the Spectra of Argon. 79 



demands a higher tension of electricity in order to start the 

 discharge, no matter how much or how little other resistance 

 is interposed ; but when the glow has once started it is con- 

 tinued by means of a much smaller electromotive force. This 

 is shown by the fact that a Thomson electrostatic voltmeter 

 connected with the terminals of the Geissler tube indicated 

 differences of potential between the ends of the tube of 

 amounts varying from six hundred and thirty volts upward. 

 De La Kue and Miiller, who found no potential-difference 

 between the ends of Geissler tubes, must have been working 

 with discontinuous discharges. 



Crookes's estimate that 27,600 volts is necessary to produce 

 the red spectrum is evidently excessive. 



The introduction of a capacity between the terminals of the 

 Geissler tube, for example two plates of metal sixteen hundred 

 square centimetres in area separated by plate glass one centi- 

 metre thick, made no difference in the red glow so long as the 

 connexions were good and the condenser quiet *. As soon as 

 a spark-gap was introduced, or the condenser began to emit 

 the humming sound peculiar to it, the beautiful blue glow so 

 characteristic of argon immediately appeared. 



If this light is examined by a revolving mirror it is seen to 

 consist of intermittent discharges. The battery charges the 

 condenser to the potential necessary to produce a spark be- 

 tween the terminals of the spark-gap. The discharge of this 

 accumulated electricity is produced in the tube and then the 

 operation is repeated. The time-interval between the dis- 

 charges is evidently a function of the capacity of the condenser, 

 as well as of the electromotive force of the battery and the 

 resistance between it and the condenser. 



The accurate determinations of the potential and current- 

 strength of the intermittent blue discharge is a matter of some 

 difficulty ; and at present we feel hardly in position to make 

 a definite statement regarding these measurements. However, 

 the potential required certainly cannot be greater than two 

 thousand volts — the electromotive force of the battery which 

 will easily produce the blue glow. Here, again, Crookes's 

 estimate of far above 27,000 volts was very much too large. 



Since it is necessary to employ a condenser to produce the 

 blue spectrum of argon, we were led to examine the electrical 

 conditions which are necessary for the discharge, hi the 

 circuit with the tube containing argon, between the tube and 

 one of the plates of the condenser, we first interposed a small 

 coil of about eight ohms resistance, having a sell-induction of 



* SirW. Thomson (Lord Kelvin), Tapers od Electrostatics and Mag- 

 netism ' (Macmillan, 1872, p. 236). 



II 2 



