569 



I have been able to corroborate the validity of this experimenter's 

 results in every respect — both on nse of concentrated and of 

 diluted sulphuric acid. A digressing l)ehaviour is shown only by ty|)ical 

 valve metals (e.g. Al and Ta). These emit a white or bluish light. 



For so far as could be ascertained, the spectrum of the yellow 

 light is continuous; often the Na-D-line is still to be observed. After 

 the experiment the electrodes surface is bright and smooth, but 

 the electrode-diameter is mostly slightly diminished. The white light 

 from valve metals is continuous, but on the boundary electrode-electro- 

 lyte-air sparks often appear then, which certainly emit line-spectra. 



The terminal voltage during the yellow luminescence (in Cu, Fe, 

 Mo, Wo, Ni, C) is about 100 Volts, the intensity of the current 

 some tenths of an Amp., i.e. on use of wire-eleclrodes of a diameter 

 of some mm., which were immersed 1 — 2 cm. deep. The temperature 

 of concentrated sulphuric acid then rises very rapidly to the boiling- 

 point, that of diluted sulphuric ncid (smaller intensity of current) 

 somewhat moie slowly. When once the boiling-point temperature has 

 been reached, the colour suddenly changes from goldish to brilliant 

 blue; at the same time the current is reduced to less than 0,1 Amp., 

 the terminal voltage rising to the total value available (225 Volts). 

 Then the well known arc-spectium of iron or tungsten is seen in 

 the spectroscope. This experiment is very suitable for demonstration. 

 Analogous phenomena can most probably also be obtained in other 

 metals, though less easily. 



^ 8. The appearance of an anodic arc of light particularly at hot 

 anodes, is, indeed comprehensible; the yellow luminescence is. how- 

 ever, less easy to understand. A purely thermal emission of the 

 metal cannot be supposed. Nor can there be any question of a 

 reaction luminescence, since the light always possesses the same 

 colour, no matter what anode material is used. Von Boi.ton 

 suggested that the anode gets covered with "a yellow incandescent" 

 oxygen mantle. In fact oxygen can be brought to an emission of a 

 yellow conlinuous light by an electric current at higher pressure 

 (13). At lower pressure a maximum of intensity in the green or 

 yellow green occurs in this continuous spectrum. It may, therefore, 

 be assumed as very probable that our electrodes are surrounded 

 by a mantle of oxygen generated electrolytically, in which the gas 

 is brought electrically to light emission under pretty high pressure. 

 At higher temperatures the pressure in this oxygen layer must 

 diminish, perhaps the layer must become quite unstable, and linally 

 conditions are reached which give rise lo a metal arc of ligiit. 



