Another remark suggested by Maxwell's theory of stress in the medium is that he considered the question from the purely statical point of view. He determined the stress so that it might produce the required forces on ponderable bodies, and be self-equilibrating in free aether. But[1] if the electric and magnetic phenomena are not really statical, but are kinetic in their nature, the stress or pressure need not be self-equilibrating, This may be illustrated by reference to the hydrodynamical models of the aether shortly to be described, in which perforated solids are immersed in a moving liquid: the ponderomotive forces exerted on the solids by the liquid correspond to those which act on conductors carrying currents in a magnetic field, and yet there is no stress in the medium beyond the pressure of the liquid.
Among the problems to which Maxwell applied his theory of stress in the medium was one which had engaged the attention of many generations of his predecessors. The adherents of the corpuscular theory of light in the eighteenth century believed that their hypothesis would be decisively confirmed if it could be shown that rays of light possess momentum: to determine the matter, several investigators directed powerful beams of light on delicately-suspended bodies, and looked for evidences of a pressure due to the impulse of the corpuscles. Such an experiment was performed in 1708 by Homberg,[2] who imagined that he actually obtained the effect in question; but Mairan and Du Fay in the middle of the century, having repeated his operations, failed to confirm his conclusion.[3]
The subject was afterwards taken up by Michell, who "some years ago," wrote Priestley[4] in 1772, "endeavoured to ascertain the momentum of light in a much more accurate manner than those in which M. Homberg and M. Mairan had attempted it." He exposed a very thin and delicately-suspended copper plate
