"Hitherto I have not been able to discover the cause of the properties of gravity." Such is the simple statement of the man who discovered those properties.
And now let us inquire a little into this law of gravity, not with the hope of explaining this great mystery of nature,--though, for my own part, I believe that the time is not far distant when the progress of discovery will enable man to make this approach towards the mystery of mysteries,--but in order to recognise the real nature of the mystery, which is a very different thing from explaining it.
In the first place the study of gravity brings us at once to the consideration of the infinitely minute,--at least of what is for us practically infinite in its minuteness. If we consider the above quotation attentively, we perceive that this quality of gravity was recognised by Newton. "It is not the quantity of the surfaces of _particles_," he says, "but the quantity of solid matter which they contain," that gives to gravity its power. Gravity resides in the ultimate particles of matter. We cannot conceive of matter so divided, no matter how finely, that non-gravitating particles could be separated from gravitating particles. Without entering into the question what atoms are, we perceive that these ultimate constituents of matter must contain, each according to the quantity of matter in it, the gravitating energy. Only, observe how incongruously we are compelled to speak. (It is always so when we deal with the infinite, whether the infinitely great or the infinitely minute.) We are speaking of atoms as the ultimate constituents of matter, and yet we are compelled, in describing their gravitating energy, to speak of the quantity of matter contained in each atom,--in other words, we speak in the same breath of an atom as not admitting of being divided or diminished, and of its containing matter by quantity, that is, by more or less. May we not, however, reasonably accept both views? The reasoning is sound by which science has proved that, so far as our material universe is concerned, there is a limit beyond which the division of matter cannot be supposed to go,--insomuch that Sir W. Thomson has indicated the actual limits of size of the atoms composing matter. Yet, passing in imagination beyond the bounds of our visible universe, and so entering into the next order of universe below it (in scale of construction),--the ether of space,--the atoms of our universe may be infinitely divisible in that universe, may be, in fact, compared with _its_ particles, as the suns and worlds of our universe are to our atoms and molecules.