CONNECTION OF ELECTRICITY AND MAGNETISM. 251 



electro dynamic potentiiil is the ouly theory hitherto known which gives 

 a complete and unobjectionable expression of all the different classes of 

 electro-dynamic phenomena. It is recommended, besides, by the extreme 

 simplicity of its fundamental law. 



Now we come to the second fundamental question. It must not be 

 forgotten that the mathematical laws hitherto spoken of giv^e not aii 

 explanation of the ultimate vera causa of electro-dynamic effects, bufe 

 only a comprehensive and precise determination of their quantitative 

 value. They perform as much for the knowledge of these actions as the 

 laws of Kepler did for astronomy. But the work of Newton has also 

 to be done for this branch of science. 



In order to find the real elementary causes of electro-dynamic effects, 

 two different ways have been followed, the one by Gauss, the other hj 

 Faraday. 



Gauss thought that the distinguishing peculiarities of electro dynamic 

 forces might depend on the time which they required to reach distant 

 points of space. Among the papers published after his death, in the col- 

 lection of his works, there are some very remarkable attempts of this 

 kind. One of these papers, written in 1834, (Gauss, Werke, bd. v, p. 

 617,) contains the nucleus of a theory similar to that published some 

 time later by his friend and coUagae, Prof. W. Weber, and perhaps 

 even less objectionable. But Gauss did not publish anything about this 

 subject. " Nil actum reputaus, si quid superesset agendum," as he says 

 himself. 



Among the theories of this kind which have received the most elabo- 

 rate development, and have met for a long time with the almost general 

 assent of continental philosophers at least, was the theory of Prof. W, 

 Weber, of Gottingen. It has played a prominent part as the theoretical 

 basis of the highly important experimental measurements by which 

 Professor Weber determined the fundamental natural constants of this 

 branch of science. According to this hypothesis, every quantum of 

 electricity exerts upon every other quantum a force the value of which 

 not only dei)ends on the actual distance of the two electric quanta, but 

 also on the velocity with which this distance is changed and on the 

 acceleration of this velocity. 



Sir W. Thomson and Prof. P. G. Tait were the first who pronounced 

 this hypothesis of Weber to be contradictory to the principle of con- 

 servation of energy, without specifying their objection. I was led to 

 the same inference hj independent investigations, as mentioned above. 



Besides this, the late Professor Eiemann and Prof. C. Neumann, jr., 

 have tried to develop the idea of Gauss. The latter has given a rela- 

 tively simple mathematical form to the assumptions necessary for this 

 end. These assumptions, interpreted iDhysically, are rather startling, 

 and, besides, this hypothesis leads to the same contradiction of the law 

 of conservation of energy as the law of Weber. Hitherto, therefore, 

 the theoretical attempts of this class have not been very successful. 



