586 Proceedings of the Royal Society 
both by their mutual collisions, and by collisions with mundane 
atoms, the whole stock of gravific energy is being gradually re- 
duced, and therefore the intensity of gravity gradually diminishing 
from age to age. 
Or, 2 d, suppose mundane matter to be spread through all space, 
but to be much denser within each of an infinitely great number of 
finite volumes (such as the volume of the earth) than elsewhere. 
On this supposition, even were there no collisions between the 
corpuscules themselves, there would be a gradual diminution in 
their gravific energy through the repeated collisions with mundane 
matter which each one must in the course of time suffer. The secular 
diminution of gravity would be more rapid according to this sup- 
position than according to the former, but still might be made as 
slow as we please by pushing far enough the fundamental assump- 
tions of very small diameters for the cage-bars of the mundane 
atoms, very great density for their substance, and very small 
volume and mass, and very great velocity for the ultramundane 
corpuscules. 
The object of the present note is to remark that (even although 
we were to admit a gradual fading away of gravity, if slow enough), 
we are forbidden by the modern physical theory of the conservation 
of energy to assume inelasticity, or anything short of perfect elas- 
ticity, in the ultimate molecules, whether of ultramundane or of 
mundane matter; and, at the same time, to point out that the 
assumption of diminished exit velocity of ultramundane corpuscules, 
essential to Le Sage’s theory, may be explained for perfectly elastic 
atoms, consistently both with modern thermodynamics, and with 
perennial gravity. 
If the gravific corpuscules leave the earth or Jupiter with less 
energy than they had before collision, their effect must be to con- 
tinually elevate the temperature throughout the whole mass. The 
energy which must be attributed to the gravific corpuscules is so 
enormously great, that this elevation of temperature would be 
sufficient to melt and evaporate any solid, great or small, in a 
fraction of a second of time. Hence, though outward-bound cor- 
puscules must travel with less velocity, they must carry away the 
same energy with them as they brought. Suppose, now, the whole 
energy of the corpuscules approaching a planet to consist of trans- 
