Maxwell. 303 



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* 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 ad- 

 herents of the corpuscular theory of light in the eighteenth 

 century believed that their hypothesis would be decisively con- 

 firmed 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,f 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.* 



The subject was afterwards taken up by Michell, who "some 

 years ago," wrote Priestley 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 



* Cf. V. Bjeiknes, Phil. Mag. ix (1905), p. 491. 

 t Histoire de 1'Acad., 1708, p. 21. 

 % J. J. (ie Mairan, Traite de V A urore boreale, p. 370. 

 History of Vision, i, p. 387. 



