244 Professor J. J. Thomson [April 13, 



one who is a member of Maxwell's University, could pass over in 

 silence the death of Hertz. 



When Hertz began his magnificent experiments on electric 

 oscillations, there were many theories of electrical action. When he 

 had finished them there was only one, Clerk Maxwell's. 



Hertz's work was done with very much quicker vibrations than 

 those produced by the apparatus now on the screen ; this, however, 

 gives rise to currents through the coil changing their direction some 

 million times a second. If we place in the coil an exhausted bulb 

 the bulb in reality will be the secondary of an induction coil, and 

 will be exposed to electromotive forces tending to produce circular 

 currents parallel to the plane of the coil. 



I will now place a bulb inside this coil, and you see that a 

 circular ring discharge passes through it, and this discharge passes 

 entirely in the gas. 



The gas in the bulb now in the coil is the vapour of silicon 

 tetrachloride ; it happens to be the bulb which gives a brighter ring 

 than any others I possess. 



If this ring discharge passes through air at different pressures, 

 the colour of the discharge changes very considerably. The first 

 bulb I put in was at fairly high pressure, about yL of a millimetre 

 or so. I will now put in another at a lower pressure, and then one at 

 a still lower pressure. Mr. Newall, who has been working at the 

 spectra of these discharges, finds that at the pressure in the first bulb 

 the spectrum is due to nitrogen ; at the second stage it is due to 

 mercury vapour ; the bulb was pumped by a mercury pump, so that 

 there is in the bulb a certain quantity of mercury vapour. 



The apple-green colour in the more highly exhausted bulb is due 

 to some compound of sulphur, which has got into the bulb from the 

 sulphuric acid used to dry the gas. Mr. Newall finds that if the 

 ordinary discharge from a coil between electrodes is taken in such a 

 bulb, there is no trace of this sulphur spectrum. He has also found 

 that when the bulb is at a pressure intermediate between what I 

 may call the mercury and the sulphur stage, when the mercury 

 and sulphur lines are both visible, these sets of lines come from 

 different layers, the sulphur lines coming from a layer nearer the 

 surface than the other. 



If we take the discharge through a bulb containing oxygen you 

 see that the ring discharge is succeeded by a bright glow ; at first the 

 colour is somewhat opaque, but gradually gets more transparent and 

 changes colour. This gives a continuous spectrum crossed by a 

 few bright lines. If we take the discharge through cyanogen you 

 see that the glow is even more persistent than the oxygen, though it 

 is not so bright ; all the gases which show this glow belong to the 

 class of substances which polymerise — that is, whose molecules can 

 combine with each other. I imagine that what takes place in bulbs 

 filled with these substances is that the discharge produces a polymeric 

 modification, and that this gradually returns to its original state, 



