built The rigidity of a rotating chain rapidly diminishes and soon disappears if we stop the machine which drives it, 5 and, similarly, though a smoke ring may travel a good many feet in still air, yet after a while it gradually falls to pieces before our eyes. This is due to friction among the rotating parts of the system. The energy of the system is gradually frittered away as heat, the motion diminishes, 10 the rings gradually lose their rigidity, and presently their component particles are once more indistinguishable from those of the surrounding air. But imagine vortex rings set up in a perfect frictionless fluid. Would not these be eternal? Or suppose the fluid to be 15 approximation to a frictionless fluid. not, at any rate, seem eternal? In short, does it not appear conceivable that the atoms of the chemist may be vortex rings formed in the ether? This was Lord Kelvin's theory, of which it has been said that it is so beautiful 20 that whether it be true or whether it be untrue, at least it deserves to be true. On this view, the atoms of the chemist, atoms of radium, atoms of oxygen, atoms of argon, helium, and all the rest of the elements are not detached particles of alien matter bedded in the ether, like plums 25 in a pudding, but differentiated portions of the ether itself. only a very near Then would they Lord Kelvin's theory accounts for much. With its aid we begin to understand, or to feel we understand, the indestructibility of atoms and their capacity for definite vibrations which is revealed to us by the spectroscope. 30 Further, this theory gratifies in a striking manner the strong sentiment in favor of a simple universe, which has been the source of so many attempts to unify our conceptions of the physical basis of the latter. But it fails to include an electric charge as part of the constitution of the sub-atoms of matter, and thus on this ground, apart from other difficulties,* fails to satisfy the demands of at least one great school of physicists. A satisfactory theory of the ether, as Dr. Larmor has pointed out, must account for the conveyance of electric 5 attraction across the ether by elastic action, and an electric field must be a field of strain. Hence each sub-atom, with its permanent electric charge, must be surrounded by a field of permanent strain in the ether. This condition requires us to reject hypotheses based upon the conception 10 of a perfectly fluid ether, and forces us to regard the ether as endowed with some quality of the nature of elasticity. "A protion," or sub-atom of matter, therefore, Dr. Larmor tells us, “must be in whole or in part a nucleus of intrinsic strain in the ether, a place at which the continuity 15 of the medium has been broken and cemented together again (to use a crude but effective image) without accurately fitting the parts, so that there is a residual strain all round the place." The ultimate element of material constitution becomes, on this view, an electric charge or 20 nucleus of permanent strain in the ether instead of a vortex ring generated out of a perfect fluid, as in the hypothesis last discussed, and we may venture to look upon molecules as composed of systems of electrically positive and negative protions in a state of steady orbital motion round about 25 each other. In short, as Dr. Larmor says in Ether and Matter, it seems as if the master key to a complete unraveling of the general dynamical and physical relations of matter may lie in the fact that it is constituted of discrete molecules "involving in their constitutions orbital systems 30 of electrons, and moving through practically stagnant ether.' I am afraid this will hardly make the matter *E. g., vortex rings at a moderate distance from each other will not exhibit in their behavior to one another anything of the nature of gravitation. clear to all; still, perhaps most of us will gather in a general way that, according to this view, the ether is not a perfectly frictionless fluid, but, on the contrary, is endowed with an elastic quality; that, somehow, electrons, which are minute 5 charged particles of matter carrying electricity or perhaps particles of electricity alone, are generated in this ether, and that systems consisting of electrons revolving round about one another may we say more or less like the systems of the heavens?-form the atoms and mole10 cules of which the familiar forms of matter are built up. And this must suffice, as it would be impossible in a short article to develop Dr. Larmor's argument fully, or to give even a sketch of the specification of an ideal medium by which he illustrates his conception of the ether as a 15 perfect fluid endowed with the rotational elasticity demanded by its more obvious properties." 66 Here, then, we have the outlines of three pictures of the eternal ether, each presenting it as it shapes itself in the mind of a great contemporary thinker. To Men20 deléeff, the chemist, it appears to be the lightest of gases, the most inactive of all the elements. Professor Osborne Reynolds, the engineer, pictures it as a mass of dense, closely packed grains; Dr. Larmor as a rotationally elastic fluid. In Professor Mendeléeff's eyes the ether seems but 25 a finer kind of matter. In those of his colleagues it is the one concrete reality; while matter, according to Professor Reynolds, consists merely of waves, and, according to Dr. Larmor, of systems of electrons, or nuclei of permanent etherial strains, in rapid motion. 30 How are we to reconcile these diverse presentments of the ether? Perhaps the best answer I can offer is to remind you that throughout the history of science truth has ever been the offspring of diversity rather than of uniformity. Three men describing a neighbor's house might very well give discordant and yet not untrue accounts of it, according as their own windows looked upon its front, its back, or one of its sides. It may be I do not venture to say it is-that each of these seemingly diverse theories of the ether expresses something that is true 5 about that aspect of the subject which chiefly has presented itself to its author. SUGGESTIONS: This is a good example of “popular” exposition of a scientific question very recent in its interest. Observe how carefully the author starts, passing from point to point, each based on what has preceded it. Into what divisions does the exposition fall? How is the subject matter proportioned ?— i. e., what portions are emphasized and why? For what sort of audience is the exposition intended? Just how clear to you is the explanation of Mendeléeff's "periodic law?" Note the clear and relatively simple vocabulary of the expositor. Observe the illuminating summary with which the exposition closes. ADAPTED SUBJECTS (1) Throw into clear and connected literary form, with familiar analogies and illustrations, some scientific explanation that you have heard in class recently. (2) Explain, for a younger person, any of the following conceptions: the theory of the tides; why we see only one side of the moon; the nebular hypothesis; liquefaction of air; the undulatory theory of light; the theory of cyclonic storms; the theory of the ice machine; electric traction; the vortex theory of matter; the X-ray; radium. MEMORY* WILLIAM JAMES MEMORY proper, or secondary memory as it might be styled, is the knowledge of a former state of mind after it has already once dropped from consciousness; 10 *A Briefer Course in Psychology. Henry Holt & Co., pp. 287-88. 5 or rather it is the knowledge of an event, or fact, of which meantime we have not been thinking, with the additional consciousness that we have thought or experienced it before. The first element which such a knowledge involves would seem to be the revival in the mind of an image or copy of the original event. And it is an assumption made by many writers that such revival of an image is all that is needed to constitute the memory of the original oc10 currence. But such a revival is obviously not a memory, whatever else it may be; it is simply a duplicate, a second event, having absolutely no connection with the first event except that it happens to resemble it. The clock strikes to-day; it struck yesterday; and may strike a million times 15 ere it wears out. The rain pours through the gutter this week; it did so last week; and will do so in sæcula sæculorum. But does the present clock-stroke become aware of the the past ones, or the present stream recollect the past stream because they repeat and resemble them? Assured20 ly not. And let it not be said that this is because clockstrokes and gutters are physical and not psychical objects; for psychical objects (sensations, for example,) simply recurring in successive editions will remember each other on that account no more than clock-strokes do. No memory is involved in the mere fact of recurrence. The successive editions of a feeling are so many independent events, each snug in its own skin. day's feeling is dead and buried; and the presence. of to-day's is no reason why it should resuscitate along 30 with to-day's. A farther condition is required before the present image can be held to stand for a past original. 25 25 Yester That condition is that the fact imaged be expressly referred to the past, thought as in the past. But how can |