IOWA ACADEMY OP SCIENCES. 
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The strain in each case extends to infinity, or as far as the 
ether extends. If the displacement of ether were prevented 
from extending on one side by a rigid imaginary wall, the 
whole strain on that side would take place between the atom 
and the wall, and would be more intense than on the opposite 
side. The atom would tend to move in such direction as to 
decrease the intensity of the strain, namely, from the wall if 
the displacement were outward, toward the wall if the displace- 
ment were inward. By the same reasoning two atoms repel 
each other if the displacement is outward, and attract if it is 
inward. The law of gravitation is thus explained on the 
hypothesis that each atom is accompanied by an inward dis- 
placement of the surrounding ether, proportional in amount to 
the mass of the atom. 
Minchin (Statics, fourth edition, vol. 2, p. 475,) by a course 
of mathematical reasoning has reached the same conclusion. 
If the atoms be regarded as cavities, the mass of an atom is 
represented by the quantity of ether removed, which repre- 
sents also the volume of the atom. Since atomic volume is not 
proportioned to atomic weight, the cavity-atom hypothesis 
must be abandoned. 
On the condensation hypothesis the mass of an atom is the 
quantity of ether condensed, its volume the space occupied on 
the average by the condensed mass which may have any kind 
of irregularity of form. 
This hypothesis implies that all atoms are built out of the 
same original stuff, and is in this respect similar to but not 
identical with Front’s hypothesis. The fact that all atoms 
attract with forces proportional to their masses shows that all 
atoms possess the same kind of mass, and ^are therefore likely 
to consist of the same sort of stuff. 
Valence, selective affinity, electric and other peculiarities of 
atoms, must, if this hypothesis of gravitation be correct, find 
their explanation in the form and density of the atom, the dis- 
tribution of its stuff in space, which can be expressed as a 
function of the three space co ordinates; together with the laws 
of energy, which express the relations of the atom to the ether. 
The field of force about an atom is also capable of represeata 
tion by a function of the space co-ordinates such that when the 
distance r from the atom is relatively great the equipotential 
surfaces are very nearly spheres. 
