which streams of particles are sweeping through uniformly in all directions, the uniform motion of the particles equally in all directions (necessary to produce gravity) being automatically kept up under the influence of the mutual collisions, in a way demonstrated to take place in the case of a gas. It should be observed that this self-adjustment of their motion by the particles is not a mere result of chance, but a rigid adjustment of such a character that, if the uniformity of the motion were artificially disturbed, the particles when left to themselves would immediately correct the irregularity. The above length of free path, though great in one sense, becomes small and suitable for a gas pervading the vast range of the visible universe. Unlike Le Sage, we do not object to the collisions of the particles among themselves; for these collisions (in the case of a medium constituted as a gas) maintain the uniformity of motion. We require no supply of matter to produce gravity, and no supply of energy. The energy is self-contained. It is simply the case of the normal motion of the particles of a gas. Motion is as natural as rest. Nothing surely could be more simple than these conditions.

12. It might be said that this theory implies a limited range to gravity. It may be extended to any desired range simply by making the particles small, and consequently the free path great. We venture to think that rather than that a theory should be required to explain that the stars gravitate, a theory should be required to explain that they do not gravitate. For surely the idea of an indefinitely extended universe all of whose parts gravitated towards each other, would represent dynamical conditions of instability on the most gigantic scale. Imagine the incongruity of the idea of the whole universe tending to agglomerate in one (perhaps infinite) mass. our mind no theory of gravity would be satisfactory that did not explain away this. The kinetic theory gets over this difficulty in a most complete manner, by allowing gravity to take place within a conformable range, without extending it to indefinite distances and thereby involving conditions of instability.

To

It

13. As we have said, we do not shirk in the slightest degree any criticism as regards this theory, but shall be glad to meet it, knowing that, if true, it will stand a full examination; and if false, the sooner it is proved so the better. There is one other point on which perhaps an objection might be raised. might be said, If a gas exists in space, how is it that we do not detect its presence in experiments on the specific heat of other gases, this gas being at the same time present? or *Of course we do not refer to double stars, in close range.

why does not some of the heat pass from the gas experimented on to this gas? In answer to this, it must be kept in view that the gravific medium, though in principle constituted as an ordinary gas, differs from an ordinary gas profoundly in several respects. First, it is necessary to assume that its particles are (as essential to the long free path) incomparably more minute than those of an ordinary gas, and the number of particles in unit of volume much greater. A molecule of an ordinary gas surrounded by the particles of the gravific medium, might be compared (as regards relative dimensions) to a visible mass surrounded by the molecules of air. Next, it is necessary to assume that the velocity of the minute particles of the gravific medium is incomparably greater than that of the relatively massive molecules of ordinary gases. Now, it is a known fact that the resistance to the passage of bodies through a medium constituted according to the kinetic theory diminishes as the normal velocity of the particles of the medium increases. By making, therefore, the normal velocity of the particles of the medium sufficiently great, all perceptible resistance to the passage of bodies through it will disappear. It is as if the medium did not exist; it becomes quite impalpable, or its presence impossible to detect. This is consistent with observation. The amount of energy, or motion, abstracted from a body passing through the medium, and given up to the medium, is exactly measured by the resistance encountered by the body. It is this transference of energy to the medium that constitutes the "resistance." If, therefore, there is no measurable resistance to the passage of the body through the medium, there is no measurable energy abstracted from the body. This gets over our difficulty; for since the molecules of ordinary gases (at their relatively slow velocity) move through the gravific medium without appreciable resistance, there is no perceptible transference of energy (i. e. “heat”) from them to the gravific medium. In other words, the presence of the gravific medium cannot interfere with the experiments on the specific heat of ordinary gases. In short, the high normal velocity of the particles of the medium necessarily renders it in all respects completely impalpable, or its presence impossible to detect by the senses. The high velocity of the particles is only naturally adapted to the minute size of the particles.

14. It would seem difficult to avoid the application of the above principles to the case of molecules in close proximity— "cohesion" or "chemical union." For, first, it would appear obvious that molecules in contact would be urged together with exceptional force, owing to the parts in contact cutting

off the entire stream of particles*. Secondly, the shapes of diverse molecules (which would have no particular influence while the molecules were at a distance) would, when the molecules are in contact, have a great influence, according to whether the solid parts (or interstices) fitted over each other, so as to afford more or less shelter from the streams of particles. Possibly this might account for (or at least throw some light upon) the extraordinary varied behaviour of chemical "affinity." If this were justified, it would certainly be a remarkably simple cause. It is just possible that a thing may be missed sometimes by looking too deep. The processes of nature are as a rule recognized to be simple, this being the necessary condition for order. Simplicity is the soul of mechanics. This view, if well founded, would have the advantage of correlating all molecular actions (including "gravity") under one cause. We have thought it just as well to mention these views in passing (without attaching the same definiteness to them as we attach to gravitation).

15. We would in conclusion make a few remarks upon a matter of principle connected with this subject. It must be evident that under a dynamical theory of gravitation, when a mass is lifted, the energy expended in lifting cannot be converted into "potential potential "energy, but must be converted into kinetic energy, in imparting motion to the particles impinging upon the upper side of the mass, and which tend to urge it downwards. Conversely, when the mass falls, kinetic energy is transferred from the particles of the medium to the mass. As a general principle, therefore, by the abandonment of the theory of "action at a distance," there can be no such entity as "potential" energy at all. We cannot avoid thinking that the very necessity to put forward a theory, that energy can possess, as it were, a double nature (kinetic, and not kinetic), in order to harmonize with the theory of "action at a distance, is by itself a sufficient logical condemnation of this latter theory. The idea of "potential" energy (i. e. an energy which is not kinetic) involves the inconceivable idea of an energy without motion, i. e. a kind of spiritual energy, whose existence or non-existence leaves matter in the same physical state. Already serious doubts have been cast upon its validity as a logical principle by some of the most eminent minds. From the prevalent use of the term "potential" energy, and at the same time the common repudiation of the theory of "action at a distance," one would be inclined to draw the inference that there was an idea to a certain extent prevalent

* We believe Le Sage called attention to this in its application to cohesion."

that this term " potential" energy could still be used in a certain sense, even after the theory of action at a distance had been abandoned. We think it can be clearly shown that this is not legitimate. For, by the rejection of the theory of "action at a distance," external matter or a medium (in a state of motion) must be concerned in developing motion in matter; and therefore it must be a case of kinetic energy, not "potential" energy. Either (for example) the motion of approach of two masses (or molecules) is developed (as supposed) without the concurrence of external matter, or (secondly) this motion is simply transferred to the masses from external matter. this latter case (which represents the case where the theory of "action at a distance" is rejected) the energy exchanged can only be the energy of motion (kinetic energy), not, therefore,

In

"potential" energy. It might, perhaps, be urged that even when the theory of" action at a distance" is rejected, a raised mass can still be said to have "potential" energy (due to its position), because it can fall. This, however, may be proved not to be legitimate. For, from the very fact that (by the rejection of action at a distance ") the energy expended in raising the mass was converted into kinetic energy, it cannot have been converted into "potential" energy (i. e. an energy which is not kinetic) as well. A double equivalent of energy cannot be generated*. We think we have clearly shown, therefore, that by the rejection of the theory of "action at a distance," the idea of "potential" energy must (to be logically consistent) be unreservedly abandoned. The rejection of "potential" energy makes all energy of one character, viz. energy of motion; and then the great principle of the indestructibility of motion inevitably presents itself for acceptance. With the theory of "action at a distance," the idea of "force" (in the old sense of an action across space without the intervention of matter) must be given up. Thus we have in the physical world, only the two great fundamental conceptions of matter and motion left; or all physical phenomena come thus to be correlated in one grand and fundamental aspect, viz. as consisting in the various exchanges and phases of motion. London, Jan. 11, 1878.

Note. We think it right to add that we make no claim to have shown (as this had been already done by others) that the molecules

To say that a raised weight tending to approach the earth by the action of the gravific medium, possessed "potential" energy because it can approach the earth, would be like saying that a ship confined by a cable and tending to approach a rock by the action of the wind, possessed "potential" energy, because it can approach the rock (by the breaking of the cable). The cases are evidently parallel.

of a gas regulate their motions so as to move in a particular manner, though we doubt whether, if we had not arrived at this conclusion independently for ourselves, we should have been able to make a practical application of it. The point it has been our object to call attention to (and which apparently has not been noticed by others) is, that the motion of the particles of a gas within the range of free path precisely satisfies all the conditions Le Sage arbitrarily assumed in order to produce gravity-or that the special character of the motion Le Sage arbitrarily assumed his streams of particles to have, actually exists within the range of free path of the particles of a gas -in other words, that all the effects of gravity can be produced by the mere existence of a gas in space, and indeed must be produced if such a gas exists.

XVIII. Electromagnetic and Calometric Absolute Measurements: the Absolute Value of Siemens's Unit of Resistance in Electromagnetic Measure; the Relation between the Current-work and the Heat-evolution in stationary Galvanic Currents; and the Absolute Values of some constant Hydroelectromotive Forces in Electromagnetic Measure. (Condensed Comparison of the Results of a Series of Investigations.) By H. F. WEBER, Professor of Mathematical and Technical Physics at the Federal Polytechnic Academy of Zurich.

MR.

[Continued from p. 43.]

III. The Heat produced by Stationary Galvanic Currents. R. JOULE, thirty-seven years since, showed by experiment that the quantity of heat which a stationary galvanic current of intensity i generates in a conductor whose resistance is w, during the time z, is proportional to wz. Sir W. Thomson then, in 1851 (and Prof. Clausius and others later), proved in the theoretical way that the value of the mechanical work which is expended in the stationary galvanic current of the intensity i, in a conductor with the resistance w, along which the electromotive force E is in action, in the time z is equal to the product iEz, or, pursuant to Ohm's law, equal to the expression wz, where the quantities E, i, w are to be taken as measured according to absolute measure. If we make the assumption that, in a stationary galvanic current in which the evolution of heat is the only action of the currentflow, the amount of heat developed in the unit of time, Q, is the full equivalent of the work expended in the same time, then we have

JG=iw=iE,

where J denotes the mechanical equivalent of the unit of heat.