XXVIII. On the forces caused by the communication of heat between a surface and a gas; and on a new photometer

In a paper read before the Royal Society, April 1874, I pointed out that the communication of heat from a solid surface to a gas, whether accompanied by evaporation or not, must, according to the kinetic theory, be attended by a reactionary force equivalent to an increase in the pressure of the gas on the surface, and, conversely, when heat is communicated from the gas to the surface the pressure against the surface is diminished; and I also suggested that these forces are the probable cause of the motion, resulting in some way from radiation, which Mr. Crookes had brought into such prominent notice. Since the publication of this paper neither my conclusions as to the existence of these “heat reactions,” nor the reasoning by which I supported them, have been controverted or even questioned; but, on the other hand, they have received important confirmation. The results at which Professors Tait and Dewar arrived after a careful investigation fully bear out my conclusions, not only as to the existence of the forces, but also as to the way in which they explain Mr. Crookes’s experiments.