V.—ON THE MECHANICAL THEORY OF CROOKES’S FORCE, 
BY 
GEORGE FRANCIS FITZGERALD, M.a., F.7.C.D. 
[Read March 18th, 1878. | 
When two surfaces at different temperatures are in presence of one another with a 
eas between them, there exists a force tending to separate them. The assumption 
of this force explains a very great number of phenomena including the motion of 
the arms in Mr. Crookes’ 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 Phil. Mag., 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 O. Reynolds, but his conclusion, that it was principally due to evaporation 
and condensation, is manifestly inadequate to explain a continuous action, such as 
that ina radiometer, and the method by which he tried to show that a surface 
when communicating heat to gas is subject to an increased presure, is open to the 
overwhelming objection, that this increased pressure would be almost imstant- 
aneously transmitted to all parts of the enclosed gas, and so could not possibly be 
the source of such a force as would explain the motion of the arms of a radiometer. 
Inamplificationof a letter I wrote to “ Nature ” onthe 17thof December, 1877, and 
which was published on the 4th of January, 1878, I now intend to prove that such 
a state of stress as Mr. Stoney’s theory requires, would exist under the assumed 
conditions. My letter contains a proposed application of Clausius’ investigation for 
finding the conducting power of a gas, as published inthe Philosophical Magazine, Vol. 
93, 4th Ser. . Mr. Stoney, in a paper read before the Royal Dublin Society, on Mon- 
day, the 18th of February, 1878, has obtained results somewhat like those obtained 
by my method, by applying a method similar to one he originally employed. 
I may first observe, that the only way in which a state of other than uniform 
stress can exist in a gas, is by the distribution of the mean velocities, and number 
of molecules being different in different directions, or, as Mr. Stoney has called it, 
by the gas being polarized. That the distribution is not uniform when heat is being 
conducted through a gas, has been pointed out long ago by both Clausius and 
Maxwell, and what is required, is to show that the distribution will then be such 
as to develope a force like Crookes’. 
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