Return to Physics of the Ether

13. We may now enumerate the main physical conditions that would require to be satisfied in order to replace the above theories by physical causes capable of a rational appreciation, and of affording an insight into the mode of working of physical phenomena.

As a first condition, we require the existence of a physical agent capable of transmitting motions to a distance with speed and facility, in order to refer to physical agency all the effects at present brought under the theory of " action at a distance." As a second condition, this physical agent must be shown to be capable of enclosing a store of motion of a very intense character, competent to produce all those forcible molecular and other movements of matter exhibited in such effects as the phenomena of chemical action, the "electric" motions, combustion, the remark- able development of molecular motion observed in the case of explosive compounds, such as in the explosion of gunpowder, and other numerous and striking phases of motion witnessed on all sides. Thirdly, this physical agent will have to be proved to be competent to exert an intense pressure upon the molecules of matter, as consonant with the very forcible character of the static effects exhibited in "cohesion," and the stable aggregation of molecules generally : and finally, these several physical qualities of the agent will have to be shown to be capable of existing harmoniously together and consistent with an extremely low density in the agent.

14. Physical Constitution of the Ether. — The ether being the means by which the entire energy received from the sun in the form of heat and light, &c, is transmitted to the earth, the ether therefore may be properly regarded as the most important of physical agents; this agent being a most influential one and everywhere present, to the ether therefore we must look as the agent concerned in the various molecular and other movements of matter, including the physical effects produced at a distance generally.

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15. Now in dealing with the problem as to the physical constitution of the ether, or the physical basis upon which its qualities depend, it may be shown as a noteworthy point that if we exclude the theory of " action at a distance," and consequently the theory of " potential energy," then this physical problem will admit of only one solution, or the problem admits of being solved but in one special way. This point is one of importance and interest, since if it be recognized that reasonable grounds exist for rejecting the above theories, then the deduction would follow with certainty that this solution to the problem is the true one or the actual fact, this following necessarily from the circumstance of the existence of but one solution.

To those who are not prepared to reject the theory of "action at a distance" on purely abstract grounds, the simple course lies open of putting the results necessarily attendant on this solution to the problem to the test of observed facts. Even if merely for the sake of argument we put for an instant the question as to the admissibility of the theory of " action at a distance " out of sight altogether, it cannot but be regarded as a point of interest that by the rejection of this theory but one solution to the problem should exist, and therefore that if this theory be unfounded, this solution to the problem must be the true one.

16. On the other hand, it may be shown that if the theory of " action at a distance " be admitted, then a final solution to the problem is no longer possible, from the fact that no definite limit exists to the number of assumptions or possible solutions, and therefore it becomes impossible to fix finally upon any one, since the same end might be attained in a great variety of ways. This will become apparent after the question has been examined.

Thus in the first place it would be impossible to decide the fundamental question whether the normal state of the component E articles of the ether is a state of motion or a state of rest; for y the hypothesis of non-material agencies, the particles might be at rest and yet be capable of expanding the agent and producing physical effects of pressure, &c. If it were assumed as a mere hypothesis that the particles of the agent were in motion in their normal state, then it would be impossible, by the admission of the theory of " action at a distance/' to fix finally upon the character of this motion; for it is evident that the particles might be either moving in straight lines, or in curved paths whose character would depend on the values or intensities assumed to the non-material agencies in different points of space, i.e. dependent on the assumed mode of variation of the intensity of the agency by a change of distance of the particles, which mode of variation the theory of " action at a distance " assumes to be purely arbitrary, or it might vary as the distance, as the square of the distance, &c.

If, again, the inquiry were directed as to the cause producing the remarkable elasticity of the ether, a fact proved by the speed

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with which this agent can propagate waves, such as waves of light, then a final solution to this question would be equally impossible, for it would be impossible to decide whether to refer the elasticity of the agent to a motion of its component particles or not, since, in accordance with the theory of " action at a distance," the particles might be at rest and yet produce any degree of elasticity; and if in motion there is scarcely an assignable limit to the variety of motion. In short, the practically unlimited field for speculation which this theory brings with it precludes the possibility of a final decision, and the maze of hypotheses presented by it renders theoretic reasoning of little avail.

17. If, on the other hand, we reject the theory of " action at a distance " and the connected theory of " potential energy," then a definite course lies open. Considering first the question whether the normal state of the component particles of the ether is a state of motion or a state of rest; then it may be shown that taking the observed fact of the elasticity of the ether, it admits of being deduced from the principle of the conservation of energy that the normal state of the ether particles is a state of motion.

18. If any portion of an aeriform medium, such for example as a portion of air be supposed confined within a receptacle which is not rigid, then the quality of elasticity enables the mass of confined air to expand when the outer air pressure is in any way removed, in which act of expansion motion is communicated to the sides of the receptacle, which yield to the internal pressure. The quality of elasticity in an aeriform medium, therefore, enables it under certain conditions to communicate or develop motion. Now it follows from the principle of conservation that the previous existence of motion is the absolutely essential condition in order for motion to be developed, for unless motion previously existed, motion could not be expended in the act of developing motion (as in the act of expansion of a gas), but this is an absolutely necessary condition, for otherwise the sum of energy could not remain constant, but there would be a creation of motion out of nothing. The quality of elasticity, therefore, in an aeriform medium by which it can expand and fill a larger portion of space than it occupies under normal conditions, and in which act motion is developed, must be dependent on a motion previously existing. Hence, since it is an observed fact that the ether possesses in its normal state the quality of elasticity, the inference necessarily follows that the normal state of the ether particles must be a state of motion.

19. We have next to inquire as to the mode or character of this motion. It is an admitted self-evident principle that a mass of matter is incapable of itself to change the direction of its motion. Absence of change in direction being the characteristic of the straight line; it therefore follows that a mass or particle of matter

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moving in absolutely free space, and unobstructed by other matter in any form, will continue to move in a straight line until by encountering matter in some form its direction of motion may be changed. From this the inference necessarily follows that the ether particles move in straight lines.

20. The only remaining question is as to the general direction of the motion. Now, it is at once clear that this motion must take place in such a way that when any appreciable portion of the medium made up of a large number of particles is considered, this portion 01 the medium can, as a whole, maintain a fixed position, or can be in equilibrium of pressure with the surrounding medium. Now, this maintenance of a fixed position by the portion of the medium or the maintenance of an equilibrium of pressure on all sides can evidently only be satisfied on the condition that the particles are moving in every possible direction, and not in any one particular direction in preference to another, the particles continually interchanging motion or re- bounding from each other in every direction, and thereby propagating an equal pressure on all sides.

There would obviously be no reason why the particles should be moving in one direction more than in another; and, moreover, it may be shown that there exists a special self-acting tendency for this motion of the particles to be maintained towards every possible direction. This results from the consideration that the equilibrium of pressure depends on the fact that the motion takes place towards every direction, and a motion of any notable number of particles towards one special direction would necessarily cause a disturbance of the equilibrium of pressure, and this, by causing a yielding of the medium in the direction of the greatest pressure, would speedily cause a readjustment of the pressure; and since this irregularity of pressure is necessarily self-righting, the irregularity of the motion of the particles on which the pressure depends must also be self-righting, equilibrium of pressure being only restored when the irregularity of motion has corrected itself, i. e. when the motion of the particles takes place towards all directions.

The existence of this mechanical self-adjusting tendency will be very apparent if we consider any special case whatever. Thus, supposing, merely for illustration, the component particles of a single cubic foot of the medium to be all moving at a given instant in one common direction, then by this procedure the transverse pressure exerted by this cubic foot of the medium would cease, for a pressure evidently cannot be exerted at right angles to the direction of motion. The pressure of the surround- ing medium would therefore cause this cubic foot of the medium to collapse laterally (due to the absence of a lateral opposing pressure), and in this act the particles of this cubic foot of the

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medium would receive a forcible transverse acceleration, which they previously wanted, the abnormal movement being thus soon corrected and the equilibrium of pressure restored. It is clear that in the actual fact no notable irregularity in the motion can accumulate, for the rapid interchange of motion going on among the particles necessarily corrects the slightest disturbance of the equilibrium of pressure immediately on its occurrence, a continual self-acting adjustment thus going on, which entirely prevents an * abnormal movement of the particles from developing itself.

21. To summarize therefore : the inferences are, first, thai the normal state of the component particles of the ether is a state of motion; second, that this motion of the particles takes place in straight lines; and third, that this motion takes place towards every possible direction. This deduction as to the physical constitution of the ether, to which we have been led without choice, since (the theory of " action at a distance " being excluded) the problem admits of but one solution, resembles in its nature the theory applied by Joule and Clausius to gases. This theory would accordingly have a general application, or would be applicable to all aeriform media, one apparent point in the theory being its extreme simplicity. We may note, that since when the medium is in its normal state the motion of the component particles takes place towards every possible direction, the particles would there- fore be accelerated and retarded both in transverse and in longitudinal directions at the passage of waves.

22. The fact of the continuance of the motion of the ether particles without decrease or loss becomes very apparent when it is considered that one particle could only lose its motion by transferring that motion to another.

The fact that a mass of matter will remain in motion until acted on by a physical cause appears sometimes to be regarded as a less self-evident truth than the fact that a mass of matter will remain at rest until acted on by a physical cause; but it may be shown that the two cases are equally axiomatic, for a change from any given degree of motion to rest is precisely the same change (though inverse) as a change from rest to that same degree of motion. Hence if it be looked "upon as self-evident that a mass of matter will not of itself change its state from rest to motion, then, to be consistent, it must be looked upon as equally self-evident that a mass of matter will not of itself change its state from motion to rest, or the mass is wholly incompetent to change its state at all.

23. It is a known fact that the quality of elasticity is developed in the ether to a high degree : hence it would be reasonable to expect from this that the motion upon which this elasticity depends must be an extremely rapid motion. The inquiry there- fore suggests itself, whether there is any observed fact that would indicate the value of this motion, or the mean speed of the ether particles in their normal state.

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Now the velocity with which the ether transmits a wave or impulse has been measured, this velocity being about 190,000 miles per second, in round numbers. The speed with which a small impulse can be conveyed from particle to particle in the normal state of the ether must clearly depend on the speed with which an interchange of motion can effect itself between the particles, i. e. must depend on the normal speed of the particles themselves. Hence the deduction follows that the normal velocity of the ether particles must at least equal that with which they can transmit a wave (such as a wave of light), for on no other physical condition could any change of velocity in the ether particles, however slight, produced by some disturbing cause, be propagated to a distance from particle to particle at the above velocity; indeed, it is necessary to infer that the actual normal velocity of the particles must be somewhat in excess of the above, since the wave is not transmitted by particles which are all moving in the direct line of propagation of the wave, but also by particles which are moving obliquely. The wave could evidently be transmitted forward at a speed equal to that of the particles themselves, only on the condition that they were all moving in the direct line of propagation of the wave, which is not the fact.

This resembles the parallel case of an air wave (such as a wave of sound), which is transmitted at a slower rate than the mean velocity of the air molecules in their normal state, for this velocity has been fixed at about 1600 feet per second, while the velocity of propagation of a wave, such as a wave of sound, is less than this, or about 1150 feet per second.

24. The above limiting value for the normal velocity of the ether particles will probably appear inordinately high on the first consideration of the subject; but it is essential that 'the special circumstances of the case should be taken into account. Thus, owing to the extreme minuteness, or very small mass, of the ether particles as compared with gross molecules, the absolute value of the energy possessed by each ether particle, even when moving at the above velocity, may be extremely small, and may possibly be even less than the energy possessed by an air molecule when moving at its normal velocity; for there is no limit to the extent to which the energy possessed by each particle may be conceived to be reduced by a sub-division of the matter of which the ether consists, and yet this does not detract in the least degree from the total sum of energy. The molecules of gases are known to possess a higher normal velocity as the molecular density diminishes. The exceptionally low density of the ether would rather lead a high normal speed for the particles to be expected than otherwise.

An extensive state of subdivision would also have the effect of so curtailing the mean length of path of the particles, or of restricting the distances which the particles can move before being intercepted by other particles within such narrow limits, as effec-

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tually to conceal the existence of this motion from the direct evidence of the senses.

25. The above deduction, as to the high speed of the ether particles in their normal state, throws at once a light upon the existence of a vast store of energy in space of a very intense character, competent to produce the most forcible observed molecular motions, such as the phenomena of chemical action, combustion, the explosion of gunpowder, and other remarkable cases of the development of motion or work, all such effects finding their explanation in an interchange of motion between the ether and the molecules of matter under special conditions, which we shall have to consider farther on. The special physical qualities of the ether may be shown to render it an agent admirably adapted, in a mechanical point of view, to fulfill its varied functions, or it may be properly contemplated as a piece of mechanism specially adapted to its work.

26. In considering the high normal velocity of the ether particles, it is to be expected beforehand that this agent must exert an extremely forcible pressure upon the molecules of matter, even if every allowance be made for the extreme low density of the agent; for it is important to note that the pressure exerted is as the square of the speed of the particles of the agent, and therefore the pressure rises in a very rapid ratio as the speed increases; so that taking into account this fact, in conjunction with the high velocity of the particles, we must be prepared to find this pressure will have a very high value. In looking to physical phenomena for an indication of this pressure, and also with the object, if possible, of arriving at a limiting value for its intensity, or the value which this pressure must at least attain on the lowest computation, we will consider one observed fact.

27. Steel of the best quality, in the form of fine wire, has been known to bear a tensile strain represented by not less than 150 tons per square inch,* and even this cannot be said to be the limit to the tensile strength of steel, since the tenacity obtainable increases as the diameter of the wire is reduced.

We will consider the case when a strain is applied to such a wire until rupture ensues; then it is necessary to conclude that the molecules of the wire are, before the strain is applied, at a certain distance from each other, for the wire can contract sensibly by a change of temperature. If, therefore, these molecules were surrounded by absolute empty space, there would be no conceivable physical cause why they should resist an attempt, however feeble, merely to change their positions in space; but it is scarcely necessary to add that the molecular interstices of the wire, as is the case with matter generally, are necessarily pervaded by the ether, which

• For a special example, see * Telegraphic Journal ' (May 1, 1874). The tenacity of the wire in question (No. 22 B.W.G. diam. = 0*03 inch) might have been in- creased, had pliability been no consideration.

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exercises its pressure upon the molecules of the wire, which molecules are also in a state of energetic vibration, as indicated by the forcible ether waves emitted (radiation of heat). The ether there- fore which pervades the wire cannot but be disturbed by the movement attendant on the separation of the molecules of the wire, and is the one agent to which the forcible resistance encountered in separating these molecules can be referred.

In order to view the subject in its simplest form, we will only regard two transverse layers of molecules forming the cross section of the wire; or we may even imagine, merely for the purpose of illustration, two such layers to have been separated from the wire, and that now a strain is applied in order to separate these two layers of vibrating molecules from each other. Then the deduction follows as a necessary one, that when by the action of the applied strain the distance of these two layers of vibrating molecules has been increased, that this change of distance has brought about a change in the distribution of the ether pressure, or has disturbed the equilibrium of this pressure, such that when the resistance has attained a maximum, a reduction of the ether pressure upon the adjacent halves of the component molecules of the two layers, amounting to 150 tons per square inch, has been effected, so that the normal ether pressure upon the outer halves of the molecules forming the two layers tends, with the above force, to resist the separation of the molecules.

As to the precise physical process by which a reduction of the pressure of the intervening medium can take place in the presence of vibrating matter, we shall rqperve the consideration of this point for the present; but it may be noted that the inference is none the less essential, that a reduction of the ether pressure does take place in the presence of the opposed vibrating molecules of the wire, since there remains no other conceivable means of explaining in a realizable manner why these portions of matter (molecules), already completely disconnected from each other in the normal state of the wire, should require this enormous force to shift their positions in the act of breaking the wire, unless in this act there were something further to be accomplished than merely to change the positions of the molecules in space.

28. jSow it is to be observed that the aoove illustrative case only gives the amount of the reduction of the ether pressure, or the difference of pressure upon the opposite halves of the molecules, and not the total or normal ether pressure, which is the point we are investigating, and it might well be that this difference of pressure only amounted to a small part of the total ether pressure.

That this difference of pressure is far from representing the total ether pressure is shown by a consideration of the case of chemically combined molecules, the force required to separate molecules " chemically combined," as it is termed, being, as a

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known fact, very much greater than in the case of " cohesion," so termed; indeed it is impossible by ordinary mechanical means to separate chemically combined molecules, whereas this result is attainable with comparative facility in the case of "cohesion." The very much greater intensity of the force that would be required to separate chemically combined molecules is well shown when the converse process takes place. Thus the explosive energy and intense heat developed at the approach of oxygen and hydrogen molecules, when the mixed gases are detonated, clearly shows the high intensity of the force that would be required to separate these molecules from each other, thereby showing the high intensity of the controlling ether pressure, and indicating that the difference of pressure to be overcome in separating the molecules in this case must be many times greater than in the case of "cohesion," as in the example given of steel. If we suppose the intensity of the pressure to be overcome in the case of the separation of the molecules of oxygen and hydrogen, oue of the most powerful cases of chemical union, to be only three times greater than that overcome by the separation of the molecules in the example given of the steel wire, then this would give 450 tons per square inch as the difference of pressure effective in the case of the molecules of oxygen and hydrogen.

This is, however, not the total or normal ether pressure, but only the effective difference of pressure; however, as our object is only to arrive at a limiting estimate for this pressure, or to fix upon the lowest value consistent with what observed physical facts would require, we will accordingly take in round numbers the estimate of 500 tons per square inch as a limiting value for the ether pressure, having thus valid grounds for concluding that this estimate is within the facts as they actually exist.

We might have noted in the example given of the steel wire that there are lateral interstices between the molecules, so that the entire estimated cross section of the wire is not available, and therefore for this cause the difference of ether pressure effective in this case must have been to a certain extent underrated.

29. We are prepared for the above estimate as to the value of the ether pressure being received with a certain amount of incredulity, at all events at the outset; indeed, the existence of such an intensity of pressure as this may well be sufficient at the first thought to strike one with astonishment; but it is to be noted that, however great this pressure might be, it could not make itself palpable to the senses, for since the ether penetrates the molecular interstices of matter, its pressure is equalized on all sides. The air cannot penetrate with freedom the molecular interstices as is the case with the ether, yet a pressure of the air amounting to several tons on the human body can exist without the perception of the senses; how much more cause, therefore, is there for the perfect concealment of the existence of the ether

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pressure, which is exercised against the molecules of matter them- selves, a perfect equilibrium existing? It is therefore certain beforehand that the mere evidence of the senses cannot affect this question at all.

From purely a priori considerations the existence of a high pressure cannot be said to be less likely than a low pressure, or indeed any one particular value for the ether pressure be said to be more probable than another. In the object, therefore, to arrive at an estimate or limiting value for this pressure, we must consider solely observed physical facts, and be guided thereby.

It will be found that after the problem has been considered in its other aspects, and the special functions of the ether in physical phenomena have been taken into account and weighed, the more apparent will become the mechanical fitness of this high pressure, whereby this great physical agent is adapted to act forcibly upon the molecules of matter; whether it be to control these molecules forcibly in stable equilibrium, as illustrated by the stable aggregation of molecules in "cohesion," "chemical union," &c. (i. e. the general phenomena of the aggregation of molecules); or to produce forcible dynamic effects when the equilibrium of pressure is disturbed, as illustrated by the forcible movements of the molecules of matter exhibited in the varied phenomena of chemical action, combustion, the effects of ex- plosives, &c.

30. Density of the Ether. — It is a known fact that the density of the ether is very low, this agent being thus suited to afford free passage to cosmical bodies travelling at high speeds, the planets, &c. In order, therefore, to be consistent with observed facts, it will be essential to show that the existence of the high static pressure as above deduced is compatible with the low density of the ether.

Though pressure is due to motion, we use the term a static " pressure for convenience, and in the same sense as it might be applied to the pressure of a confined gas, due to the motion of its molecules,

31. The mean density of the air at the earth's surface is known, and the normal velocity of the air molecules giving a pressure of 15 lb. per square inch has been deduced as 1600 feet per second, in round numbers. The pressure of the ether as of any aeriform medium being proportional to the square of the velocity of the component particles, and in the ratio of the density: taking, therefore, as previously, the measured speed of a wave of light as the lowest limit for the speed of the ether particles, we will, in the first place, consider what the density of the ether would be if it only gave a pressure equal to that of the atmosphere. It is evident, therefore, that in order for the ether to produce a pressure equal to that of the atmosphere, the ether density would require to be so much less than the atmospheric density, as the square of.the

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normal velocity of the ether particles is greater than the square of the normal velocity of the air molecules. Taking, therefore, the square of the velocity of the ether particles (in feet per second), and dividing it by the square of the velocity of the air molecules

or this result expresses the number of times the ether density would require to be less than the atmospheric density, in order for the ether to give a pressure equal to that of the atmosphere (15 lb. per square inch). This density expresses such an infinitesimal amount or almost vanishing quantity, viz. a density upwards of three hundred and ninety thousand millions of times less than that of the atmosphere, that the result can scarcely but be regarded as a fiction; indeed, a bare consideration of the result by itself would almost warrant the inference that the ether pressure must be a considerable multiple of the atmospheric pressure, if only to bring up the ether density to a reasonable amount.

32. We will, accordingly, now take the value for the ether pressure (500 tons per square inch) previously fixed upon as the lowest limiting value, and we may then deduce what the density would amount to, in order to give this pressure. Pressure being directly proportional to density, the aoove value for the ether density corresponding to a pressure of 15 lb. per square inch would therefore, to give a pressure of 500 tons per square inch, have to be increased in the ratio of these pressures : or we have 1 v 500 x 2240 1 _ u . ,.

Hence we may observe, therefore, the perfect mechanical possibility of the existence of a very forcible ether pressure, consistent with the known fact that the density of the ether is extremely low; a pressure of an intensity adequate to account for the stable and forcible character of the effects exhibited in the phenomena of u cohesion," and the general phenomena of the aggregation of molecules, which may be classed under the " static " effects of the ether; as the other physical effects, such as the general phenomena of chemical action, combustion, &c, may be classed under the "dynamic" effects of the ether. The fact that pressure increases in the rapid ratio of the square of the speed of the

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particles of the agent is a most important one, as this renders possible the attainment of a very forcible pressure with the aid of but a small quantity of matter whose normal velocity is a high one.

33. In connection with this subject, it may be of interest to contemplate the possibility of the following as a physical problem. If we suppose a given mass of matter and a given volume of space, the volume of the space being supposed vastly greater than that of the mass of matter. Then it becomes possible, by the sub- division of this mass of matter (which may be readily conceived to be carried out to any extent), to pervade the entire volume of the space with matter, or there is no limit to the degree of close proximity into which the particles of matter pervading this volume of space may be brought by continued subdivision, the approach of the particles going on continuously without limit as the sub- division progresses. Thus, with a given mass of matter there is no limit to the extent of space that may be pervaded by matter by continued subdivision, or there may be no appreciable portion of the vast volume of space but what contains myriads of particles of matter. The normal state of this finely subdivided matter may be conceived to be a state of motion or a state of rest; if a state of motion, then we may observe the physical possibility of the existence of a store of energy of an extreme intensity, and which, from the minuteness and small length of path of the moving particles, must be concealed; this motion being also necessarily attended by the production of an intense and at the same time evenly balanced pressure, the smoothness and uniformity of the pressure, and the consequent concealment of its existence from the senses, being more and more complete as the subdivision progresses.

34. Many considerations point to the conclusion that the state of subdivision in the case of the ether is extreme, or that the ether particles are incomparably more minute than the molecules of matter. In the first place, the ether could not well otherwise penetrate with facility the molecular interstices. Secondly, the known fact that the density of the ether is very low, points to the conclusion that the ether particles are very minute, for if this were not the case, the mean distance of the particles would necessarily be inordinately great. Now, the observed extreme steadiness of the effects due to the ether pressure, as exhibited in the phenomena of "cohesion," &c, makes the inference necessary that the mean distance of the ether particles cannot be great, but that the mean distance of the particles is probably contained many times within the limits of space which a molecule of matter would occupy, and that, therefore, so much the more must the particles themselves be small compared with molecules.

It is clear that if the mean distance of the ether particles bore any near proportion to the dimensions of molecules, then the number of particles impinging against the molecule would neces-

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sarily fluctuate or vary considerably at each instant, which would be quite incompatible with the maintenance of a steady pressure against the molecule. The mechanical condition required for the maintenance of a steady and constant pressure against a surface must evidently be that the number of particles impinging against the surface is on an average the same at each instant, and this condition can only be fulfilled when the mean distance of the impinging particles is small relatively to the extent of the surface, and the number of the particles is therefore great; just as, for example, the force with which two hemispherical cups, when evacuated, are pressed together by the impacts of the air molecules upon their outer surfaces, could not possibly be steady and constant, if the mean distance of the air molecules were not incomparably less than the dimensions of the surfaces against which they impinge. The same considerations precisely in principle apply in the case of the action of the pressure of the ether upon molecules, or the mean distance of the ether particles must, to render possible the maintenance of a steady pressure, be much less than the dimensions of molecules. If, therefore, the mean distance of the ether particles be small compared with the dimensions of molecules, how much more must the particles themselves be small compared with molecules, and this more especially since the known low density of the ether renders the inference necessary that the dimensions of the ether particles must be small compared with their mean distance ? Analogy would also be in favour of the above conclusions, for just as the mean distance of the air molecules is extremely small compared with the length of a wave of sound, so the mean distance of the ether particles is extremely small compared with the length of a wave 01 light. This extreme minuteness of dimensions is at the same time the fitting mechanical condition adapted to a high speed for the particles, as by this extremely subdivided state of the matter forming the ether, the energy almost vanishes as regards each ether particle taken separately, and yet subsists as a whole to its full value; or by this minuteness of the particles or extensive state of subdivision, the motion goes on, as it were, so smoothly and equably as to be imperceptible, and the equilibrium of masses and molecules of matter (excepting under special conditions) is not disturbed thereby.

35. Let us suppose, for the purpose of illustration, that it has been possible to form an air vacuum corresponding to 17A00 of an inch of mercury. Then the amount of air thus left in the receiver, or the sum of the masses of the residual air molecules, would equal the sum of the masses of the ether particles, or the quantity of matter represented by the ether contained within the same receiver, this being in accordance with the limiting value for the ether density already fixed upon. Now the matter represented by these air molecules contained within the receiver when

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compared with the equal quantity of matter iu the form of ether contained within the same space, represents, on the one hand, a few extremely large masses of matter possessing an extremely slow speed; on the other hand, a large number of extremely small masses of matter possessing a very high speed.

If now we imagine, merely for illustration, each t of these air molecules to be subdivided into a million parts, and that the speed of each component part has been increased a thousand times. The presence of the ether within the receiver may be left out of account for the present. Then by this imaginary process of sub- division, the mean distance of these parts of matter (which we shall term "particles") would be so reduced as to bring these particles into closer proximity than the molecules of air outside the receiver (the mean distance of the particles of the subdivided matter being inversely as the cube root of their number). The pressure against the interior of the receiver, which is as the square of the speed of the particles, would now be increased a million-fold; and yet this result is attained without any increase in the absolute value of the energy of each particle, for the energy has, by the reduction of mass, remained precisely the same for each particle as before, although the total energy has become vastly greater, this energy being now subdivided among a large number of particles, and the pressure maintained by a greatly increased number of moving particles.

We might imagine this process of subdivision, or this reduction of mass combined with increase of speed, to go on progressively, and thus the total energy would be continually increasing, and the mean distance and mean length of path of these small moving masses or particles would be continually diminishing, and therefore the concealment of this motion from the senses would be more and more complete. The pressure would continually rise in intensity, and at the same time become more even and perfectly balanced as the number of particles increased.

We might thus imagine this process to go on progressively, until at length the dimensions, mean distance, and speed of the ether particles themselves had been reached; or it is possible thus step by step to arrive at a just conception of the wonderful intensity of the store of energy that is rendered physically practicable, and the high static value of the pressure that may be reached, under the simple mechanical conditions of an extensive subdivision of matter combined with a high speed.

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