AND MODERN PHYSICS. 203 



has mot with much opposition, directed not against its failure 

 to explain the phenomena, but against its assumption of the 

 existence of a medium in which light is propagated. 



"We have seen that the mathematical expression for 

 electro-dynamic action led, in the mind of Gauss, to the con- 

 viction tliat a theory of the propagation of electric action in 

 time would 1x3 found to be the very key-stone of electro- 

 dynamics. Now we are unable to conceive of propagation in 

 time, except either as the flight of a material substance through 

 space, or as the propagation of a condition of motion, or stress, 

 in a medium already existing in space. 



" In the theory of Neumann, the mathematical conception 

 called ]K>tential, which we are unable to conceive as a material 

 substance, is supjKxsed to lx projected from one particle to 

 another in a manner which is quite indej>emlent of a medium, 

 and which, as Neumann has himself pointed out, is extremely 

 different from that of the propagation of light. 



41 In the theories of Kiemnnn and Hetti it would ap[ear 

 that the action is supjKxsed to be propagated in a manner 

 somewhat more similar to that of light 



" Hut in all of these theories the question naturally occurs : 

 If something is transmitted from one particle to another at a 

 distance, what is its condition after it has left one particle and 

 before it has reached the other? If this something is the 

 |K>tential energy of the two particles, as in Neumann's theory, 

 how are we to conceive this energy as existing in a point of 

 space, coinciding neither with the one jKirticle nor with the 

 other ? In fact, whenever energy is transmitted from one Inxly 

 to another in time, there must lie a medium or .substance in 

 which tl.e energy exists after it leaves one body and before it 

 reaches the other, for energy, a* Torricelli* remarked, 'is a 

 quintessence .of so subtle a nature that it cannot IKJ contained 

 in any vessel except the inmost substance of material things. 1 

 Hence all these theories lead to a conception of a medium in 

 which the propagation takes place, and if we admit this 

 medium as an hyi>othesis, I think it ought to occupy a pro- 

 minent place in our investigations, and that we ought to 

 " Lczioni Accadeiniche * (Fircnzc, 1715), p. 25 k 



