AT THE PHYSICO-TECHNICAL INSTITUTE 411 



at Chicago on August 21. The final decision was a little 

 difficult, since, though I do not yet feel myself an old man, or 

 recognize the manifold little infirmities of advancing years as 

 any serious hindrance, my wife and my friends are somewhat 

 perturbed over my resolution to undertake such a journey in 

 my seventy-second year. 



* I am convinced that America represents the future of civilized 

 Humanity, and that it contains a vast number of interesting 

 men, while in Europe we have only chaos or the supremacy of 

 Russia to look forward to.' 



The anxiety of Helmholtz's family and of his friends was, 

 however, so great that the Ministry, at his request, increased the 

 allowance made for travelling expenses, so that his wife was 

 able to accompany him. 



And thus Helmholtz prepared for the journey. He concluded 

 his Lectures earlier than usual, and made an important com- 

 munication to the Academy on July 6, with the title ( Conclusions 

 from Maxwell's Theory as to the Motions of Pure Ether'. 



In Maxwell's Theory of Electrodynamics, mobility was 

 ascribed to the ether as the carrier of electrical and magnetic 

 forces, and values were also given for the direction and intensity 

 of the motive forces which act upon it. This assumption 

 entails no difficulty, so long as we conceive the ether as 

 permeated by ponderable substances, which move with it. 

 Helmholtz, however, maintains that the case is different for 

 space that is free from ponderable bodies, and filled with ether 

 only, such as we imagine interstellar space, and also the inter- 

 stices between the molecules of heavy bodies, to be. In these 

 cases we must ask whether pure ether is altogether free from 

 all inertia, and able to satisfy the equations of Maxwell, and 

 what are the motions it must perform. This is closely related 

 to the question whether it yields to the ponderable bodies 

 moving through it, or permeates them ; that is, whether it is 

 wholly at rest, or partly moves with them, partly yields to them, 

 according to Fresnel's hypothesis. 



Helmholtz makes the assumption that pure ether, regarded 

 mechanically, has the properties of a frictionless, incompressible 

 fluid, but that it is wholly without inertia, and finds that the laws 

 proposed by Maxwell, and completed by Hertz with the explicit 

 introduction of velocity-components, are adequate on this 



