80 Messrs. Trowbridge and Richards on 



•015 of u henry. The blue glow changed to the red glow. 

 We then modified the self-induction, and discovered that even 

 the self-induction of the leads to the tube, which consisted of 

 a few feet of uncoiled wire, undoubtedly modified the blue 

 discharge, for an amount of induction equivalent to +'000051 

 henry had a marked effect in diminishing the brilliancy of 

 the blue glow. 



A comparatively small ohmic resistance substituted for the 

 impedance of the self-induction between the tube and one 

 plate of the condenser produced precisely the same effect as 

 this coil, causing a complete transformation from blue to red. 

 The change from blue to red is so marked that a tube of argon 

 may well serve as an inductometer of some sensitiveness, as 

 well as a means of comparing the influence of self-induction 

 with ohmic resistance. The effect of impedance or resistance 

 must be to prolong or damp the oscillations. Indeed the 

 resistance of the tube itself may be so great as to damp the 

 oscillations without the need of the introduction of outside 

 resistance or self-induction ; therefore argon at high tension 

 gives the red glow with a condenser and rate of oscillation 

 which are quite capable of producing the blue glow in a tube 

 of lower tension. 



Kayser* criticises Crookes's statement that a condenser 

 and a spark-gap are necessary for the production of the blue 

 spectrum. He finds that with a lower pressure in the tube 

 than 2 milliin. the other spectrum can be readily obtained 

 without condensers and spark-gaps. He also states that it is 

 much easier to produce the pure blue spectrum than the pure 

 red. In order to obtain the red spectrum the strength of the 

 current must be adapted to the gas pressure. Kayser em- 

 ployed an induction-coil. The condenser, however, in the 

 primary of an induction-coil sends an oscillatory discharge 

 through the secondary. Although Kayser did not employ a 

 condenser in the manner recommended by Crookes, he still 

 had a condenser in his electrical system, and the resistance of 

 his Geissler tube was probably so proportioned that the secon- 

 dary circuit was in resonance with the primary circuit. To 

 prove this w r e placed a tube containing argon across the ter- 

 minals of the secondary of an induction-coil, and having 

 removed the condenser attached to the primary we sent a 

 discharge through the tube by means of the break in the 

 induction-coil. The light of the discharge was red, and wdien 

 it was examined by a revolving mirror no trace of blue was 

 seen in the capillary portion of the tube. An adjustable 



* Sitzungsberichte der kdniglich preussischen Altad. der Wissenschaften 

 zu Berlin, May 7, 1896. 



