IOWA ACADEMY OF SCIENCES. 
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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 
il00,000 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 v/ay 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 ail 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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