LMCLENNAN-EDWARDS] ABSORPTION SPECTRA OF MERCURY 



169 



aluminium terminals in water as will be seen from the second spectrum 

 in the jfigure gives a continuous spectrum of remarkable extent. It 

 can be obtained with ease down to A = 2150 A°.U. 



The arrangement for producing the Henri spark is shown in Fig. 

 4. The terminals of the induction coil AB were joined to the spark 

 gap at CD and to the inside coatings of two one-gallon Leyden jars 



Figure 4 



EF. The outside coatings of these jars were joined to two rods of alu- 

 minium MN. These rods constituted the terminals of the spark 

 gap which was the light source and they were short circuited by a coil 

 of small self-induction GH, 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 they were held were provided with threads which enabled one 

 to readily alter the distance between the sparking points. When the 

 spar ku was in action the terminals MN were immersed to a depth of 

 about 5 cms. 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 focussed with a cylindrical quartz lens upon 

 the slit of the spectrograph. The third spectrum in Fig. 3 is the 

 mercury vapour absorption spectrum taken with the light from the 

 Henri spark. In the region below A = 2150 A°.U. it will be seen 

 there is complete absorption. The asymmetrical nature of the ab- 

 sorption at A = 2536 • 72 A°.U. is also brought out. At A = 2338 A°.U. 

 it will be seen that the absorption is complex, and consists of four 

 bands, one extending from A = 2313 A°.U. to A = 2320 A°.U., one at 

 A = 2322 A°.U. another at A = 2326 A°.U. and a wider one between 

 A = 2330 A°.U. and A = 2338 A°.U. The absorption moreover is 

 strong and sharply edged on the longer wave side but it weakens out 

 and is less clearly defined on the side of the shorter waves. 



It will be noted the third spectrum in Fig. 3 also shows a narrow 

 absorption band at A = 2288 A°.U. As cadmium vapour has an 

 absorption band at this point its occurrence in this photograph was 

 ascribed to the presence of a trace of cadmium in the mercury. 



