from a Zinc Plate by Ultra- Violet Light. 



303 



The above experiments were all performed with a pressure 

 of about 5 mm. of mercury in the detector. As this pressure 

 could be easily reduced to about 1 mm. by means of the 

 Fleuss pump which was used in the experiments, it was thought 

 desirable to find what effect, if any, this change of pressure 

 had on the results. 



D. — The relation between E and I for a pressure of 1 mm. 

 is shown in the following table : — 



r. 



I. E. 



E-M. 



10 

 110-2 



126 211 

 1 1 



1-67 

 1-00 



The variation in E/I is still very marked and for the same 

 range of intensity almost equal to that in C. Too much 

 stress, however, should not be laid on this coincidence 

 inasmuch as the initial intensity of the light is not necessarily 

 the same in both cases. All that can justly be deduced is 

 that in all cases the ratio E/I increases with increase of intensity 

 at a gradually increasing rate. 



Absorption Experiments. 



As the apparatus employed in the preceding experiments 

 was well adapted for investigating the absorption of the light 

 by different gases, the following cases were considered, viz., 

 the absorption due to air and hydrogen of the light produced 

 by sparking between : — 



(a) aluminium terminals in air ; 



(b) iron terminals in hydrogen. 



The method of procedure with a given spark was as follows : 

 One of the tubes of length d was introduced between the 

 detector and the spark-gap and readings were taken : 



(1) when the tube was exhausted ; 



(2) when the tube was filled with air at atmospheric 



pressure. 



(3) when the tube was filled with hydrogen at the same 



pressure. 



If I x I 2 I 3 denote the successive readings, the absorption 

 due to a column of air of length d was assumed to be — j — - . 



