Spectrum of Mercury, Cadmium, and Zinc Vapours. 697 



X = 1849*6 A.U. was the most strongly absorbed by o mercury 

 vapour. That in the neighbourhood o£ \= 2536*72 A.U. came 

 next, while high vapour densities were required to bring out 

 the absorption at \=2338 A.U. 



In the third experiment a large Hilger quartz spectrograph 

 was used. With this instrument the arc spectrum of mercury 

 from a quartz lamp was first taken, then the spark spectrum 

 from aluminium terminals in water after the manner devised 

 by Henri*, and then the spectrum of the light from the 

 spark between these aluminium terminals in water, after 

 it had passed through a heated clear fused quartz evacuated 

 tube containing mercury vapour of high density. These 

 three photographs are shown in fig. 3. The spark from 

 aluminium terminals in water, as will be seen from the 

 second spectrum in the figure, gives a continuous spectrum 

 of remarkable extent. It can be obtained with ease down 

 to\ = 2150A.U. 



The arrangement for producing the Henri spark is shown 

 in fig. 4. The terminals of the induction-coil AB were 



Fig*. 4. 





\ 1 



1 



G 





A 



J 



1* 



fg 



L 





1 





H 



1 N 



B 



I ! 





joined to the spark-gap at CD and to the inside coatings of 

 two one-gallon leyden-jars EF. The outside coatings of these 

 jars were joined to two rods of aluminium MN. These rods 

 constituted the terminals of the spark-gap which was the 

 light source, and as shown they were short-circuited by a 

 coil of small self-induction (tH. The aluminium terminals 

 MN were rods about 1 cm. in diameter. They were conically 

 pointed, and were held clamped in a vertical plane inclined 

 at 45° to each other. The clamps in which tbey were held 

 were provided with threads which enabled one to readily 

 alter the distance between the sparking-points. When the 

 spark was in action the terminals MN were immersed to a 

 depth of about 5 cm. in a vessel of water. The light from 

 the spark passed through the water and out of a quartz 

 window sealed into the side of the vessel. It was then 



* Henri, Phys. Zeit. No. 12, p. 516, June 15th, 1913. 

 Phil. Mag. S. 6. Vol. 30. No. 179. AV. 1915. 2 Z 



