344 POPULAR SCIENCE MONTHLY 



particles in suspension; this is the Brownian movement. He first 

 thought this was a vital phenomenon, but soon he saw that the in- 

 animate 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; one concentrates the 

 light to illuminate the microscopic preparation, thought they; with 

 light goes heat; thence inequalities of temperature and in the liquid 

 interior currents which produce the movements referred to. 



It occurred to M. Gouy to look more closely, and he saw, or thought 

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

 brisker 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 without cease, without borrowing anything from 

 an external source of energy, what ought we to believe? To be sure, 

 we should not on this account renounce our belief in the conservation 

 of energy, but we see under our eyes now motion transformed into 

 heat by friction, now inversely heat changed into motion, and that 

 without loss since the movement lasts forever. This is the contrary 

 of Carnot's principle. If this be so, to see the world return backward, 

 we no longer have need of the infinitely keen eye of Maxwell's demon; 

 our microscope suffices. 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 behold already 

 one of our principles in peril. 



The Principle of Relativity. — Let us pass to the principle of rela- 

 tivity: this not only is confirmed by daily experience, not only is it 

 a necessary consequence of the hypothesis of central forces, but it is 

 irresistibly imposed upon our good sense, and yet it also is assailed. 

 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 shall measure the velocity of the earth; 

 not its velocity in relation to the sun or the fixed stars, but its ab- 

 solute velocity. 



I well know what will be said: It is not its absolute velocity that 

 is measured, it is its velocity in relation to the ether. How unsatisfactory 

 that is ! Is it not evident that from the principle so understood we 

 could no longer infer anything? It could no longer tell us anything 

 just because it would no longer fear any contradiction. If we succeed 



