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1878.] Past Changes in the Universe. 
some influence on the proper motion of the stellar suns, in 
addition to the repulsive action of the heat generated by the 
mutual collisions. 
In the absence of any explanation of the mechanism of 
gravity, or idea of the nature of gravific energy, it naturally 
becomes impossible to see how heat once dissipated in the 
ether could ever be recovered again, or how heat-energy de- 
rived from gravific energy could ever be re-converted into 
gravific energy. But when it comes to be recognised (as it 
logically and inevitably must) that gravific energy resides 
in a medium pervading space, and that therefore this 
medium is necessarily immersed in the ether or heat-con- 
veying medium, then we have data for new conclusions. It 
appears clear, or almost a necessary deduction, that the 
waves of heat cannot be conveyed through the gravific 
medium (in which the ether is immersed) without becoming 
to some extent dissipated in the gravific medium : for it 
seems incredible how two media should be immersed in each 
other without interfering with each other’s motions at all. 
If we admit that the waves of heat, though no perceptible 
diminution occurs in planetary distances, are at last in the 
course of ages frittered away or dissipated in the gravific 
medium in which the ether is immersed, then, since energy 
cannot be annihilated, the full equivalent of heat-energy 
(represented by the waves) is finally converted into gravific 
energy (by dissipation in the gravific medium). The energy, 
therefore, which came from the gravific medium, and was 
converted into heat-energy in the adtion of gravity at the 
approach of the masses, is finally restored to the gravific 
medium again, in the frittering away of the waves of heat 
in that medium. Thus the total amounts of gravific energy 
and of heat-energy may remain constant in the Universe, 
the one being convertible into the other, and back again. It 
may be observed that, besides the above apparently unavoid- 
able conversion of heat-energy into gravific energy by dissi- 
pation of the heat-waves in the gravific medium (and the 
consequent equalisation of the energy in the two media), it 
would seem a reasonable conclusion that two media (the 
gravific medium and the heat-conveying medium), immersed 
in each other, must naturally maintain an equilibrium of 
motion or energy between themselves, as, for example, is 
known to be the fadt in the case of two gases immersed in 
each other or mixed, however diverse their qualities may 
be. It should be observed that the gravific medium must 
be constituted as a gas (according to the kinetic theory) in 
order to accord with the observed effects of gravity. 
