weight. Alas for the good old times! for our comparison, apt as it is, is too probably thrown away on the degenerate inhabitants of (once) merry England, erewhile the home of the "Miller," with his honest quarterstaff, of jolly and chivalrous wrestlers, boxers, and bowmen, now the hell of running-kicks, garrotting, gouging, and stabbing.

Aetas parentum, pejor avis, tulit

nos nequiores, mox daturos

progeniem vitiosiorem.

The dissipation of energy is a great fact in a moral as well as in a physical sense. In those good old times men fought with men, -irrepressible energy, rather than any sordid passion or uncontrolled vice, constantly pulling the trigger! Now creatures in the likeness of men vent their despicable passions in murderous assaults upon women and children. But science hints at an effectual cure. It is probable that before many years have passed, electricity, which by some mysterious means enables our nerves to call our muscles into play, which enables us to converse with one another at distances of thousands of miles, which alike plates the teaspoon and illumines the lighthouse, will be called upon by an enlightened legislature to produce absolutely indescribable torture (unaccompanied by wound or even bruise), thrilling through every fibre of the frame of such miscreants.]

139. After inertia, which is not accounted for by any of the hypotheses as to the ultimate nature of matter which we have just given, the most general property of matter which we recognise is that of universal gravitation, in virtue of which portions of matter, if situated at a distance from one another, are possessed of potential energy. We are apt to hold exaggerated notions of the immense power of gravity;

but a little consideration will show us that it is in reality one of the most trivial of the forces to which matter is directly or indirectly subject.

Think for a moment of the fundamental experiments in electricity and magnetism, known to men for far more than 2000 years, the lifting of light bodies in general by rubbed amber, and of iron filings by a loadstone. To produce the same effects by gravitation-attraction,—at least if the attracting body had the moderate dimensions of a handspecimen of amber or loadstone, we should require it to be of so dense a material as to weigh at the very least 1,000,000,000 lbs., instead of (as usual) a mere fraction of a pound. Hence it is at once obvious that the imposing nature of the force of gravity, as usually compared with other attractive forces, is due not to its superior qualitative magnitude, but to the enormous masses of the bodies which exercise it.

In fact, the excessively delicate Torsion-balance of Michell was absolutely requisite to demonstrate, much more to measure, the mutual attraction between a large and a small leaden sphere. And (unless the third of the hypotheses as to the nature of matter above given be correct, in which case the form of our statement would require modification) small or even moderately large pieces of matter are held together entirely by cohesion, gravitation being absolutely insensible; though in a huge mass like the earth, the force exerted by one hemisphere on the other (i.e. the force which would be called into play to prevent its being split in two) depends mainly upon gravitation, in comparison with whose enormous amount even a cohesive force of 500 lbs. per square inch over a circular surface of 4000 miles radius sinks into utter insignificance!

140. One only of the many hypotheses which have been advanced to explain the cause of gravitation has succeeded in passing the first preliminary tests. Of course, the assumption of action at a distance may be made to account for anything; but it is impossible (as Newton long ago pointed out in his celebrated letters to Bentley) for any one "who has in philosophical matters a competent faculty of thinking" for a moment to admit the possibility of such action.

Hence we have but two ways of accounting for gravitation either it is due to differences of pressure in a substance continuously filling all space, except where matter displaces it (?), or it is due to impacts, in some respects. analogous to those of the particles of a gas which have been found to be capable of accounting for gaseous pressure.

Now, all attempts as yet made to connect it with the luminiferous ether, or the medium required to explain electric and magnetic distance-action, have completely failed; so that we are apparently driven to the impact theory as the only tenable one.

141. To this theory Le Sage of Geneva devoted a singularly acute mind during the whole of his exceptionally long life*; but, for all that, his posthumous tract on the subject is but little in advance of the results he had arrived at in his eighteenth year.

He assumes the existence of ultra-mundane corpuscles; in infinite numbers, even compared with those of the particles of matter; of dimensions excessively small, but flying about in all directions with velocities enormously great. Portions gross matter virtually screen one another to a certain extent from the pressure due to this perpetual rain of corpuscles; but only on the sides turned towards one another. Hence a lone body would be equally battered on all sides;

of

but the introduction of a second mass interferes with this arrangement, and diminishes the pressure on the side next it. It is easy to show that the amount of this diminution, for given small masses, is inversely as the square of their relative distance. But when larger masses are taken account of, this diminution of pressure will not be (as gravity is) directly as the quantities of matter present, unless the further assumption is made that matter, whether by the great distance between its particles, or by the cage-like form of these particles, is almost perfectly permeable to the corpuscles; so that, practically, the corpuscles rain upon the interior particles of a mass as freely as if each of them had been alone in space.

Some of the postulates of this theory are hard to grant, and there is additional difficulty as to the mode in which the supply of energy of the corpuscles is to be kept up. Το enter into details on this subject is not in accordance with our plan. We therefore refer the reader to Sir W. Thomson's account of Le Sage's theory (Proc. R.S.E., 1871), and his suggestions for its improvement, based upon his theory of

vortex-atoms.

142. But we must make one remark. If Le Sage's theory, or anything of a similar nature, be at all a representation of the mechanism of gravitation, a fatal blow is dealt to the notion of the tranquil form of power we have called potential energy. Not that there will cease to be a profound difference in kind between it and ordinary kinetic energy; but that BOTH will be henceforth to be regarded as kinetic. What we now call kinetic energy is that of visible motions, also of motions of the smaller parts of bodies, and of the luminiferous ether, etc., each of these being more refined, as it were, than the preceding. But if Le Sage's theory be true,

potential energy of gravitation is a kinetic form still further refined than any of these. of these. And the conservation of energy may perhaps once more be completely and accurately expressed as the conservation of vis viva, though the term will of course have then a meaning incomparably more extensive than its original one.

143. But, in speculations like these, we have soared far beyond that which may be called the first refinement on ordinary gross matter; i.e. the luminiferous, probably also the electric and magnetic, medium, provisionally the Ether. To the consideration of its principal properties we now turn our attention.

These are, at first sight at least, of an apparently incongruous character; for, from one point of view, the ether appears as a fluid, from another as an elastic solid. Nothing is more certainly established in physical astronomy than the excessive minuteness of the resistance offered by the ether to the planetary motions, if, indeed, there be such a resistance at all appretiable, even when these motions are, as in the case of the earth, somewhere about 100,000 feet per second! On the other hand, we learn from physical optics that light, transmitted with a velocity of 188,000 miles per second, depends upon transverse disturbances of some kind or other; while several optical phenomena indicate that a disturbance of the nature of compression (if such be possible) would be transmitted with velocity almost infinitely great, in comparison even with this enormous velocity.

144. Stokes, however, has given a very ingenious illus tration which enables us to see that such an extraordinary combination of apparently irreconcilable properties is by no means without analogy, even in common matter. He takes the case of a solution of glue, or isinglass, or jelly, in