ELECTRIC TRANSMISSION IN VACUA. 147 



accompany the discharge, strongly resembling the cor- 

 uscations of the aurora polaris. Hence tubes, used for 

 this purpose, have been called aurora tubes. 



GEISSLER TUBES Improved tubes of this 

 kind, called from their inventor Geissler tubes, 

 are constructed with fine platinum wire sealed 

 into their extremities; the points projecting 

 inwards, and loops formed outside for the 

 attachment of conducting cords or wires. The 

 glass is bent into a variety of graceful curves 

 and folds: small tubes, bent in this manner, 

 being inclosed, for protection, in large straight 

 ones; and thus long, frail tubes are reduced to 

 compact, convenient forms, in which they can 

 be safely handled, as shown in Fig. 44. 



The air is exhausted from them by a mercury 

 pump, after which they are hermetically sealed. 

 The expansion of the fine platinum wires being 

 very slight and nearly the same as that of the 

 glass, is not sufficient to cause fracture, hence 

 the vacuum produced in well-constructed tubes 

 remains permanent for years. 



Beautiful fluorescent effects are obtained by 

 constructing such tubes of uranium glass. Sim- 

 ilar effects are also obtained by introducing into 

 them various solids and gases; as sulphate of 

 quinine, fluoride of boron, fluoride of silicon, 

 iodine, hydrogen, and nitrogen ; which give 

 certain characteristic colors, when subjected to 

 electric action. ^ g . 43- 



The effect of the discharge is greatly increased Tube - 

 if a break be made in the connection between one end 

 of the tube and the machine, so as to introduce a short 



