Messrs. Roscoe and Thorpe on Absorption- Spectra. 467 



Neglecting the slightly conical character of the surface of the 

 water column, and assuming (as the result of experiments in which 

 the motion of a smoke cloud was observed) that the movement 

 of the air was throughout in lines parallel to the axis of the tube 

 along which it flowed, and showing that the pressure does not vary- 

 along the length of the tube, the author proceeds to discuss the 

 hydrodynamic equations expressing the conditions of the problem 

 (the motion of the air being uniform and independent of time), and 

 represents the volume of air A passing through the tube in a second 

 as : — 



A=W 



|_2r 2 



2 (logE-logr) J' 



~W being the weight of water, in grammes, discharged in a second, 

 r the radius of the jet in turns of the micrometer-screw (6*8 

 turns of which correspond to.l centim.), E being the radius of the 

 aspirating tube. 



The results obtained by observation accorded well with those 

 given by this equation, so long as the value of E did not exceed 

 the limit within which the suppositions regarding the motion of 

 the air hold good. 



The question was considered whether the results might not be 

 brought into even closer accord with theory by the assumption 

 that a slipping action takes place between the air and the water- 

 jet on the one hand, and between the air and the tube on the other, 

 instead of the assumption previously made that the air adhered alike 

 to the water and to the tube in its passage. The result of the 

 calculation, however, led to no nearer approximation; and, finally, 

 experiments with other materials for the tube and other gases 

 (namely, coal-gas and carbonic anhydride) were made, without re- 

 sulting in any marked difference from the results obtained with air 

 and glass. 



May 4. — Capt. E. J. O. Evans, E.N., C.B., Vice-President, in the 



Chair. 



The following paper was read : — 



" On the Absorption-Spectra of Bromine and Iodine Monochloride." 

 By H. E. Eoscoe, E.E.S., and T. E. Thorpe. 



The paper contains the results of an exact series of measure- 

 ments 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 molecular weights and colour 

 of the vapours being almost identical. The two spectra, which are 

 both channelled, were compared simultaneously 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 pur* 



2H2 



