Sir William Thomson on Vortex Statics. 97 



2nd. Instead of the relative chemism of the chlorine atom, 

 as estimated by its iodine-liberating power under these condi- 

 tions, being 1*5 in all three cases, it is 6 with the electric and 

 10 with the magnesium light. The examination of these dif- 

 ferences, as related to the actinic forces of different sources of 

 light and to different acid and basic radicals, is being investi- 

 gated further. 



Finally, the influence of absorbing media upon the in- 

 visible rays of the sun and electric light were determined — 

 the comparison-tubes being surrounded by a thickness of 

 3 cubic metres of ammonio-sulphate of copper, neutral 

 potassium, chromate, and fuchsine, each solution being brought 

 to the apparent degree of translucency for the blue, yellow, 

 and red respectively. The exposure to the sun was from 9.30 

 a.m. to 5*30 p.m., to the electric light 20 minutes; but both 

 are reduced to the interval of 1 hour. 



Blue. H 2 S0 4 + KI 

 HC1 + „ 



Yellow. H 2 S0 4 + KI 

 HC1 + „ 



Bed. H 2 SO, + KI 

 HC1 + „ 



Sun. Electric light. 



0-74 1-80 



1-12 6-75 



0-11 0-00 



0-25 0-125 



0-28 0-60 



0-56 2-25 



These figures show that the selective action of absorbing 

 media upon the invisible rays of different illuminants varies 

 greatly, and suggests the employment of this method for the 

 mapping out of absorption-spectra for the actinic portion. 



Stevens Institute of Technology, 

 June 1880. 



XV. Vortex Statics. By Sir William Thomson*. 



THE subject of this paper is steady motion of vortices. 

 1. Extended definition of " steady motion." The motion 

 of any system of solid or fluid or solid and fluid matter is said 

 to be steady when its configuration remains equal and similar, 

 and the velocities of homologous particles equal, however the 

 configuration may move in space, and however distant indivi- 

 dual material particles may at one time be from the points 

 homologous to their positions at another time. 

 2. Examples of steady and not steady motion : — 

 (1) A rigid body symmetrical round an axis, set to rotate 

 round any axis through its centre of gravity, and left free, 



* From the Proceedings of the Royal Society of Edinburgh, Session 

 1875-76. Communicated by the Author. 



