336 Discharge of Electricity through Exhausted Tubes. 



if it were, the glow would travel with the velocity of light. 

 It is necessary to mention this point, for the light from these 

 discharges has great powers of producing phosphorescence. 



The glow seems to consist of gas which has been in the path 

 of the discharge, and whose molecules have been split up by 

 it and projected from the line of discharge. This gas which, 

 when projected, is in a peculiar state, by a process of chemical 

 combination gradually returns to its original condition, and 

 it is while it is in this state of transition from its new condi- 

 tion to the old that it phosphoresces. If this is the case we 

 should expect that the period of phosphorescence would be 

 shortened by raising the temperature. On trying the experi- 

 ment I found that this took place to a very marked extent. 

 A discharge-bulb filled with oxygen at a low pressure was 

 placed over a Bunsen burner ; before the bulb got hot each 

 bright discharge was succeeded by a bright afterglow, but 

 as the bulb got hotter and hotter the glow became fainter 

 and fainter, and at last ceased to be visible, though the bright 

 ring was still produced at each discharge of the jar. When 

 the Bunsen was taken away and the bulb allowed to cool, 

 the glow reappeared. 



The spectrum of the afterglow is a continuous spectrum, 

 in which I could not detect the superposition of any bright 

 lines. The only gas besides oxygen in which I have been 

 able to detect any afterglow is air, though in this case the 

 range of pressure within which it is exhibited is exceedingly 

 small ; indeed it is often by no means an easy matter to get 

 a bulb filled with air into the state in which it shows the 

 glow. The spectrum of the air-glow showed bright lines ; I 

 thought myself that I could see a very faint continuous 

 spectrum as well. Some friends, however, who were kind 

 enough to examine the spectrum, though they could see the 

 bright lines clearly enough, were of opinion that there was 

 nothing else visible. I endeavoured to photograph it, but 

 without success, so that the existence of a continuous spectrum 

 for this glow must be considered doubtful. 



When the discharge passes through acetylene, the first two 

 or three discharges are a bright apple-green, the subsequent 

 ones, however, are white, and as the green discharge does 

 not reappear, we must conclude that the acetylene is decom- 

 posed by the discharge. 



[To be continued.] 



