1894.] 



on the Work of Hertz. 



329 



violet, surprisingly far beyond the limits of the visible spectrum* 

 (Fig. 9). m 



This is rather a digression, but I have taken some pains to show- 

 it properly because of the interest betrayed by Lord Kelvin on this 



V 



JAt:-1 



Fig, 



Zinc Knob discharging Negative Electricity in the very Ultra-violet Light 

 of a Spectrum formed by a Quartz Train. Right position of Knob shown. 



matter, and the caution which he felt about accepting the results of 

 the Continental experimenters too hastily. 



It is probably a chemical phenomenon, and I am disposed to 



* While preparing for the lecture it occurred to me to try, if possible during 

 the lecture itself, some new experiments on the effect of light on negatively 

 charged bits of rock and ice, because if the effect is not limited to metals it must 

 be important in connection with atmospheric electricity. When Mr. Branly 

 coated an aluminium plate with an insulating varnish, he found that its charge 

 was able to soak in and out of the varnish during illumination (' Comptes Rendus,' 

 vol. ex. p. 898, 1890). Now, the mountain tops of a negatively charged earth are 

 exposed to very ultra-violet rays, and the air is a dielectric in which quiet up- 

 carrying and sudden downpour of electricity could go on in a manner not very 

 unlike the well-known behaviour of water vapour ; and this, perhaps, may be the 

 reason, or one of the reasons, why it is not unusual to experience a thunderstorm 



