Intelliaence and Miscellaneous Articles, 75 



generally assumed to be the resultants of forces which act between 

 particles of the rod. Thus the observed action between two dis- 

 tant particles is in this instance removed from the class of direct 

 action at a distance by referring it to the intervention of the rod ; 

 ~~the action of the rod is explained by the existence of internal forces 

 in its substance ; and the internal forces are explained by means of 

 forces assumed to act between the particles of which the rod is 

 composed — that is, between bodies at a distance which, though 

 small, must be finite. 



" The observed action at a considerable distance is therefore ex- 

 plained by a great number of forces acting between bodies at very 

 small distances, for which we are as little able to account as for 

 the action at any distance, however great. 



" Nevertheless the consideration of the phenomenon, as explained 

 in this way, leads us to investigate the properties of the rod, and 

 to form a theory of elasticity which we should have overlooked if 

 we had been satisfied with the explanation by action at a distance."* 



But even granting that such an explanation as this is no help at 

 all towards getting rid of action at a distance, we are not limited 

 to direct impact as the only possible explanation. If we conceive 

 beings living in a continuous fluid the existence of which they were 

 not capable of perceiving, two vortex rings in the fluid would to 

 them appear to act on one another directly at a distance, and yet 

 there is no real action at a distance, and no impact of hard bodies 

 on one another. Is it not possible that in the future the actions 

 of the particles of bodies on one another maj be explained in some 

 such way as this ? or even that the phenomena of direct impact 

 themselves may be so accounted for ? 



A CONTRIBUTION TO THE THEORY OF SO-CALLED ELECTRICAL 

 EXPANSION OR ELECTROSTRICTION. BY PROF. L. BOLTZMANN, 

 OF GRAZ. 



Let the interval between two concentric conducting spherical 

 shells with radii a and a-\-cc be filled with a solid elastic dielectric 

 of which the constant of dielectricity is D. Let its interior be 

 charged with the potential p, and its outer surface connected to 

 earth. If we use the notation employed in Lame's Lecons to the 

 elastic -traction force 



K 1 = \0 + 2/ ?T 



the expression 



in which 



is then added. 



dr 



8tt>-V ' 

 (D-i)(D-l) 



Electricity and Magnetism/ vol. i. p. 122. 



