NATUKE 747 



tricity and moving with very high velocity. He looked upon them, in 

 other words, as currents of convection. Now, these cathodic rays are 

 deviated by the magnet. In virtue of the principle of action and re- 

 action, they should in their turn deviate the magnetic needle. It is 

 true that Hertz believed he had proved that the cathodic rays do not 

 carry negative electricity, and that they do not act on the magnetic 

 needle; but Hertz was wrong. First of all, Perrin succeeded in col- 

 lecting the electricity carried by these rays electricity of which 

 Hertz denied the existence; the German scientist appears to have 

 been deceived by the effects due to the action of the X-rays, which 

 were not yet discovered. Afterwards, and quite recently, the action 

 of the cathodic rays on the magnetic needle has been brought to light. 

 Thus all these phenomena looked upon as currents of convection, 

 electric sparks, electrolytic currents, cathodic rays, act in the same 

 manner on the galvanometer and in conformity to Eowland's law. 



VI. Lorentz's Theory. We need not go much further. Accord- 

 ing to Lorentz's theory, currents of conduction would themselves be 

 true convection currents. Electricity would remain indissolubly con- 

 nected with certain material particles called electrons. The circula- 

 tion of these electrons through bodies would produce voltaic currents, 

 and what would distinguish conductors from insulators would be that 

 the one could be traversed by these electrons, while the others would 

 check the movement of the electrons. Lorentz's theory is very attrac- 

 tive. It gives a very simple explanation of certain phenomena, which 

 the earlier theories even Maxwell's in its primitive form could 

 only deal with in an unsatisfactory manner; for example, the aberra- 

 tion of light, the partial impulse of luminous waves, magnetic polari- 

 zation, and Zeeman's experiment. 



A few objections still remained. The phenomena of an electric sys- 

 tem seemed to depend on the absolute velocity of translation of the 

 centre of gravity of this system, which is contrary to the idea that we 

 have of the relativity of space. Supported by M. Cremieu, M. Lipp- 

 man has presented this objection in a very striking form. Imagine 

 two charged conductors with the same velocity of translation. They 

 are relatively at rest. However, each of them being equivalent to a 

 current of convection, they ought to attract one another, and by 

 measuring this attraction we could measure their absolute velocity. 

 " N"o !" replied the partisans of Lorentz. " What we could measure in 

 that way is not their absolute velocity, but their relative velocity with 

 respect to the ether, so that the principle of relativity is safe." What- 

 ever there may be in these objections, the edifice of electro- dynamics 

 seemed, at any rate in its broad line?, definitively constructed. Every- 

 thing was presented under the most satisfactory aspect. The theories 

 of Ampere and Helmholtz, which were made for the open currents 

 that no longer existed, seem to have no more than purely historic in- 



