The law of gravity, or of the mutual attraction of masses of matter upon each other, accounts so perfectly for all the observed motions of the heavenly bodies, that we are apt to regard Newton's discovery of the great law as though it had finally solved the mystery of these motions. Many accept the verdict given by the poet Pope in the famous epitaph which he suggested for Newton—

      "Nature and Nature's laws lay hid in night:

      God said, Let Newton be! and all was Light."

      But Newton, who probably knew as much about his work as Pope, was of another opinion. Every one knows how he compared himself to a child who had picked up a few shells on the shore, while the ocean of truth lay unexplored before him. He has, however, spoken definitely of the great discovery which has rendered his name illustrious, in terms which show that he did not find that all was light. Among the questions which he specially would have had answered, amongst the secrets of nature concealed beneath the ocean of truth, the mystery of gravity was probably the chief. When Newton asked of the Ocean of Truth what Mrs. Hemans later said, and in another sense, of the natural sea—

      "What hidest thou in thy treasure-caves and cells,

      Thou hollow-sounding and mysterious main?"

      he had in his thoughts the very power which he is commonly supposed to have explained, but which was in truth for him, more than for any man that had ever lived, the mystery of mysteries.

      It may be well to consider the very words of the great philosopher, so far at least as our more diffuse language can present the concise expressions of the original Latin:

      "Hitherto we have explained," he says, "the phenomena of the heavens and of our sea by the power of gravity, but have not yet assigned the cause of this power. This is certain" (we must hearken attentively here, for when a man like Newton speaks of aught as certain, we have sure ground to go upon)—"this is certain, that it must proceed from a cause that penetrates to the very centres of the sun and planets, without suffering the least diminution of its forces; that operates, not according to the quantity of surfaces of particles on which it acts (as mechanical causes usually do), but according to the quantity of the solid matter which they contain, and propagates its virtue on all sides to immense distances, decreasing always as the squares of the distances. Gravitation towards the sun is made up of the gravitations towards the several particles of which the body of the sun is composed, and in receding from the sun decreases accurately as the square of the distances as far as the path of Saturn … , nay, and even to the remotest parts of the paths of comets. … But hitherto I have not been able to discover the cause of those properties of gravity from phenomena; and I frame no hypotheses:[4] for, whatever is not deduced from phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy. … To us it is enough that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies and of our sea."

      "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.

      But while gravity thus draws us to the contemplation of the infinitely minute, it also leads us to the consideration of what is for us the infinitely vast.

      Newton was only able to speak confidently of the action of gravity at the distance of Saturn, the remotest planet known in his day. He did, indeed, refer to the comets as probably obeying, even in the remotest parts of their paths, the force of the sun's gravity; but he could not be certain on that point, because in his time no comet had been proved to travel back to the sun after receding to the remotest portion of its track. We now know not only that the