696 Prof. McLennan and Mr. Edwards on the Absorption 



to have been examined in detail. In regard to the band 

 noted by Wood at A, = 2140 A.U., especially as it was obtained 

 with high vapour densities, it appeared to the writers that 

 it might be connected with the absorption observed by 

 Kirschbaum at \ = 1849*6 A. U. Some experiments were 

 made by us to test this view, and also to study the character 

 of the absorption band at \ = 2338 A.U., and these will be 

 described in what follows. 



II. Absorption Spectrum of Mercury. 



In the first experiments the light from a quartz mercury 

 arc-lamp was projected through an evacuated clear fused 

 quartz tube containing a little mercury. The mercury was 

 gradually heated, and a series of photographs was taken with a 

 small Hilger quartz spectrograph. A reproduction of one of 

 these photographs is shown in fig. 1, Plate XVI. The upper 

 spectrum is that of the mercury arc alone, the second is that 

 obtained when the quartz tube was moderately heated, and 

 the third is that obtained when the mercury vaponr density 

 was considerably higher. The asymmetrical character of the 

 absorption band is clearly brought out by the photograph. 



In the second experiments a photograph was first taken of 

 the spark spectrum of mercury in air in a manner already 

 described in a previous communication by one of us*. 



Photographs were also taken of the spectrum of the light 

 from the spark between terminals of cadmium in air after it 

 passed through the mercury vapour in the exhausted quartz 

 tube mentioned above. These were taken with gradually in- 

 creasing vapour density, and are shown in fig. 2. In this photo- 

 graph the mercury spectrum is shown at the top w r ell down 

 into the ultraviolet and the strong lines at X= 1942*1 ALT. 

 and \= 1849*6 A.U. are clear and distinct. The succeeding 

 four spectra show that even with small vapour density the 

 absorption was such as to cut off the light of wnve-lengths in 

 the region of X = 1942*l A.U. and \ = 1849*6 A.U. In the 

 second last spectrum, absorption at \ = 2536*72 A.U. can just 

 be detected, but in the last one it is well marked. The 

 absorption band at \=2338 A.U. o also comes out in this 

 spectrum, and that at \ = 1849 A.U. has widened out so 

 that on the side of longer wave-lengths it has reached 

 \ = 2144*0 A.U. From the general appearance of the photo- 

 graph it will be seen that the absorption at \= 1849*6 A.U. 

 develops symmetrically with increasing vapour density. This 

 photograph also shows that light of wave-lengths near to 



* McLennan, Proc. Roy. Soc. A. vol. xci. p. 26 (1914). 



