242 DT.SCHAKGE OF ELECTKICITY. 



before the bulb got Lot each biiylit discbarge Avns .succeeded by ii briglit 

 after- glow, but as the bulb got hotter midhotter the glow became faiuter 

 and faiuter, and at last ceased to be visible, though the bright riug 

 was still produced at each discharge of the jar. Wheu the Buiiseu 

 was takeu away aud the bulb allowed to cool the glow re-ai)])eared. 



The spectrum of the after-glow is a continuous spectrum, in which 

 I could not detect the super-position of any bright lines. The only gas 

 beside oxygen in which I have been able to detect any after-glow is air, 

 though in this case the range of x>ressure within which it is exhibited 

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

 get a bulb lilled 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 si)ectrum, though they 

 could see the bright lines clearly enough, were of opinion that there 

 was nothing else visible. I endeavored to photograph it, but without 

 success, so that the existen{ie 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 decomposed by the discharge. 



Phosphorescence produced by the discharge. — The discharge without 

 electrodes produces a very vivid phosphorescence in the glass of the 

 vessel in which the discharge takes place; the phosphorescence is green 

 when the bulb is made of German glass, blue wheu it is nmde of lead 

 glass, "N^ot only does the bulb itself i)horphoresce, but a piece of ordi- 

 nary glass tubing held outside the bulb and about a foot from it ])hos- 

 phoresces brightly; while uranium glass will phosphoresce at a dis- 

 tance of several feet from the discharge. Similar effects, but to a 

 smaller extent, are produced by the ordinary si)ark between the poles 

 of an electrical machine. 



The vessel in which the discharge takes place may be regarded as 

 the secondary of an induction coil, and the discharge in it shows similar 

 properties to those exhibited by currents in a metallic secondary. Thus 

 no discharge is produced unless there is a free way all round the tube; 

 the discharge is stopped if the tube is fused up at any point. In order 

 that tlie discharge may take place, it is necessary that the molecules 

 of the gas shall be able to form a closed chain without the interposition 

 of any non-conducting substance; indeed, the discharge seems to be 

 hindered by the presence in such a chain of any second body, even 

 though it may be a good conductor of electricity. Thus, when a tube 

 such as that in Fig. 7 is used, which has a barometer tube attached 

 to it, so that by raising or lowering the vessel into which the tube 

 dips a mercury pellet may be introduced into the discharge circuit, 



