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| History of theories of Aether and Electricity | |||||||||
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The Theory of Aether in the seventeenth Century
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THE observation of the heavens, which has been pursued continually from the earliest ages, revealed to the ancients the
regularity of the planetary motions, and gave rise to the
conception of a universal order. Modern research, building on
this foundation, has shown how intimate is the connexion
between the different celestial bodies. They are formed of the
same kind of matter ; they are similar in origin and history ;
and across the vast spaces which divide them they hold
perpetual intercourse. Until the seventeenth century the only influence which was known to be capable of passing from star to star was that of light. Newton added to this the force of gravity ; and it is now recognized that the power of communicating across vacuous regions is possessed also by the electric and magnetic attractions. It is thus erroneous to regard the heavenly bodies as isolated in vacant space; around and between them is an incessant conveyance and transformation of energy. To the vehicle of this activity the name aether has been given. The aether is the solitary tenant of the universe, save for that infinitesimal fraction of space which is occupied by ordinary matter. Hence arises a problem which has long engaged attention, and is not yet completely solved : What relation subsists between the medium which fills the interstellar void and the condensations of matter that are scattered throughout it? The history of this problem may be traced back continuously to the earlier half of the seventeenth century. It first emerged clearly in that reconstruction of ideas regarding the physical universe which was effected by Rene Descartes. Descartes was born in 1596, the son of Joachim Descartes, Counsellor to the Parliament of Brittany. As a young man he followed the profession of arms, and served in the campaigns of Maurice of Nassau, and the Emperor ; but his twenty-fourth year brought a profound mental crisis, apparently not unlike those which have been recorded of many religious leaders ; and he resolved to devote himself thenceforward to the study of philosophy. The age which preceded the birth of Descartes, and that in which he lived, were marked by events which greatly altered the prevalent conceptions of the world. The discovery of America, the circumnavigation of the globe by Drake, the overthrow of the Ptolemaic system of astronomy, and the invention of the telescope, all helped to loosen the old foundations and to make plain the need for a new structure. It was this that Descartes set himself to erect. His aim was the most ambitious that can be conceived ; it was nothing less than to create from the beginning a complete system of human knowledge. Of such a system the basis must necessarily be metaphysical ; and this part of Descartes' work is that by which he is most widely known. But his efforts were also largely devoted to the mechanical explanation of nature, which indeed he regarded as one of the chief ends of Philosophy.[1] The general character of his writings may be illustrated by a comparison with those of his most celebrated contemporary [2]. Bacon clearly defined the end to be sought for, and laid down the method by which it was to be attained; then, recognizing that to discover all the laws of nature is a task beyond the powers of one man or one generation, he left to posterity the work of filling in the framework which he had designed. Descartes, on the other hand, desired to leave as little as possible for his successors to do ; his was a theory of the universe, worked out as far as possible in every detail. It is, however, impossible to derive such a theory inductively unless there are at hand sufficient observational data on which to base the induction ; and as such data were not available in the age of Descartes, he was compelled to deduce phenomena from preconceived principles and causes, after the fashion of the older philosophers. To the inherent weakness of this method may be traced the errors that at last brought his scheme to ruin. The contrast between the systems of Bacon and Descartes is not unlike that between the Roman republic and the empire of Alexander. In the one case we have a career of aggrandizement pursued with patience for centuries ; in the other a growth of fungus-like rapidity, a speedy dissolution, and an immense influence long exerted by the disunited fragments. The grandeur of Descartes' plan, and the boldness of its execution, stimulated scientific thought to a degree before unparalleled ; and it was largely from its ruins that later philosophers constructed those more valid theories which have endured to our own time. Descartes regarded the world as an immense machine, operating by the motion and pressure of matter. " Give me matter and motion," he cried, " and I will construct the universe." A peculiarity which distinguished his system from that which afterwards sprang from its decay was the rejection of all forms of action at a distance ; he assumed that force cannot be communicated except by actual pressure or impact. By this assumption he was compelled to provide an explicit mechanism in order to account for each of the known forces of nature - a task evidently much more difficult than that which lies before those who are willing to admit action at a distance as an ultimate property of matter. Since the sun interacts with the planets, in sending them light and heat and influencing their motions, it followed from Descartes' principle that interplanetary space must be a plenum, occupied by matter imperceptible to the touch but capable of serving as the vehicle of force and light. This conclusion in turn determined the view which he adopted on the all- important question of the nature of matter. Matter, in the Cartesian philosophy, is characterized not by impenetrability, or by any quality recognizable by the senses, but simply by extension ; extension constitutes matter, and matter constitutes space. The basis of all things is a primitive, elementary, unique type of matter, boundless in extent and infinitely divisible. In the process of evolution of the universe three distinct forms of this matter have originated, correspond ing respectively to the luminous matter of the sun, the transparent matter of interplanetary space, and the dense, opaque matter of the earth. " The first is constituted by what has been scraped off the other particles of matter when they were rounded ; it moves with so much velocity that when it meets other bodies the force of its agitation causes it to be broken and divided by them into a heap of small particles that are of such a figure as to fill exactly all the holes and small interstices which they find around these bodies. The next type includes most of the rest of matter ; its particles are spherical, and are very small compared with the bodies we see on the earth ; but nevertheless they have a finite magnitude, so that they can be divided into others yet smaller. There exists in addition a third type exemplified by some kinds of matter namely, those which, on account of their size and figure, cannot be so easily moved as the preceding. I will endeavour to show that all the bodies of the visible world are composed of these three forms of matter, as of three distinct elements ; in fact, that the sun and the fixed stars are formed of the first of these elements, the interplanetary spaces of the second, and the earth, with the planets and comets, of the third. For, seeing that the sun and the fixed stars emit light, the heavens transmit it, and the earth, the planets, and the comets reflect it, it appears to me that there is ground for using these three qualities of luminosity, transparence, and opacity, in order to distinguish the three elements of the visible world [3]. According to Descartes' theory, the sun is the centre of an immense vortex formed of the first or subtlest kind of matter [4]. The vehicle of light in interplanetary space is matter of the second kind or element, composed of a closely packed assemblage of globules whose size is intermediate between that of the vortex-matter and that of ponderable matter. The globules of the second element, and all the matter of the first element, are constantly straining away from the centres around which they turn, owing to the centrifugal force of the vortices [5]; so that the globules are pressed in contact with each other, and tend to move outwards, although they do not actually so move [6]. It is the transmission of this pressure which constitutes light ; the action of light therefore extends on all sides round the sun and fixed stars, and travels instantaneously to any distance [7]. In the Dioptrique[8] vision is compared to the perception of the presence of objects which a blind man obtains by the use of his stick ; the transmission of pressure along the stick from the object to the hand being analogous to the transmission of pressure from a luminous object to the eye by the second kind of matter. Descartes supposed the " diversities of colour and light " to be due to the different ways in which the matter moves [9]. In the Meteores[10], the various colours are connected with different rotatory velocities of the globules, the particles winch rotate most rapidly giving the sensation of red, the slower ones of yellow, and the slowest of green and blue - the order of colours being taken from the rainbow. The assertion of the dependence of colour on periodic time is a curious foreshadowing of one of the great discoveries of Newton. The general explanation of light on these principles was amplified by a more particular discussion of reflexion and refraction. The law of reflexion - that the angles of incidence and refraction are equal - had been known to the Greeks ; but the law of refraction that the sines of the angles of incidence and refraction are to each other in a ratio depending on the media was now published for the first time [11]. Descartes gave it as his own ; but he seems to have been under considerable obligations to Willebrord Snell (b. 1591, d. 1626), Professor of Mathematics at Leyden, who had discovered it experimentally (though not in the form in which Descartes gave it) about 1621. Snell did not publish his result, but communicated it in manuscript to several persons, and Huygens affirms that this manuscript had been seen by Descartes. Descartes presents the law as a deduction from theory. This, however, he is able to do only by the aid of analogy ; when rays meet ponderable bodies, " they are liable to be deflected or stopped in the same way as the motion of a ball or a stone impinging on a body " ; for " it is easy to believe that the action or inclination to move, which I have said must be taken for light, ought to follow in this the same laws as motion."[12] Thus he replaces light, whose velocity of propagation he believes to be always infinite, by a projectile whose velocity varies from one medium to another. The law of refraction is then proved as follows [13]: Let a ball thrown from A meet at B a cloth CBE, so weak that the ball is able to break through it and pass beyond, but with its resultant velocity reduced in some definite proportion, say 1 : k. Then if BI be a length measured on the refracted ray equal to AB, the projectile will take k times as long to describe BI as it took to describe AB. But the component [1] Of the works which bear on our present subject, the Dioptrique and the Meteores were published at Leyden in 1638, and the Principia Philosophiae at Amsterdam in 1644, six years before the death of its author. [2] The principal philosophical works of Bacon were written about eighteen years before those of Descartes. [3] Principia, Part iii, 52. [4] It is curious to speculate on the impression which would have been produced had the spirality of nebulae been discovered before the overthrow of the Cartesian theory of vortices. [5] Ibid., 55-59. [6] Ibid., 63. [7] Ibid., 64. [8] Discours premier. [9] Principia, Part iv, 195. [10] Discours Huitieme [11] Dioptrique, Discours second. [12] Ibid., Discours premier. [13] Ibid., Discours second. |