February 28, 1890.] 



SCIENCE. 



H9' 



quality of the elastic medium given to it by the mere presence 

 of the other body. We have seen, in the case of the two bodies 

 connected by an elastic string, that the stress is due to the 

 relative positions of the two bodies. They were brought into 

 their position by motion ; but, so soon as this motion was over- 

 come by the elastic resistance of the string, the force applied 

 became potential, not kinetic. 



Now, supposing one of these bodies to remain stationary while 

 the other is moved farther away, the stress between the two is 

 increased. . The push on the one has increased the pull on the 

 other, yet there has been no motion of the latter. The only 

 motion there was in the system was that imparted to the 

 former ; and the motion was away from the latter, not towards 

 it. A new static condition has been induced upon both of 

 them, but can it be said to have been propagated from one to 

 the other? I do not think it can. 



Now, these two bodies thus connected may be moved frem 

 one place to another, yet, so long as they maintain their same 

 relative positions, the stress or tension of the elastic between 

 them will not vary. So, if a form of stress between two bodies 

 in space may be conceived to be entirely independent of the 

 presence of any other bodies in space, a relative motion of these 

 two bodies between themselves involves a change of stress 

 without propagation ; and, again, if these two be relatively 

 still, they may move relatively to all other bodies without 

 changing their stress or altering their static condition with 

 regard to stress; yet in one sense (and I conceive in this sense 

 alone) is the stress properly said to be propagated. 



Professor Dolbear says, "One may call it potential or kinetic 

 energy if he chooses: a static condition will presently be 

 reached, but not instantly." 



Right here, I suppose, lies the gist of the whole thing. The 

 point I wished to make was that Professor Dolbear did not 

 distinguish between the condition of the medium in the two 

 states of motion and rest. A potential condition involves 

 motion only in so far as motion was necessary to bring it into 

 being. It may in one sense be considered as stored-up motion, 

 as it is capable of giving out again an equivalent quantity o 

 motion, but it is not motion itself. He implies, and I agree 

 with him, that motion precedes the potential condition. Now, 

 this motion, or energy if you choose, may be of any known 

 kind (not necessarily electrical, but may be) ; and when, by 

 reason of the work done, we have produced a condition of 

 matter of a certain kind, — when all the work has been done, 

 — we have a conditon that is called electrification. 



What I have been contending for is that the magnetic field 

 and the fields of electrification and gravity are those fields 

 when they are established (before that, in the interval during 

 which they are being established, the fields are kinetic, not 

 potential) ; that in the case' of electrification the movement 

 necessary or involved in its establishment is not the electric 

 field, but is the electric current which can be propagated, and 

 the condition of stress produced by that motion is the electric 

 field. 



It is possible, however, that I have not fully understood 

 him. Perhaps he means something like this: imagine, for 

 instance, a row of material points numbered from 1 to 10 

 spread out in a row, and connected by elastic thread. If 1 and 

 10 are stationary, and the intermediate numbers are strung 

 along the string, the tension upon the string is uniform 

 throughout its length. If, now. No. 1 be moved a little farther 

 from No. 3 than originally, the tension on the string between 

 the two would be greater than before. This would cause a 

 slight movement of No. 3; and so on to No. 10, which we have 

 imagined stationary. Would it take time for the pull on No. 

 1 to become apparent on No. 10? It would with all material 

 elastics, because of their viscosity and inertia ; but, if we use as 

 our elastic the luminiferous ether, I maintain that it would 

 not, for one of the main features assigned to it is absolute and 

 perfect elasticity. If No. 1 were pushed nearer to No 2, the 

 lessening tension would exist at No. 10 at the same instant, 

 because another property of the ether is absolute incompressi- 

 bility. 



Taking this latter characteristic of the ether, — incompressi-^ 

 bility, — if we had a long pole (say, a thousand miles long), 

 if we should cause one molecule of that pole to change its? ' 

 position, would not every other molecule, even those a thousand 

 miles away, be caused to move also? and would there be any 

 time lost in their responding? Or, to put it in another way, if 

 we should premise that the farthermost molecule should not 

 move, would it be possible in any way to move the nearer- 

 molecules? I contend that it would not, and that if one moved, 

 all must move. 



He says, "Mr. Perry seems to say, that, if there was but one- 

 body in the universe, it could not have an electric field, even if 

 it could be electrified. ' ' That is not my statement, nor is it my 

 idea. My idea is, that in the case of static fields, under which- 

 head I would include electrification, magnetism, and gravity, 

 two exactly equal and opposite conditions are inevitable. I 

 cannot conceive of there being a push without there being a 

 corresponding and equivalent pull, without the destruction of 

 equilibrium ; and, if the equilibrium be destroyed, we have 

 motion which may be in any direction whatever. This is what 

 I conceive to be the difference between the two kinds of fields. 

 As before stated, I do not believe a pull can exist without a 

 corresponding push and yet maintain equilibrium. If the one 

 exceeds the other, there will be motion towards the greater, — 

 there will be more positive than negative electricity; there will 

 be more north-seeking magnetism than south-seeking magnet- 

 ism, or vice versa; and by the same token one may exist with- 

 out the other. By the same course of reasoning, if a stress can 

 be propagated (as I use the term "stress"), there is implied 

 an existence of one form (the positive) before the existence ot 

 the other (the negative), as the element of time is involved. 

 Faraday distinctly states that there cannot be an absolute 

 charge of matter. I stated my belief that a stress could not 

 exist unless there were two particles. I should also have added 

 that there must be a connecting medium between those two 

 particles. If this be so, then it is clear without reasoning^ 

 that, if either one of the particles or the connecting medium 

 be wanting, the conditions for stress are wanting, and it can-, 

 not exist. A material body having two points and a connect- 

 ing medium between the two is therefore capable of either 

 magnetism or electrification. If one of the points be wanting, 

 and energy be applied to the other, this energy, instead of 

 being stored up by the tension of the elastic medium, and pro- 

 ducing stress as before, — which would be capable of giving out 

 again an equivalent amount of energy or motion, which stress- 

 might constitute electrification or magnetism, — produces no 

 stress, but motion at once, which may be heat, light, or the- 

 electric current, or give rise to these. 



Again : if there were but a single body in space, its physical 

 field would, I think, be confined within itself, and not radiate- 

 outward indefinitely. Let us imagine space to consist of an. 

 elastic jelly : then all particles of matter in space are connected' 

 with each other by elastic bonds. One particle cannot be 

 moved from its position without setting up stresses in space 

 between itself and all the other particles. But it is evident 

 that the algebraic sum of all the stresses is zero. If the stress 

 be wholly positive on one body or particle, the stresses on all the 

 other particles will be negative and exactly equal in amount,, 

 whether there be a million or only one other particle. If there 

 be but one other particle or body, all of the negative stress — 

 electricity, if you choose — will be upon it, or perhaps rather in 

 the medium joining the two. Now, since the stress lies 

 wholly between these two (they are in no way connected with 

 any other particles, or, in other words, they are the only two 

 particles in space) , they maj' be moved in any way, providing 

 their positions relatively to each other remain the same, 

 without altering the stress of the surrounding medium. Since 

 they do move, there is kinetic energy; but this movement does 

 not alter their relations to other particles, because there are no 

 other particles: hence no additional stresses are set up. Their 

 movement does not convert the potential energy stored up 

 between them into kinetic energy, although a movement of one- 

 relative to the other would do so; and the resulting kinetic- 



