82 The Spectra of Argon. 



are connected with the terminals of an electrical machine the 

 pure red spectrum is obtained. If a spark-gap is interposed 

 in such a manner that a condenser charged by the machine 

 can discharge through the tube, the blue discharge imme- 

 diately results. The condenser discharge oscillates through 

 the gas. 



The oscillatory discharge of the condenser is evidently an 

 important factor in producing the blue spectrum of argon. 

 According to Lord Kelvin's law, if R denotes the resistance 

 of the circuit, L the self-induction, and C the capacity of the 

 circuit, the discharge of the condenser becomes non-oscillatory 



when R > \f __ _. It may be, therefore, that an estimate of 



the resistance in the tube can be obtained by measuring the 

 self-induction which is required to change from the blue dis- 

 charge to the red. 



When the tube containing argon at a suitable pressure is 

 brought near a Hertz oscillator giving a rate of about 

 115,000,000 oscillations per second, it immediately shows the 

 blue colour. In this case the oscillator consisted of two zinc 

 plates about 40 centim. square with a spark-gap between 

 them. The capacity and impedance of the circuit were ex- 

 tremely small. 



The extreme sensitiveness of an argon tube to oscillatory 

 discharges leads us to believe that it will be of great use in 

 the study of wave-motions of electricity. As we have seen, 

 it is competent to show when the Hertz oscillator is working 

 properly, that is, sending forth electrical oscillations and not 

 unidirectional discharges. The change of colour in the tube 

 from red to blue is so marked that an argon tube reveals 

 what is not shown in a conspicuous manner by other gases. 

 We have thought this remarkable property of an argon 

 tube worthy of being distinguished by a name which might 

 describe it, and we have therefore called an argon tube fitted 

 for the study of electrical waves a Talantoscope {raXdvTwaL^) . 



In an oscillatory discharge the molecules receive powerful 

 electrical impulses of opposite sign. These impulses are 

 separated, it may be, by one millionth of a second. It is 

 significant that the shorter wave-lengths of light accompany 

 these electrical oscillations. It is our purpose to extend our 

 study of the effect of electrical oscillations through more 

 highly rarefied media in which arise the Rontgen rays. 

 These rays are probably highly modified by the oscillatory 

 discharge. A battery of a large number of cells now at our 

 command will afford the best means of studying this subject ; 



