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VI. . On the Absorption-Spectra of Bromine and of Iodine Monochloride. 
By H. E. Roscoe, F.B.S., and T. E. Thorpe, F.B.S. 
Received March 16, — Read May 4, 1876. 
[Plate 4.] 
The element bromine and the compound iodine monochloride possess nearly the same 
molecular weight. The colours of their vapours appear almost identical, and a rapid 
glance at the complicated absorption-spectra afforded by the two gases fails to detect 
any difference between them. It becomes, therefore, a matter of importance to ascer- 
tain whether the molecules of the two bodies, when gaseous, vibrate identically or 
similarly. We have accordingly undertaken a series of exact measurements of the 
absorption-spectra of these two gases, the results of which we have now the honour to 
communicate to the Royal Society. 
The two spectra were compared simultaneously by means of one of Kirchhoff’s 
model spectroscopes, 4 flint-glass prisms of 60° and 45° and a magnifying-power of 40 
being employed. The position of the lines and of the well-defined edges of the bands 
in both spectra were read off by reflexion upon a fixed arbitrary scale ; the positions of 
27 air-lines lying between the extremes of the absorption-spectra were then determined 
upon the same scale, both the scale and prisms remaining untouched during the different 
series of observations. In order to determine the wave-lengths of each of the absorp- 
tion-bands in the spectra of bromine and iodine monochloride, the wave-length of each 
of the 27 air-lines was ascertained by reference to the measurements of Thalen ; in 
three cases, marked H, in the accompanying Table the numbers given by Huggins have 
been used. * 
Colour of ray. 
Micrometer- 
scale reading. 
Wave-length, 
Colour of ray. 
Micrometer- 
scale reading. 
Wave-length. 
f 
42*48 
6602 
r- 
53-06 
5666 
Red -i 
1 
42-96 
6562 
54-81 
5549 
l 
43-61 
6480 
55-00 
5541 
1 
Orange ...«^ 
1 
46-81 
49-30 
49-47 
49-54 
49-68 
6171 
5949 
5942 
5932 
5929 
Yellow ...< 
55-09 
55-17 
55-25 
55-61 
55- 86 
56- 18 
5534 
5530 
5524 H. 
5495 
5479 
5462 
51-68 
5767 
56-29 
5453 
51-96 
5745 
58-10 
5350 H. 
Yellow ...< 
52-48 
52-83 
5711 
5686 
Green ■ 
f 
58-27 
5339 
52-88 
5680 H. 
58-59 
5320 
53-00 
5675 
2 g 2 
