396 Prof. Challis on a Theory 



surface. The incident light thus produces an abnormal state 

 of the atoms at and near the superficies of the vane, analogous 

 in some degree to the state of the superficial atoms of a body 

 electrified by friction. Such disturbance of the atoms at the 

 surface must be accompanied by change in the interior, having 

 the effect of causing the atoms to be somewhat more crowded 

 together at one part than at another. (See on these points the 

 article on the Force of Electricity in the Philosophical Maga- 

 zine for October 1860, and the Theory of Electric Force in 

 'The Principles of Physics,' pp. 505-555.) With respect to 

 the vanes of the radiometer in their state of continuous pertur- 

 bation as to temperature, it is clear that the crowding of the 

 atoms will be towards the bright or cooler surface, and that 

 there will be an abnormal increment of density, regular or not, 

 from the blackened to the white surface. 



This being admitted, I may proceed to state the theory which 

 I have elsewhere proposed for accounting for the maintenance 

 of the interior gradation of atomic density. I consider that 

 this theory is best given in a " New Discussion of the Hydro- 

 dynamical Theory of Magnetism," contained in the Philoso- 

 phical Magazine for June 1872 ; but it should be noticed that 

 in a magnet the interior gradation of atomic density results 

 from the peculiar constitution of hardened steel, and not from 

 abnormal superficial conditions. The case of our problem is 

 more analogous to the production of thermo-galvanic currents 

 by Seebeck's experiment. In any case, however, of such grada- 

 tion the theory supposes that atomic repulsion acting from denser 

 towards rarer parts is equilibrated by molecular attraction acting 

 from rarer towards denser parts. According to the hydrody- 

 namical theory of Attractive and Repulsive Forces (which is 

 given more completely in the Philosophical Magazine for Sep- 

 tember 1872 than in my previous discussions of it), these forces 

 result from the dynamical action of cctherial undulations — those 

 of small breadth and intensity causing repulsion in the direc- 

 tion of their propagation, and those of which the breadth and 

 maximum intensity are large, causing attraction in the direc- 

 tion opposite to that of propagation. Now in all cases of the 

 equilibration of a congeries of atoms by these opposing forces, 

 when terms including the square of the velocity of the aether 

 are taken into account, the mathematical reasoning (as given 

 in the Philosophical Magazine for June 1872) shows that the 

 aether itself is accelerated by reason of the contraction of chan- 

 nel due to the constancy of magnitude and form of the atoms, 

 and the increment of their number towards the denser parts. 

 These impressed accelerations maintain a steady circulating motion 

 in the midst of the aether, which is always tending, by the inertia 

 of its unlimited mass, to put a stop to such motion. According 



