574 Messrs. Petavel and Hutton on the 



The two iron rods soon became heated throughout the greater 

 part o£ their length and a fused bead of metal occasionally 

 droi)ped from the end of the smaller one. This glow, which 

 was of a bright yellow colour, was very luminous and easy to 

 photograph. The electromotive force required seemed to be 

 practically independent of the distance apart of the electrodes. 

 During most of the experiments this distance was betwc^en 

 1 and 2 inches, at which length the current remained 

 fairly constant. The E.M.F. was between 550 and 650 volts 

 on the terminals of the alternator, a non-inductive resistance 

 of 45 ohms being placed in series with the arc. The current 

 varied from 6 to 9 amperes. 



The first spectrum was taken in air at a pressure of 17 mm., 

 the pressure being increased for each successive photograph 

 wdthout interrupting the current. The exact values for each 

 experiment are given in the subjoined table referring to 

 Plate XXy. It is worthy of note that after the arc was 

 stopped the pressure fell to half its former value. 



So far as the spectra are concerned, the most noticeable 

 difference from that usually observed lies in the fact that so 

 few of the lines come out with any marked intensity. With 

 the relatively large power used the entire discharge seems to 

 be taken by the iron vapour, the gas-spectrum being altogether 

 absent. Direct measurement and comparison with the 

 ordinary iron arc shows that the simplification already 

 noticeable in the self-induction spark is carried still further in 

 the case of the glow- discharge, for whereas a small number 

 of the brighter iron lines retain their intensity unimpared, 

 the greater number are considerably diminished or are 

 altogether absent. This is particularly remarkable in view 

 of the recent work of de Watteville"^, who has shown that 

 the spectrum of iron as obtained in the cone of the Bunsen 

 flame presents an appearance, so far as the relative intensity 

 and general character of the iron lines is concerned, essentially 

 similar to that seen in the iron spark with self-induction, and 

 thus is still much more complicated than the one with which 

 w^e are now dealing. Although most of the iron lines found 

 by Hartley f in the oxy-hydrogen flame-spectrum occur also 

 in the glow-spectrum, the intensities and appearance are widely 

 diff'erent and no systematic connexion between the two could 

 be found. 



Broadly speaking the effect of increase of pressure within 

 these limits (17 to 105 mm.) is to still further simplify the 

 spectrum. At the higher pressures, however, the first traces 



* C. de Watteville, Coinptes Rendus, vol. cxxxy. p. 1329 (1902). 

 t Hartley, Pliil. Trans. 1894, A. p. 161. 



