III. Dynamical Theories. In more recent years the

suggestion was made that the so-called atoms may be forms of motion (ex. gr. permanent eddies) in one elementary material known as the ether ; on this view all the atoms are constituted of the same matter, but the physical conditions are different for the different kinds of atoms. It has been said that there is an
* Theoretische Ojptik, Braunschweig, 1885.
CH. XXl] MATTER AND ETHER THEORIES 463
initial difficulty in any such hypothesis, since the all-pervading elementary fluid must possess inertia, so that to explain matter we assume the existence of a fluid possessing one of the chief characteristics of matter. This is true as far as it goes, but it is not more unreasonable than to attribute all the fundamental properties of matter to the atoms themselves, as is done by many writers. The next paragi-aph contains a statement of one of the earliest attempts to formulate a dynamical atomic hypothesis.
(i) The Vortex Ring Hypothesis. This hypothesis assumes that each atom is a vortex ring in an incompressible frictionless homogeneous fluid.
Vortex rings — though, since friction is brought into play, of an imperfect character — can be produced in air by many smokers. Better specimens can be formed by taking a card- board box in one side of which a circular hole is cut, filling it with smoke, and hitting the opposite side sharply. The tendency of the particles forming a ring to maintain their annular connection may be illustrated by placing such a box on one side of a room in a direct line with the flame of a lighted candle on the other side. If properly aimed, the ring will travel across the room and put out the flame. If the box is filled only with air, so that the ring is not visible, the experiment is more effective.
In 1858 von Helmholtz* showed that a closed vortex filament in a perfect fluid is indestructible and retains certain characteristics always unaltered. In 1867 Sir William Thomson propounded f the idea that matter consists of vortex rings in a fluid which fills space. If the fluid is perfect we could neither create new vortex rings nor destroy those already created, and thus the permanence of the atoms is explained. Moreover the atoms would be flexible, compressible, and in incessant vibration
* Crelle\^ Journal, 1858, vol. lv, pp. 25 — 55 ; translated by Tait in the Philosophical Magazine, .June, 1867, supplement, series 4, vol. xxxiii, pp. 485 — 512.
t Proceedings of the Royal Society of Edinburgh, Feb. 18, 1867, vol. vi, pp. 94—105.
464 MA.TTER AND ETHIOR THEORIES [CH. XXI
at a definite fundamental rate. This rate is very rapid, and Sir William Thomson gave the number of vibrations per second of a sodium ring as probably being greater than 10".
By a development of this hypothesis Sir J. J. Thomson* showed, some years ago, that chemical combination may be explained. He supposed that a molecule of a compound is formed by the linking together of vortex filaments representing atoms of different elements : this arrangement may be compared with that of helices on an anchor ring. For stability not more than six filaments may be combined together, and their strengths must be equal. Another way of explaining chemical combination on the vortex atom hypothesis has been suggested by W. M. Hicks. It is known f that a spherical mass of fluid, whose interior possesses vortex motion, can move through liquid like a rigid sphere, and he has shown that one of these spherical vortices can swallow up another, thus forming a compound element.
(ii) The Vortex Sponge Hypothesis. Any vortex atom hj^pothesis labours under the difficulty of requiring that the density of the fluid ether shall be comparable with that of ordinary matter. In order to obviate this and at the same time to enable it to transmit transversal radiations Sir William Thomson suggested what has been termed, not perhaps very happily, the vortex sponge hypothesis |: this rests on the assumption that laminar motion can be propagated through a turbulently moving inviscid liquid. The mathematical difficulties con- nected with such motion have prevented an adequate dis- cussion of this hypothesis, and I confine myself to merely mentioning it.
These hypotheses, of vortex motion in a fluid, account for the indestructibility of matter and for many of its properties. But in order to explain statical electrical attraction it would
* A Treatise on the Motion of Vortex Rings, Cambridge, 1883.
+ See a memoir by M. J. M. Hill in the Pliilosophical Transactions of the Royal Society, London, 1894, part i, pp. 213—246.
J Philosophical Magazine, London, October, 1887, series 5, vol. xxiv, pp. 342 —353.
CH. XXl] MATTER AND ETHER THEORIES 4G5
seem necessary to suppose that the ether is elastic; in other words, that an electric field must be a field of strain. If so, complete fluidity in the ether would be impossible, and hence the above theories are now regarded as untenable.
(iii) The Ether-Squirts Hypothesis. Karl Pearson* sug- gested another dynamical theory in which an atom is conceived as a point at which ether is pouring into our space from space of four dimensions.
If an observer lived in two-dimensional space filled with ether and confined by two parallel and adjacent surfaces, and if through a hole in one of these surfaces fresh ether were squirted into this space, the variations of pressure thereby produced might give the impression of a hard impenetrable body. Similarly an ether- squirt from space of four dimensions into our space might give us the impression of matter.
It seems necessary on this hypothesis to suppose that there are also ether-sinks, or atoms of negative mass ; but as ether- squirts would repel ether-sinks we may suppose that the latter have moved out of the universe known to our senses.
By defining the mass of an atom as the mean rate at which ether is squirting into our space at that point, we can deduce the Newtonian law of gravitation, and by assuming certain periodic variations in the rate of squirting we can deduce some of the phenomena of cohesion, of chemical action, and of electro- magnetism and light. But of course the hypothesis rests on the assumption of the existence of a world beyond our senses.
(iv) Tlie Electron Hypothesis. MacCullagh, in 1837 and 1839, proposed to account for optical phenomena on the as- sumption of an elastic ether possessing elasticity of the type required to enable it to resist rotation. This suggestion has been recently modified and extended by Sir Joseph Larmorf, and, as now enunciated, it accounts for many of the electrical and magnetic (as well as the optical) properties of matter.
* American Journal of Mathematics, 1891, vol. xiii, pp. 309 — 362.
t Philosophical Transactions of the Royal Society, Loudon, 1894, pp. 719
822 ; 1895, pp. 695—743.
B. li. 30
466 MATTER AND ETHER THEORIES [CH. XXI
The hypothesis is however very artificial. The assumed ether is a rotationally elastic incompressible fluid. In this fluid Larmor introduces monad electric elements or electrons, which are nuclei of radial rotational strain. He supposes that these electrons constitute the basis of matter. He further supposes that an electrical current consists of a procession of these electrons, and that a magnetic particle is one in which these entities are revolving in minute orbits. Djmamical considera- tions applied to such a system lead to an explanation of nearly all the more obvious phenomena. By further postulating that the orbital motion of electrons in the atom constitute it a fluid vortex it is possible to apply the hydrodynamical pulsatory theory of Bjerknes or Hicks and obtain an explanation of gravitation.
Thus on this view mass is explained as an electrical mani- festation. Electricity in its turn is explained by the existence of electrons, that is, of nuclei of strain in the ether, which are supposed to be in incessant and rapid motion. Whilst, to render this possible, properties are attributed to the ether which are apparently inconsistent with our experience of the space it fills. Put thus, the hypothesis seems very artificial. Perhaps the utmost we can say for it is that, from some points of view, it may, so far as analysis goes, be an approximation to the true theory; in any case much work will have to be done before it can be considered established even as a working hypothesis.
(v) Recent Developments. Most of the above was written in 1891. Since then investigations on radio-activity have opened up new avenues of conjecture which tend to strengthen the electron theory as a working hypothesis. More than thirty years ago Clerk Maxwell had shown that light and electricity were closely connected phenomena. It was then believed that both were due to waves in the hypothetical ether, but it was supposed that the phenomena of matter on the one side and of light and electricity on the other were sharply distinguished one from the other. The difterences, however, between matter and light tend to disappear as investigations proceed. In 1895
CH. XXl] MATTER AND ETHER THEORIES 4G7
Rontgen established the existence of rays which could produce light, which had the same velocity as light, which were not affected by a magnet, and which could traverse wood and certain other opaque substances like glass. A year later Becquerel showed that uranium was constantly emitting rays which, though not affecting the eye as light, were capable of producing an image on a photographic plate. Like Rontgen rays they can go through thin sheets of metal ; like heat rays they burn the skin; like electricity they generate ozone from oxygen. Passed into the air they enable it to conduct the electric current. Their speed has been measured and found to be rather more than half that of light and electricity. It was soon found that thorium possessed a similar property, but in 1903 Curie showed that radium possessed radio-activity to an extent previously unsuspected in any body, and in fact the rays were so powerful as to make the substance directly visible. Further experiments show^ed that numerous bodies are radio-active, but the effects are so much more marked in radium that it is convenient to use that substance for most experimental pur- poses.
Radium gives off no less than three kinds of rays besides a radio-active emanation. In these discharges there appears to be a gradual change from what had been supposed to be an elementary form of matter to another. This leads to the belief that of the known forms of matter some, perhaps even all, are not absolutely stable. On the other hand, it may be that only radio-active bodies are unstable, and that in their disintegration we are watching the final stage in the evolution of stable and constant forms of matter. It may, however, in any case turn out that some, or perhaps all, of the so-called elements may be capable of resolution into different combinations of electrons or electricity.
At an earlier date J. J. Thomson had concluded that the glow, seen when an electric current passes through a high vacuum tube, is due to a rush of minute particles across the tube. He calculated their weight, their velocity, and the charge of electricity transported by or represented by them, and found
SO— 2
468 MATTER AND ETHER THEORIES [CH. XXI
these to be constant. They were deflected like Becquerel rays. All space seems to contain them, and electricity, if not identical with them, is at least carried by them. This suggested that these minute particles might be electrons. If so, they might thus give the ultimate explanation of electricity as well as matter, and the atom of the chemist would be not an irreducible unit of matter, but a system comprising numerous such minute particles. These conclusions are consistent with those subse- quently deduced from experiments with radium. In 1904 the hypothesis was carried one stage further. In that year J. J. Thomson investigated the conditions of stability of certain systems of revolving particles; and on the h3^pothesis that an atom of matter consists of a number of particles carrying negative charges of electricity revolving in orbits within a sphere of positive electrification he deduced many of the pro- perties of the different chemical atoms corresponding to different possible stable systems of this kind. His scheme led to results agreeing closely with the results of Mendelejev's periodic hypo- thesis according to which some or all of the properties of an element are a periodic function of its atomic weight. An interesting consequence of this view is that Franklin's description of electricity as subtle particles pervading all bodies may turn out to be substantially correct. It is also remarkable that corpuscles somewhat analogous to those whose existence was suggested in Newton's corpuscular theory of light should be now supposed to exist in cathode and Becquerel rays.
(vi) These facts have been utilized by G. Le Bon who suggested that electricity and matter may be regarded as intermediate stages in the flux of the ether, the former being one stage in the incessant dissociation of matter which arises from ether and ultimately is resolved again into ether. The theory is ingenious but appears untenable.
(vii) Ether as matter. Recently another hypothesis as to the nature of the ether was put forward by J). I. Mendelejev, the distinguished chemist. In the grouping of the elements ac- cording to his periodic law, there were originally twelve series