Intelligence and Miscellaneous Articles. 539 



opposite - each other, exceeds a certain width, as the rarefaction 

 proceeds the positive light, if it does not wholly disappear, is re- 

 duced to a thin layer on the surface of the anode, and the anode 

 *' glows." In rarefied air, which is assumed to be the medium of 

 these experiments, the colour is peach-blossom. If an electrode is 

 made of pieces of different metals, for instance if a disk is used 

 half of aluminium and half of silver, the kathode light is generally 

 distributed over both halves of such an electrode, but shows far 

 greater brightness on the aluminium half. This probably depends, 

 as has already been observed by Hittorf, on the fact that the re- 

 sistance to passage is considerably greater on a silver kathode than 

 on an aluminium one. If such an electrode, hitherto unused, 

 having a perfectly fresh surface is used as the anode, the glow 

 covers the whole of the anode side facing the kathode ; and assu- 

 ming that both halves are symmetrical with respect to the kathode, 

 they show more or less approximately equal intensity. This 

 extension of the glow is also maintained if the discharge is 

 allowed to pass in the same direction, and occurs also if the 

 passage of the current is broken and then re-established in the 

 same direction. But if the current is reversed for a few seconds, 

 so that the compound electrode acts as a kathode, and then the 

 former direction is restored, the glow is bright on the silver half of 

 the anode, but on the aluminium half it is either quite destitute 

 of light, or so pale that special attention is necessary to see 

 the light. The boundary of light exactly corresponds to the line 

 of separation of the two surfaces. The discharge light at the 

 anode accordingly despises that metal which is favoured by the 

 kathode light, and occurs with brightness on that surface on which 

 the kathode light shows the least intensity. 



The phenomenon was also demonstrated also with a series of 

 other electrode surfaces. In an aluminium circular disk of 4 centim. 

 diameter, a star-shaped surface of silver was let in. The glow at 

 the anode was limited to the silver star, the whole surrounding 

 surface being destitute of light. 



If in an aluminium disk, several centimetres in diameter, nume- 

 rous detached silver points are inlaid in circles of j millim. to 2J 

 millim. in diameter, the anode luminosity occurs at all the silver 

 points, while the intermediate aluminium parts remain dark. By 

 arranging the silver points, any given luminous figures can be ob- 

 tained, stars, arabesques, &c. 



If, on the other hand, aluminium parts are let in a silver disk, 

 the whole of the disk is luminous except the aluminium, which re- 

 mains dark. If the anode consists of a chain of alternate links of 

 aluminium and silver, only the silver ones are luminous. Silver 

 when used as kathode in rarefied air soon tarnishes, becoming 

 steel-yellow 7 to steel-blue. 



The author will elsewhere enter more minutely on the pheno- 

 menon described, especially as to the behaviour of other metals and 

 in other gases. — Verhandl. def Plujs. Gesdlschaft Berlin, Dec. 10, 

 181)2; Wiedemann's Annalen, May 18H;3. 



