On the Mechanical Theory of Crookes's Force. 15 



He, likewise, controverts the view that the seat of the elec- 

 tromotive force in gas batteries is the place of contact of 

 metal, liquid, and gas ; but he comes to the conclusion which 

 in the present communication I have declared is not univer- 

 sally valid — that the entire current of the gas battery owes its 

 rise to the dissolved gases. At the same time he does not 

 admit that the gradual falling-ofF of the current of a closed 

 gas battery is to be attributed to polarization coming in, but 

 seeks its cause solely in the diminution of the volume of gas 

 dissolved in the liquid. As, however, he does -not measure 

 the electromotive forces by momentary closings of the circuit, 

 as Gaugain and I have done, but calculates them from the 

 current-intensity observed during a continued closing, and 

 from the resistance, it is not possible from his measurements 

 to distinguish the primary from the secondary actions. That 

 a mixture of this sort has not been avoided is shown also by 

 the proposition at which Mr. Morley arrives : — that the elec- 

 tromotive force of the gas battery is not constant, but increases 

 with the resistance. 



Munich, May 1878. 



II. On the Mechanical Theory of Crookes's Force. 

 By George Francis Fitzgerald, M.A., F.T.C.D.* 



WHEN two surfaces at different temperatures are in pre- 

 sence of one another with a gas between them, there 

 exists a force tending to separate them. The assumption of 

 this force explains a very great number of phenomena, inclu- 

 ding the motion of the arms in Mr. Crookes's radiometers, 

 and the so-called spheroidal state of liquids. That this force 

 was due to some sort of unequal stress in the gas between the 

 two surfaces, was pointed out by Mr. Stoney in the Philoso- 

 phical Magazine, March and April 1876, where he attempted 

 to show that such a state of stress would arise. An attempt 

 to explain the motion of the arms of a radiometer had been 

 made previously by Professor 0. Keynolds ; but his conclu- 

 sion, that it was principally due to evaporation and condensa- 

 tion, is manifestly inadequate to explain a continuous action, 

 such as that in a radiometer ; and the method by which he 

 tried to show that a surface, when communicating heat to gas, 

 is subject to an increased pressure, is open to the overwhelming 

 objection that this increased pressure would be almost instan- 

 taneously transmitted to all parts of the enclosed gas, and so 



* From the Scientific Transactions of the Koyal Dublin Society for 

 October 1878. Communicated by the Author. 



