself has preceded it, consequently above the stripe of 

 metallic silver. Where, however, the arc is interrupted by sud- 



