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| History of Theories of Aether and Electricity | |||||||||
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Electric and magnetic Science prior to the introduction of the potentials
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rapid progress. It was frankly advocated by another member
of the Dutch school, Hermann Boerhaave [1] (6. 1668, d. 1738),
Professor in the University of Leyden, whose treatise on
chemistry was translated into English in 1727. Somewhat later it was found that the heating effects of the rays from incandescent bodies may be separated from their luminous effects by passing the rays through a plate of glass, which transmits the light, but absorbs the heat. After this discovery it was no longer possible to identify the matter of heat with the corpuscles of light ; and the former was consequently accepted as a distinct element, under the name of caloric. [2] In the latter part of the eighteenth and early part of the nineteenth centuries [3] caloric was generally conceived as occupying the interstices between the particles of ponderable matter - an idea which fitted in well with the observation that bodies commonly expand when they are absorbing heat, but which was less competent to explain the fact [4] that water expands when freezing. The latter difficulty was overcome by supposing the union between a body and the caloric absorbed in the process of melting to be of a chemical nature; so that the consequent changes in volume would be beyond the possibility of prediction. As we have already remarked, the imponderability of heat did not appear to the philosophers of the eighteenth century to be a sufficient reason for excluding it from the list of chemical elements ; and in any case there was considerable doubt as to whether caloric was ponderable or not. Some experimenters believed that bodies were heavier when cold than when hot; others that they were heavier when hot than when cold. The century was far advanced before Lavoisier and Rumford finally proved that the temperature of a body is without sensible influence on its weight. Perhaps nothing in the history of natural philosophy is more amazing than the vicissitudes of the theory of heat. The true hypothesis, after having met with general acceptance throughout a century, and having been approved by a succession of illustrious men, was deliberately abandoned by their successors in favour of a conception utterly false, and, in some of its developments, grotesque and absurd. We must now return to s'Gravesande's book. The phenomena of combustion he explained by assuming that when a body is sufficiently heated the light-corpuscles interact with the material particles, some constituents being in consequence separated and carried away with the corpuscles as flame and smoke. This view harmonizes with the theory of calcination which had been developed by Becher and his pupil Stahl at the end of the seventeenth century, according to which the metals were sup- posed to be composed of their calces and an element phlogiston. The process of combustion, by which a metal is changed into its calx, was interpreted as a decomposition, in which the phlogiston separated from the metal and escaped into the atmosphere ; while the conversion of the calx into the metal was regarded as a union with phlogiston. [5] s'Gravesande attributed electric effects to vibrations induced in effluvia, which he supposed to be permanently attached to such bodies as amber. " Glass," he asserted, " contains in it, and has about its surface, a certain atmosphere, which is excited by Friction and put into a vibratory motion ; for it attracts and repels light Bodies. The smallest parts of the glass are agitated by the Attrition, and by reason of their elasticity, their motion is vibratory, which is communicated to the Atmosphere abovementioned : and therefore that Atmosphere exerts its action the further, the greater agitation the Parts of the Glass receive when a greater attrition is given to the glass." The English translator of s'Gravesande's work was himself destined to play a considerable part in the history of electrical science. Jean Theophile Desaguliers (b. 1683, d. 1744) was an Englishman only by adoption. His father had been a Huguenot pastor, who, escaping from France after the revocation of the Edict of Nantes, brought away the boy from La Rochelle, concealed, it is said, in a tub. The young Desaguliers was afterwards ordained, and became chaplain to that Duke of Chandos who was so ungratefully ridiculed by Pope. In this situation he formed friendships with some of the natural philosophers of the capital, and amongst others with Stephen Gray, an experimenter of whom little is known [6] beyond the fact that he was a pensioner of the Charterhouse. In 1729 Gray communicated, as he says, [7] " to Dr. Desaguliers and some other Gentlemen " a discovery he had lately made, " showing that the Electrick Vertue of a Glass Tube may be conveyed to any other Bodies so as to give them the same Property of attracting and repelling light Bodies as the Tube does, when excited by rubbing : and that this attractive Vertue might be carried to Bodies that were many Feet distant from the Tube." This was a result of the greatest importance, for previous workers had known of no other way of producing the attractive emanations than by rubbing the body concerned. [8] It was found that only a limited class of substances, among which the metals were conspicuous, had the capacity of acting as channels for the transport of the electric power ; to these Desaguliers, who. continued the experiments after Gray's death in 1736, gave [9] the name non-electrics or conductors. After Gray's discovery it was no longer possible to believe that the electric effluvia are inseparably connected with the bodies from which they are evoked by rubbing ; and it became necessary to admit that these emanations have an independent existence, and can be transferred from one body to another. Accordingly we find them recognized, under the name of the electric fluid [10] as one of the substances of which the world is constituted. The imponderability of this fluid did not, for the reasons already mentioned, prevent its admission by the side of light and caloric into the list of chemical elements. The question was actively debated as to whether the electric fluid was an element sui generis, or, as some suspected, was another manifestation of that principle whose operation is seen in the phenomena of heat. Those who held the latter view urged that the electric fluid and heat can both be induced by friction, can both induce combustion, and can both be transferred from one body to another by mere contact ; and, moreover, that the best conductors of heat are also in general the best conductors of electricity. On the other hand it was contended that the electrification of a body does not cause any appreciable rise in its temperature; and an experiment of Stephen Gray's brought to light a yet more striking difference. Gray, [11] in 1729,. made two oaken cubes, one solid and the other hollow, and showed that when electrified in the same way they produced exactly similar effects ; whence he concluded that it was only the surfaces which had taken part in the phenomena. Thus while heat is disseminated throughout the substance of a body, the electric fluid resides at or near its surface. In the middle of the eighteenth century it was generally compared to an enveloping atmosphere. " The electricity which a non-electric of great length (for example, a hempen string 800 or 900 feet long) receives, runs from one end to the other in a sphere of electrical Effluvia" says Desaguliers in 1740 [12] and a report of the French Academy in 1733 says : [13] " Around an electrified body there is formed a vortex of exceedingly fine matter in a state of agitation,, which urges towards the body such light substances as lie within its sphere of activity. The existence of this vortex is more than a mere conjecture ; for when an electrified body is brought close to the face it causes a sensation like that of encountering a cobweb. " [14] The report from which this is quoted was prepared in connexion with the discoveries of Charles-Francois du Fay (b. 1698, d. 1739), superintendent of gardens to the King of France. Du Fay [15] accounted for the behaviour of gold leaf when brought near to an electrified glass tube by supposing that at first the vortex of the tube envelopes the gold-leaf, and so attracts it towards the tube. But when contact occurs, the gold-leaf acquires the electric virtue, and so becomes surrounded by a vortex of its own. The two vortices, striving to extend in contrary senses, repel each other, and the vortex of the tube, being the stronger, drives away that of the gold-leaf. " It is then certain" says du Fay, [16] " that bodies which have become electric by contact are repelled by those which have rendered them electric ; but are they repelled likewise by other electrified bodies of all kinds ? And do electrified bodies differ from each other in no respect save their intensity of electrification ? An examination of this matter has led me to a discovery which I should never have foreseen, and of which I believe no one hitherto has had the least idea." [1] Boerhaave followed Homberg in supposing the matter of heat to be present ia all so-called vacuous spaces. [2] Scheele in 1777 supposed caloric to be a compound of oxygen and phlogiston, and light to be oxygen combined with a greater proportion of phlogiston. [3] In suite of the experiments of Benjamin Thompson, Count Eumford (b. 1753, .d. 1814), in the closing years of the eighteenth century. These should have sufficed to re-establish the older conception of heat. [4] This had been known since the time of Boyle. [5] The correct idea of combustion had been advanced by Hooke. "The dissolution of inflammable bodies," he asserts in the Micrographia, " is performed by a substance inherent in and mixed with the air, that is like, if not the very same with, that which is fixed in saltpetre." But this statement met with little favour at the time, and the doctrine of the compound nature of metals survived in full vigour until the discovery of oxygen by Priestley and Scheele in 1771-5. In 1775 Lavoisier reaffirmed Hooke's principle that a metallic calx is not the metal minus phlogiston, but the metal plus oxygen; and this idea, which carried with it the recognition of the elementary nature of metals, was generally accepted by the end of the eighteenth century. [6] Those who are interested in the literary history of the eighteenth century will recall the controversy as to whether the verses on the death of Stephen Gray were written hy Anna Williams, whose name they bore, or by her patron Johnson. [7] Phil. Trans, xxxvii (1731), pp. 18, 227, 285, 397. [8] Otto von Guericke (b. 1602, d. 1686) had, as a matter of fact, observed the conduction of electricity along a linen thread ; but this experiment does not seem to have been followed up. Cf. Experimenta nova magdeburgica, 1672. [9] Phil. Trans, xli. (1739), pp. 186, 193, 200, 209: Dissertation concerning Electricity, 1742. [10] The Cartesians defined a fluid to be a body whose minute parts are in a continual agitation. [11] Phil. Trans, xxxvii., p. 35. [12] Phil. Trans, xli., p. 636. [13] Hist, de 1'Acad., 1733, p. 6. [14] This observation had been made first by Hawksbee at the beginning of the century. [15] Mem. de 1'Acad. des Sciences, 1733, pp. 23, 73, 233, 457 ; 1734, pp. 341, 503; 1737, p. 86 ; Phil. Trans, xxxviii. (1734), p. 258. [16] Mem. de 1'Acad., 1733, p. 464. |