through Exhausted Tubes without Electrodes. 335 



discharge, the spectrum is very much like that of the 

 ordinary jar-discharge in air. When, however, the pressure is 

 so low that the discharge passes with difficulty, a few lines in 

 the spectrum shine out very brightly, whilst others become 

 faint, so faint indeed sometimes that if the air-spectrum were 

 not thrown into the field of view of the spectroscope at the 

 same time, they might pass unnoticed. Three lines which are 

 very persistent, the first a citron-green, the second a more 

 refrangible green, and the third a blue, I am inclined to 

 think must be due to mercury vapour from the pump. 



I am indebted to Prof. Liveing for the loan of a very fine 

 direct-vision spectroscope, and to him and Mr. Robinson, of 

 the Cambridge Chemical Laboratory, for valuable advice in 

 the attempts which I made to photograph the spectra of 

 some phosphorescent glows mentioned below. 



I should like to call attention to the advantages for spectro- 

 scopic purposes which attend this method of producing the 

 discharge ; it is easily done either by an ordinary electrical 

 machine or an induction-coil. An intensely bright discharge 

 is got, and there is no danger of complication arising from 

 the spectrum of the gas getting mixed with that of the 

 electrodes. 



Discharge in Oxygen. 



By far the most remarkable appearance is presented when 

 the discharge passes through oxygen, for in this gas the 

 bright discharge is succeeded by a phosphorescent glow 

 which lasts for a considerable time; indeed, with a strong 

 discharge it may remain visible for more than a minute. 

 When the discharges succeed one another pretty rapidly, the 

 phosphorescence is so strong that it hides the successive 

 bright discharges, and the tube seems permanently full of a 

 bright yellow fog. We can thus by the use of this gas con- 

 vert the intermittent light given by the bright discharge into 

 a continuous one. 



Perhaps the most striking way of showing this phosphor- 

 escence is to use a long tube, about a metre long and 6 or 

 7 centimetres in diameter, with a bulb blown in the middle, 

 the primary coil being twisted round this bulb. Then, when 

 the sparks pass between the jars, a bright ring- discharge 

 passes through the bulb, from which, as if shot out from the 

 ring, the phosphorescent glow travels in both directions along 

 the tube, moving slowly enough for its motion to be followed 

 by the eye. It cannot, therefore, be produced by the direct 

 action of the light from the spark on the gas in the tube, for 



