Ultra-violet Light in different Gases. 511 



the spectrum. It has been found that air absorbs all light 

 of wave-length below about 185 fifi, but that it is highly 

 transparent to light of wave-lengths about 200 fi/uu. Quartz 

 also absorbs light of shorter wave-lengths than this, though 

 hydrogen is transparent to light of wave-length as short as 



100 flfJL. 



The method used in these experiments was to interpose a 

 brass vessel, V (see Fig. 1), 10 cms. in length and 4 cms. in 

 diameter, closed at either end by a quartz window, which 

 served to contain the various gases, between the source of ultra- 

 violet light, L, and an apparatus, A, in which the intensity of 

 illumination was measured. 



The source of ultra-violet light used was a spark between iron 

 terminals in pure dry hydrogen, the arrangement being the same 

 as that described in a previous article by the author, " On the 

 Photo-electric Discharge from Metallic Surfaces in Different Gases" 

 {Tram. Roy. Soc. Vol. 202 A, p. 439, 1903). The iron terminals 

 were connected to the seconclaiy terminals of a 6" induction coil, 

 in parallel with which were also three large Leyden jars. An 

 alternating current of 3'5 amps, were sent through the primary 

 of the coil. 



The intensity of the ultra-violet light was measured by the 

 magnitude of the photo-electric current it produced from a zinc 

 surface, negatively charged, situated 3 mms. behind a fine gauze, 

 which served as the positive electrode. The vessel containing 

 these electrodes was closed in front by a quartz window and was 

 kept exhausted to a pressure of about 15 mms. The zinc was 

 usually charged to a potential of — 40 volts by means of a battery 

 of small secondary cells. 



The gauze was connected to one pair of quadrants of an 

 electrometer, and the photo-electric current measured by noting 

 the deflection obtained when the zinc was illuminated for 10 sees., 

 the method used being that described in the paper " On the 

 Photo-electric Discharge," quoted above. 



A quartz lens, Q, was used to throw a parallel beam of rays 

 through the vessel V on to the zinc electrode of the leak 

 apparatus A. 



This method of measuring the intensity of the light does not, 

 of course, admit of the absorption of light of any one particular 

 wave-length being measured, but only the absorption of that light 

 which produces the photo-electric discharge from the zinc surface. 

 The method is, however, obviously capable of modification and 

 extension, but for the purpose in view the above is all that is 

 required. 



The results obtained for each gas are given below. 



