8 Preliminary Remarks. 



immediate testimony of the senses. I have said that th'e particles of 

 all known bodies exert upon one another attractive and repulsive 

 forces. On the other hand when we can demonstrate the exis- 

 tence of these forces in an unknown principle, we infer that this 

 principle is material. Thus, light is not tangible, it is not, so far 

 as we can perceive, extended ; it has no weight, or at least none 

 capable of being appreciated by our balances. It is so subtle as to 

 elude all the ordinary methods by which matter manifests itself 

 to the senses. But by causing it to pass through transparent 

 bodies, as glass, water, &c., it deviates from a direct course in its 

 passage, and is bent precisely as if it were repelled by a force 

 proceeding from the surface, and attracted on the other hand 

 within by the particles which compose the transparent body. 

 We know also that it employs a certain time, very short indeed, but 

 yet capable of being estimated, in passing from luminous bodies 

 to us. In fine, by subjecting rays of light to certain tests, we 

 find that transparent bodies attract and repel them differently 

 on certain sides from what they do on others. From these prop- 

 erties, taken together, we are led to conclude that light is a ma- 

 terial substance, composed of particles extremely small, the form 

 of which is symmetrical on certain faces, which are susceptible 

 of particular attractions and repulsions, and which move in free 

 space, and through transparent bodies, with a given and deter- 

 ciinable velocity. 



1 1 . There are still other principles which act upon material 

 bodies without being either visible or tangible, or susceptible 

 of being weighed by our balances, which even present much 

 fewer indications of materiality than light, and which not- 

 withstanding are believed to be material substances. Such 

 is the unknown principle of electricity. Nothing absolutely ma- 

 terial has yet been detected in the cause of electrical phenomena, 

 nothing indeed which does not admit of being explained without 

 the supposition of matter. Still in its distribution over bodies, 

 in its passage from ont to the other through the obstacles which 

 separate them, this principle acts in a manner so exactly con- 

 formable to the laws of equilibrium and motion which belong to 

 fluid substances, that we can on this hypothesis calculate with 

 the utmost precision, and in all their details, the phenomena that 

 are to take place under given circumstances. It seems extreme- 

 ly probable, therefore, that the principle in question is a flu- 

 id, and that it is accordingly material. The same reasoning 



