produced in Air by Ultra-Violet Light. 491 



The piece of fluorite used in the experiment was the only 

 piece which would transmit ultra-violet light capable of producing 

 positive ions in any quantity. Many other pieces cut fi'om different 

 crystals seemed opaque to the radiation producing the positive 

 ions, but in every case the negative (surface) ions were obtained 

 though in less quantity than before. In this connection Lyman's 

 work shows that the limit of the spectrum transmitted by fluorite 

 varies considerably in different specimens. The shortest wave 

 length transmitted was X 1230. These results suggest that the 

 ionisation of air by ultra-violet light sets in at some wave length 

 between X, 1230 and A, 1450 and increases very rapidly with 

 decreasing wave length. 



I have great pleasure in thanking Professor Sir J. J. Thomson 

 for suggesting the investigation to me and for his interest in the 

 course of the work. 



