The Attraction and Repulsion caused by the Radiation of Heat

WILL you allow me to say a few words in reference to the report of Mr. Crookes's paper which appeared in NATURE, vol. xi. p. 494. Apparently Mr. Crookes does not understand the nature of the forces which I have shown to result from the communication of heat between a gas and a surface; otherwise he would not bring forward as conclusive against the supposition that the phenomena which he has discovered are due to these forces, experiments which show entirely the other way. As I have previously explained, it follows as a direct result of the kinetic theory of gas, that if such forces as I have supposed exist for a certain tension of the gas surrounding the surface, they will not be diminished by diminishing the tension of the gas; and consequently no amount of pumping would destroy such forces where they once existed. Whereas the smaller the tension of the gas the freer the surface will be to move, and the less its motion would be opposed by convection currents; hence, on the supposition that the motion is due to these forces, the only effect of improving the vacuum would be to intensify the action. An this being the case, it is clear that Mr. Crookes's experiments, in which he finds that the action still remains in the most perfect vacuum which he has obtained, tend to support and not to upset my conclusion that the actions are due to these forces. The fact that Mr. Crookes finds it impossible to conceive this only shows, as I have said, that he does not comprehend the nature of the forces; for it certainly presents no greater difficulty than the fact that the velocity of sound is independent of the tension of the gas through which it is transmitted.