4 



On Absorption- Spectra. 



[May 4, 



or once and a half tke yoluuie of air in the room. If the inmates have 

 each 1000 cubic feet of space, it follows that either their supply of fresh 

 air is short by 1500 cubic feet per head per hour, or else that there are 

 sources of excessive humidity mthin the air-space which demand im- 

 mediate removal. 



II. On the Effect of Heat on the Chloride^ Bromide^ and Iodide 

 of Silver.^' By G. F. Rodwell, F.B.A.S.., F.C..S., Science 

 Master in Marlborough College. Communicated by Professor 

 Tyndall, F.B.S. Received March 10, 1876. 



III. On the Effects of Heat on some Chloro-brom-iodides of 

 Silver.^^ By G. F. Rodwell, F.R.A.S., F.C.S., Science 

 Master in Marlborough CoUege. Communicated by Frederick 

 GuTHRiE_, F.R.S._, Professor of Physics in the Royal School of 

 Mines. Received April 13_, 1876. 



IV. ''^ On the Absorption- Spectra of Bromine and Iodine Mono- 

 chloride.'^ By H. E. RoscoE, F.R.S., and T. E. Thorpe. 

 Received March 16, 1876. 



(Abstract.) 



The paper contains the results of an exact series of measurements of 

 the absorption-spectra of the vapours of the element bromine and of 

 the compound iodine monochloride, made with the object of ascertaining 

 whether the molecules of these two gases vibrate identically or similarly, 

 their ruolecular weights and colour of the vapours being almost identical. 

 The two spectra, which are both channelled, were compared simultane- 

 ously by means of one of Kirchhoff's 4-prism spectroscopes, the position 

 of the lines being read off by reflection on an arbitrary scale. In order 

 to determine the wave-lengths of these bands, the wave-length of each 

 of 27 air-lines lying between the extremes of the absorption-spectra was 

 ascertained by reference to Thalen's numbers ; whilst for the purpose of 

 reducing the readings of the absorption-bands to wave-lengths a graphical 

 method was employed, the details of which are given in the paper. This 

 method appears to be one of general appHcabihty for the plotting of 

 spectra. 



Tables then follow givmg the wave-lengths of 66 bands of each 

 absorption- spectrum ; and a map accompanies the text in w^hich the bands 



o 



are drawn to a scale one half that of Angstrom's " Spectre JN'ormal." 



A careful comparison of these Tables and of the map show^s that, al- 

 though both spectra contain a large number of lines which are nearly 

 coincident, the spectra as a whole are not identical, either when the 

 vapours are examined at high or low temperatures, or w^hen the lengths 

 of the columns of absorbing gas are varied. 



