610 APPLIED MATHEMATICS 



But here the scene changes. 



The biologist, armed with his microscope, long ago noticed in his 

 preparations disorderly movements of little particles in suspension: 

 this is the Brownian movement; he first thought this was a vital 

 phenomenon, but he soon saw that the inanimate bodies danced with 

 no less ardor than the others; then he turned the matter over to the 

 physicists. Unhappily, the physicists remained long uninterested in 

 this question; the light is focused to illuminate the microscoprc pre- 

 paration, thought they; with light goes heat; hence inequalities of 

 temperature and interior currents produce the movements in the 

 liquid of which we speak. 



M. Gouy, however, looked more closely, and he saw, or thought 

 he saw, that this explanation is untenable, that the movements 

 become more brisk as the particles are smaller, but that they are not 

 influenced by the mode of illumination. 



If, then, these movements never cease, or rather are reborn with- 

 out ceasing, without borrowing anything from an external source 

 of energy, what ought we to believe? To be sure, we should not 

 renounce our belief in the conservation of energy, but we see under 

 our eyes now motion transformed into heat by friction, now heat 

 changed inversely into motion, and that without loss since the move- 

 ment lasts forever. This is the contrary of the principle of Carnot. 



If this be so, to see the world return backward, we no longer 

 have need of the infinitely subtle eye of Maxwell's demon; our 

 microscope suffices us. Bodies too large, those, for example, which 

 are a tenth of a millimeter, are hit from all sides by moving atoms, 

 but they do not budge, because these shocks are very numerous and 

 the law of chance makes them compensate each other: but the 

 smaller particles receive too few shocks for this compensation to 

 take place with certainty and are incessantly knocked about. And 

 thus already one of our principles is in peril. 



We come to the principle of relativity : this not only is confirmed 

 by daily experience, not only is it a necessary consequence of the 

 hypothesis of central forces, but it is imposed in an irresistible way 

 upon our good sense, and yet it also is battered. 



Consider two electrified bodies; though they seem to us at rest, 

 they are both carried along by the motion of the earth; an electric 

 charge in motion, Rowland has taught us, is equivalent to a current; 

 these two charged bodies are, therefore, equivalent to two parallel 

 currents of the same sense and these two currents should attract 

 each other. In measuring this attraction, we measure the velocity 

 of the earth; not its velocity in relation to the sun or the fixed stars, 

 but its absolute velocity. 



I know it will be said that it is not its absolute velocity that 

 is measured, but its velocity in relation to the ether. How unsatis- 



