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of expression and formulation of our knowledge, and also tion of this atmosphere, and its nucleus may be left as regards points of detail. This great advance, which is out of our discussions; but in the problems of bio ogy the result of the gradual focussing of a century’s work in great tracts of invariable correlations have to be dealt the minute exploration of the exact laws of optical and with, which seem hopelessly more complex than any known electric phenomena, clearly carries with it deeper insight or humanly possible physical scheme. To make room tor into the physical nature of matter itself and its modes of these we have to remember that the atomic nucleus has inanimate interaction. remained entirely undefined and beyond our problem ; so If we rest on the synthesis here described, the energy of that what may occur, say when two molecules come into the matter, even the thermal part, appears largely as potenclose relations, is outside physical science—not, however, tial energy of strain in the aether, which interacts with altogether outside, for we know that when the vital nexus m the kinetic energy associated with disturbances involving any portion of matter is dissolved, the atoms will remain, finite velocity of matter. It may, however, be maintained in their number, and their atmospheres, and all inorganic that an ultimate analysis would go deeper, and resolve all relations, as they were before vitality supervened. phenomena of elastic resilience into consequences of the Nature of Properties of Material Bodies. It thus kinetic stability of steady motiohal states, so that only appears that the doctrine of atomic material constitution motions, but not strains, would remain. On such a view and the doctrine of a universal aether stand to each other the aether might conceivably be a perfect fluid, its fundain a relation of mutual support; if the scheme of physica mental property of elastic reaction arising (as suggested by laws is to be as precise as observation and measurement Kelvin and FitzGerald) from a structure of tangled or inappear to make it, both doctrines are required in our efforts terlaced vortex filaments pervading its substance, which towards synthesis. Our direct knowledge of matter can, might conceivably arrange themselves into a stable conhowever, never be more than a rough knowledge o f16 figuration and so resist deformation. This raises the general average behaviour of its molecules; for the smallest further question as to whether the transmission of gramaterial speck that is sensible to our coarse perceptions vitation can be definitely recognized among the procontains myriads of atoms. _ The properties of the most perties of an ultimate medium; if so, we know that it minute portion of matter which we can examine are thus must be associated with some feature, perhaps very deepof the nature of averages. We may gradually invent seated, or on the other hand perhaps depending simply on means of tracing more and more closely the average drifts incompressibility, which is not sensibly implicated m the of translation or orientation, or of changes of arrange- electric and optical activities. With reference to all such ment, of the atoms; but there will always remain an further refinements of theory, it is to be borne in mind that unaveraged residue devoid of any recognized regularity, the perfect fluid of hydrodynamic analysis is not a merely which we can only estimate by its total amount. Thus, if passive inert plenum ; it is also a continuum, with the prowe are treating of energy, we can separate but mechanical perty that no finite internal slip or discontinuity of motion and electric and other constituents in it; and there will can ever arise in it through any kind of disturbance; and be a residue of which we know nothing except its quantity, this property must be postulated, as it cannot be explained. and which we may call thermal. This merely thermal Motion of Material Atoms through the jEther.—An energy—which is gradually but very slowly being restricted important question arises whether, when a material body in amount as new subsidiary organized types become recog- is moved through the sether, the nucleus of each atom nised in it—though transmutable in equivalent quantities carries some of the surrounding aether along with it; or with the other kinds, yet is so only to a limited extent, whether it practically only carries on its strain-form or the tracing out of the laws of this limitation belongs to the physical atmosphere, which is transferred from one porscience of thermodynamics. It is the business of that tion of aether to another after the manner of a shadow, science to find out what is the greatest amount of thermal rather like a loose knot which can slip along a rope energy that can possibly be recoverable into organized kinds or without the rope being required to go with it. We can under given circumstances. The discovery of definite obtain a pertinent illustration from the motion of a vortex laws in this region might at first sight seem hopeless ; but ring in a fluid; if the circular core of the ring is thin the argument rests on an implied postulate of stability compared with its diameter, and the vorticity is not very and continuity of constitution of material substances, so o-reat, it is the vortical state of motion that travels across that after a cycle of transformations we expect to recover the fluid without transporting the latter bodily with it them again as they were originally—on the postulate, in except to a slight extent very close to the core. . We might fact, that we do not expect them to melt out of organized existence in our hands. The laws of thermodynamics, thus imagine a structure formed of an aggregation of very including the fundamental principle that a physical pro- thin vortex rings, which would move across the fluid withsensibly disturbing it; on the other hand, if formed of perty, called temperature, can be defined, which tends out stronger vortices, it may transport the portion of the fluid towards uniformity, are thus relations between the properties of types of material bodies that can exist perma- that is within its own structure along with it as if it were nently in presence of each other; why they so maintain a solid mass, and therefore also push aside the surrounding themselves remains unknown, but the fact gives a point fluid as it passes. The motion of the well-known steady vortex is an example of the latter case. d’appui. Returning now to the aether, on our present spherical Convection of Optical Waves. —The nature of the point of view no such complications there arise ; it must be regarded as a continuous uniform medium free from any motion, if any, that is produced in the surrounding complexities of atomic aggregation, whose function is regions of the aether by the translation of matter through confined to the transmission of the various types of it, can be investigated by optical experiment. The obvious physical effect between the portions of matter. The body to take in the first instance is the Earth itself, problem of its constitution is thus one which can be which on account of its annual orbital motion is travelling attacked and continually approximated to, and which through space at the rate of about 18 miles per second. may possibly be definitely resolved. It has to be com- If the surrounding aether is thereby disturbed, the waves petent to transmit the transverse waves of light and of light arriving from the stars will partake of its movethe ascertained phemonena of the astronomical electricity, and the other known radiant and electric ment; aberration of light show that the rays travel to the actions • the way in which this is done is now in the main known, though there are still questions as to the mode I observer, across this disturbed aether near the Earth, m
