radiation; this mass is in general exceedingly small; for example, we find by the application of the rule we have just given that the mass emitted by each square centimetre of surface of a body at the temperature of the sun is only about 1 milligram per year. We should expect that when some of the ether, bound to a body by its lines of force is carried off by the radiation, other portions of ether which will not be connected with the body will flow in to take its place. Thus, in consequence of the radiation which proceeds from all bodies the ether around them will be set in motion in much the same way as if a series of sources and sinks were distributed throughout the bodies.
Though the actual mass of the ether travelling with a wave of light is exceedingly small, yet its velocity is so great, being that of light, that even a very small mass possesses an appreciable amount of momentum. When the light is absorbed in its passage through a medium which is not perfectly transparent this momentum will also be absorbed and will be communicated to the medium, and will tend to make it move in the direction in which the light is travelling; the light will thus appear to exert a pressure on the medium; the pressure, which is called the pressure of radiation, has been detected and measured by Lebedew, Nicols and Hull and Poynting. All the phenomena associated with this pressure may be explained very simply by the view that light possesses momentum in the direction in which it is travelling. The possession of momentum by light, supposing light to be an electric phenomenon, has been deduced by somewhat abstruse consideration. On the old Newtonian emission theory it is obvious at once that this momentum must exist, for it is just the momentum of the particles which constitute the light. It is remarkable how recent investigations have shown that many of the properties of light which might be supposed to be peculiar to a process similar to that contemplated on the emission theory, would also be possessed by the light if it were an electric
