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m t : I E R 105 Newton’s discovery of the actual facts regarding universal aether will involve a synthesis or intimate correlation of gravitation. Although, however, gravitation has formed the various types of physical agencies, which appear so the most perfect instance of an influence completely different to us mainly because we perceive them through expressible, up to the most extreme refinement of accuracy, different senses. The evidence for this view, that all these in terms of laws of direct action across space, yet, as is agencies are at bottom connected together and parts of well known, the author of this ideally simple and perfect the same scheme, was enormously strengthened during the theory held the view that it is not possible to conceive of latter half of the 19th century by the development of a reladirect mechanical action independent of means of trans- tion of simple quantitative equivalence between them; it mission. In this belief he differed from his pupil Cotes, has been found that we can define quantities relating to and from most of the great mathematical astronomers of them, under the names of mechanical energy, electric the 18th century, who worked out in detail the task energy, thermal energy, and so on, so that when one of sketched by the genius of Newton. They were content them disappears it is replaced by the others to exactly with a knowledge of the truth of the principle of gravita- equal amount. This single principle of energy has transtion ; instead of essaying to explain it further by the formed physical science by making possible the construcproperties of a transmitting medium, they in fact modelled tion of a network of ramifying connexions between its the whole of their natural philosophy on that principle, various departments : it thus stimulates the belief that and tried to express all kinds of material interaction in these constitute a single whole, and encourages the search terms of laws of direct mechanical attraction across space. for the complete scheme of interconnexion of which the If material systems are constituted of discrete atoms, principle of energy and the links which it suggests form separated from each other by many times the diameter of only a single feature. any of them, this simple plan of exhibiting their interAtomic Structure of Matter.—The greatest obstacle to actions in terms of direct forces between them would such a search is the illimitable complexity of matter, as indeed be exact enough to apply to a wide range of contrasted with the theoretical simplicity and uniformity questions, provided we could be certain that the laws of of the physical agencies which connect together its different the forces depended only on the positions and not also parts. It has been maintained since the times of the early on the motions of the atoms. The most important Greek philosophers, and possibly even more remote ages, example of its successful application has been the theory that matter is constituted of independent indestructible of capillary action elaborated by Laplace • though even units, which cannot ever become divided by means of any here it appeared, in the hands of Young, and in complete mutual actions they can exert. Since the period, a century fulness afterwards in those of Gauss, that the definite ago, when Dalton and his contemporaries constructed from results, attainable by the hypothesis of mutual atomic this idea a scientific basis for chemistry, the progress of attractions really reposed on much wider and less special that subject has been wonderful beyond any conception principles—those, namely, connected with the modern doc- that could previously have been entertained; and the trine of energy. atomic theory in some form appears now to be an indisIdea of an jEther.—The wider view, according to which pensable part of the framework of physical science. Now the hypothesis of direct transmission of physical influences this doctrine of material atoms is an almost necessary expresses only part of the facts, is that all space is filled corollary to the doctrine of a universal aether. For if with physical activity, and that while an influence is pass- we held that matter is continuous, one of two alternatives ing across from a body A to another body B, there is some would be open. We might consider that matter and aether dynamical process in action in the intervening region, can co-exist in the same space ; this would involve the though it appears to the senses to be mere empty space. co-existence and interaction of a double set of properties, The question is whether we can represent the facts more introducing great complication which would place any simply by supposing the intervening space to be occupied coherent scheme of physical action probably beyond the by a medium which transmits physical actions after the powers of human analysis. Or we might consider that manner that a continuous material medium, solid or liquid, aether exists only where matter is not, thus making it a transmits mechanical disturbance. Various analogies of very rare and subtle and elastic kind of matter j then we this sort are open to us : for example, the way in which a should have to assign these very properties to the matter fluid medium transmits pressure from one immersed solid itself where it replaces aether, in addition to its more to another—or from one vortex ring belonging to the fluid familiar properties, and the complication would remain. to another, which is a much wider and more suggestive case The other course is to consider matter as formed of ultimate or the way in which an elastic fluid like the atmosphere atoms, each the nucleus or core of an intrinsic modification transmits sound; or the way in which an elastic solid impressed on the surrounding region of the aether; this transmits waves of transverse as well as longitudinal dis- might conceivably be of the nature of vortical motion of a placement. It is on our familiarity with modes of trans- liquid round a ring-core, thus giving a vortex atom, or of mission such as these, and with the exact analyses of them an intrinsic strain of some sort radiating from a core, which the science of mathematical physics has been able which would give an electric atom. We recognize an atom to make, that our predilection for filling space with an only through its physical activities as manifested in its adhereal transmitting medium, constituting a universal interactions with other atoms at a distance from it; this connexion between material bodies, largely depends; field of physical activity would be identical with the perhaps ultimately it depends most of all, like all our surrounding field of sethereal motion or strain that is physical conceptions, on the intimate knowledge that associated with the atom, and is carried on along with it we can ourselves exert mechanical effect on outside bodies as it moves. Here then we have the basis of a view only through the agencies of our limbs and sinews. The in which there are not two media to be considered, but pioblem thus arises, Can we form a consistent notion of one medium, homogeneous in essence and differentiated such a connecting medium ? It must be a medium which as regards its parts only by the presence of nuclei of can be effective for transmitting all the types of physical intrinsic strain or motion—in which the physical activiaction known to us : it would be worse than no solution ties of matter are identified with those arising from the to have one medium to transmit gravitation, another to atmospheres of modified aether which thus belong to transmit electric effects, another to transmit light, and so its. atoms. As regards laws of general physical interon. Thus the attempt to find out a constitution for the actions, the atom is fully represented by the constituS. I. — 14