IOWA ACADEMY OF SCIENCES. 49 



moving bodies whose velocity is relatively small, not exceed- 

 ing a few hundred kilometers per second, but acts as a solid 

 toward such high velocities as that of light, which is nearly 

 yOO,000 kilometers per second. Copper, again, is a familiar 

 example of a metal having nearly perfect elasticity within a 

 certain limit of strain. Beyond that limit it yields to pressure 

 like a fluid. The ether shows the same combination of proper- 

 ties with a wider limit of strain. Ether in a vacuum will bear 

 a very great electrical strain without yielding; so that the most 

 perfect vacuum attainable is an all but perfect non- conductor; 

 but if atoms be present the ether gives way to the stress and a 

 current passes very much more readily. This indicates that 

 there is some sort of discontinuity at or near the surface of the 

 atoms. 



One of the oldest theories of gravitation was proposed by 

 Le Sage and elaborated by him for a lifetime. He supposed 

 the atoms to have an open structure, something like wire 

 models of solid figures, and to be exposed to a continuous storm 

 of exceedingly minute " ultramundane corpuscles" which he 

 assumed to be flying about in all directions with inconceivable 

 velocity. Two atoms shelter each other from this storm in 

 direct proportion to the quantity of matter in each and inversely 

 as the square of their distance apart, and are therefore driven 

 together in accordance with Newton's law. The ultramundane 

 corpuscles are supposed so small that no atomic vibrations cor- 

 responding to heat or light are caused by their impact. 



Le Sage's theory is unsatisfactory because it takes no 

 account of the ether, which for such high velocities acts as a 

 solid and would bring the little flying corpuscles to compara- 

 tive rest in a small fraction of a second. 



Kelvin has proposed a modification of Le Sage's theory in 

 order to accommodate it to the existence of the ether. He first 

 showed that vortex rings have some of the properties of elastic 

 solids, and in a perfect fluid would be indestructible; then sug- 

 gested that atoms may be vortex rings of ether, and the ultra- 

 mundane corpuscles very much smaller vortex rings having 

 high velocities of translation. In order to account for the 

 permanence of atoms and corpuscles, this view presupposes a 

 practically frictionless fluid ether, which does not at all corre- 

 spond with the actual ether. 



Maxwell, after deducing the mathematical theory of elec- 

 tricity from the hypothesis of ether strain, showed that gravi- 

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