them, not to be terminated in a mathematical superficies, but to grow gradually into one another; the external ether beginning to grow rarer, and the internal to grow denser, at some little distance from the superficies of the body, and running through all intermediate degrees of density in the intermediate spaces." 75

Newton then propounds in terms of this conception of the ether an elaborate explanation of the refraction of light, cohesion, and the action of acids upon various substances. As he nears the end of the letter, the notion of the ether as being graduated in density according to its distance from the central pores of solid bodies has evidently suggested to his mind the simple explanation of gravity referred to.

"I shall set down one conjecture more, which came into my mind now as I was writing this letter; it is about the cause of gravity. For this end I will suppose ether to consist of parts differing from one another in subtilty by indefinite degrees; that in the pores of bodies there is less of the grosser ether, in proportion to the finer, than in the regions of the air; and that yet the grosser ether in the air affects the upper regions of the earth, and the finer ether in the earth the lower regions of the air, in such a manner, that from the top of the air to the surface of the earth, and again from the surface of the earth to the centre thereof, the ether is insensibly finer and finer. Imagine now any body suspended in the air, or lying on the earth, and the ether being by the hypothesis grosser in the pores, which are in the upper parts of the body, than in those which are in its lower parts, and that grosser ether being less apt to be lodged in those pores than the finer ether below, it will endeavour to get out and give way to the finer ether below, which cannot be, without the bodies descending to make room above for it to go out into.

"From this supposed gradual subtilty of the parts of ether some things above might be further illustrated and made more intelligible; but by what has been said, you will easily discern whether in these conjectures there be any degree of probability, which is all I aim at. For my own part, I have so little fancy to things of this nature, that had not your encouragement moved me to it, I should never, I think, have thus far set pen to paper about them." 76

This rather crude hypothesis of gravity was much pondered over by Newton, and assumed a more mature form in query twenty-one of his Opticks, from

which we shall quote below. These citations from Newton's early correspondence clearly indicate that while his opinions fluctuated as to the detailed method of applying the theory of the ether to such phenomena and hence because of his avowed experimentalism always presented such opinions tentatively and with some diffidence; yet as to the existence of such a medium and the legitimacy of the appeal to it for a solution of certain difficulties he had no doubt whatsoever. For More, the world would fly to pieces without the ethereal spirit; for Newton it would run down and become motionless if it were not for the continual recruiting of motion in these various ways by active principles lodged in the ether. And he never gave up hope that experimental evidence might eventually be secured which would establish or definitely overthrow some of these specific conjectures 77. It was in this spirit and to this purpose that he proposed many of the thirty-one queries attached to the Opticks.

This judgment of Newton's ethereal hypothesis is interestingly confirmed by the last paragraph of the Principia.

;

"And now we might add something concerning a certain most subtle spirit which pervades and lies hid in all gross bodies; by the force and action of which spirit the particles of bodies mutually attract one another at near distances, and cohere if contiguous; and electric bodies operate to greater distances, as well repelling as attracting the neighbouring corpuscles; and light is emitted, reflected, refracted, inflected, and heats bodies and all sensation is excited, and the members of animal bodies move at the command of the will, namely by the vibrations of this spirit, mutually propagated along the solid filaments of the nerves, from the outward organs of sense to the brain, and from the brain into the muscles. But these are things that cannot be explained in few words, nor are we furnished with that sufficiency of experiments which is required to an accurate determination and demonstration of the laws by which this electric and elastic spirit operates.

78

In other words, the existence of this spirit and its

77 Opticks, p. 369.

78 Principles, II, 314.

causal relation to such phenomena is assumed to be indubitable; the only uncertainty, and hence the reason why these matters cannot be properly treated in the Principia, is that we have so far been unable to obtain accurate experimental laws expressing the operations of this pervasive medium. It is worthy of note that here also there is no hint of the manifold distinctions about the ether made in his letter of 1675; it appears to be conceived as a single medium.

(C) Development of a More Settled Theory

It is in the Opticks, and especially in one of those queries which were last appended to the work, that Newton's final statements on the nature and functions of the ether are proffered. Here we find his earlier suppositions clarified and developed in greater detail; here also the explanation of gravity hit upon in the course of his letter to Boyle is presented in a refined and simplified form.

The passage opens with the statement of an interesting fact for explanation 79: A thermometer enclosed in a vacuum and carried from a cold place into a warm one" will grow warm as much and almost as soon as the thermometer which is not in vacuo. . . . Is not the heat of the warm room conveyed through the vacuum by the vibrations of a much subtiler medium than air, which after the air was drawn out remained in the vacuum ?.... And is not this medium exceedingly more rare and subtle than the air, and exceedingly more elastic and active? And doth it not readily pervade all bodies? And is it not (by its elastic force) expanded through all the heavens ?

"Is not this medium much rarer within the dense bodies of the sun, stars, and planets and comets, than in the empty celestial space between them? And in passing from them to great distances, doth it not grow

"Opticks, p. 323, ff.

denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer? For if this medium be rarer within the sun's body than at its surface, and rarer there than at the hundredth part of an inch from its body, and rarer there than at the fiftieth part of an inch from its body, and rarer there than at the orbit of Saturn; I see no reason why the increase of density should stop anywhere, and not rather be continued through all distances from the sun to Saturn, and beyond. And though this increase of density may at great distances be exceeding slow, yet if the elastic force of this medium be exceeding great, it may suffice to impel bodies from the denser parts of the medium towards the rarer, with all that power which we call gravity. And that the elastic force of this medium is exceeding great, may be gathered from the swiftness of its vibrations ". Newton here cites the velocity of sound and of light by way of illustration, and enters upon a disquisition in which he repeats some of his earlier speculations on the possibility of explaining refraction, sensation, animal motion, magnetism, and the like by the aid of the ether. He then launches on a further description of the medium. "And if

any one should suppose that ether (like our air) may contain particles that endeavour to recede from one another (for I do not know what this ether is), and that its particles are exceedingly smaller than those of air, or even than those of light: the exceeding smallness of its particles may contribute to the greatness of the force by which those particles may recede from one another, and thereby make that medium exceedingly more rare and elastic than air, and by consequence exceedingly less able to resist the motions of projectiles, and exceedingly more able to press upon gross bodies, by endeavouring to expand itself.

May not planets and comets, and all gross bodies, perform their motions more freely, and with less resistance, in this ethereal medium than in any fluid which fills all space adequately without leaving any pores, and by consequence is much denser than quicksilver or gold? And may not its resistance be so small, as to be inconsiderable? For instance: if this ether (for so I will call it) should be supposed 700,000 times more elastic than our air, and above 700,000 times more rare; its resistance would be above 600,000,000 times less than that of water.

And so small a resistance would scarce make any sensible alteration in the motions of the planets in ten thousand years."

Newton's ether as finally portrayed is thus a medium of essentially the same nature as air, only much rarer. Its particles are very small and are present in greater quantity according as they are more distant from the inner pores of solid bodies. They are elastic, i.e., they possess mutually repulsive powers, being constantly in the endeavour to recede one from another, which endeavour is the cause of the phenomena of gravitation. Other phenomena of the types above noted are attributed to additional active powers possessed by the ether, or are occasionally spoken of as following likewise from the operation of these repulsive forces. But the active powers apparently cannot be dispensed with, inasmuch as the universal machine is on the decline and the ether is burdened with the responsibility of constantly replenishing the vigour and motion of the cosmos through the exercise of these active principles. It is interesting biographically to observe that in Newton's later writings the number of inexplicable elements or qualities that are invoked to account for the variety of extra-mechanical phenomena is greatly reduced as compared with the early attempts. In fact, in one instructive section of the Opticks he repeats in the form of a vast