Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/14

Otto von Guericke (I802 86). strong spirit of nitre, glass of lead, caput mortuum of amber, white sapphire, white amethyst, diaphanous ore of lead, carnelian, and a green stone supposed to be a sapphire. To these discoveries of Boyle his contemporary Otto von Guericke added the highly important one of electric light (Experimenta Nova Magdeburg ica, lib. iv. cap. 15). Having cast a globe of sulphur in a glass sphere, and broken off the glass, he mounted the sulphur ball upon a revolving axis, and excited it by the friction of the hand. By this means he discovered that light and sound accom panied strong electrical excitation, and he compares the light to that which is exhibited by breaking lump sugar in the dark. With this powerful apparatus Guericke verified on a greater scale the results obtained by his predecessors, and obtained several new ones of very considerable import ance. He found that a light body, when once attracted by an excited electric, was repelled by it, and was in capable of a second attraction until it had been touched by some other body ; and that light bodies suspended within the sphere of influence of an excited electric possessed the same properties as if they had been excited. To our illustrious countryman Sir Isaac Newton the science of electricity owes some important observations. He used in his electrical experiments a globe of glass rubbed by the hand instead of the sulphur globe of Von Guericke. It would appear that Newton was the first to use glass in this way (Optics, query 8th). We owe also to Sir Isaac a beautiful experiment on the excitation of elec tricity which has since become very popular. Having fixed a round disc of glass in a short brass cylinder, he placed small pieces of thin paper within the cylinder and upon a table, so that the lower surface of the glass was one-eighth of an inch distant from the table. He then rubbed the upper surface of the glass, and he observed the pieces of paper " leap from one part of the glass to the other, and twirl about in the air." This experiment, after a previous unsuccessful trial, was repeated by the Royal Society in 1676 (Brewster s Life of Neivton, p. 307). Francis Hawksbee, one of the most active experi mental philosophers of his age, added many new facts to the science. In 1705 he communicated to the Royal Society several curious experiments on what he calls " the mercurial phosphorus." He showed that light could be produced by passing common air through mer cury placed in a well-exhausted receiver. The air rushing through the mercury, blew it up against the sides of the glass that held it, " appearing all around like a body of fire, consisting of abundance of glowing globules." The phenomenon continued till the receiver was half full of air. These phenomena had been observed in the Torricel lian vacuum before Hawksbee s time, and various explana tions suggested. He suspected that they were due to electricity, and remarked their resemblance to lightning. Like Newton he used a revolving glass globe rubbed by the hand to generate electricity. Besides the experiment above alluded to he made many others on the electric light and on the attractions of electrified bodies. Descrip tions of these will be found in his Physico- Mechanical Experiments, 1709, and in several memoirs in the Philo sophical Transactions about 1707. About the same time Dr Wall (Phil Trans., 1708) observed the spark and crackling sound accompanying the electrical excitation of amber, and compared them to thunder and lightning. One of the most ardent experimentalists of his time was Stephen Gray, a Fellow of the Royal Society. In his first paper, published in 1720, he showed that electricity could be excited by the friction of feathers, [HISTORY. hair, silk, linen, woollen, paper, leather, wood, parchment, and gold-beaters skin. Several of these bodies exhibited light in the dark, especially after they had been warmed ; but all of them attracted light bodies, and sometimes at the distance of eight or ten inches. An epoch was made in the history of electricity by the discovery of Gray in 1729, that certain bodies had, while others had not, the power of conveying electricity from one body to another, i.e., in modern phrase, conducting it. Gray experimented with a glass tube, into the ends of which were fastened two corks ; into one of these he fastened a fir rod, and to the end of the rod an ivory ball. On rubbing the glass he found that the ball attracted the light bodies as vigorously as the glass itself. He made a variety of experiments with rods of different length, and with a packthread, by which he suspended his ball from the balcony of an upper story of his house, all with the same result. He then attempted to carry the electricity horizontally on a packthread which he suspended with hempen strings ; but the experiment failed. On the occasion of a repetition of the experiments at the hcuse of his friend Wheeler, silk strings were suggested as a support, and found to answer, while metal wires failed. Gray and Wheeler were thus led to the con clusion that it was the material of the supports that was in question, and that whereas packthread had, silk had not the power of transmitting electricity to a distance. Gray and Wheeler managed, by supporting a packthread by silk loops, to convey electricity from a piece of rubbed glass to a distance of 886 feet. The con ducting power of fluids, and of the human body, was established by Gray. He also made many curious experi ments on the electrical properties of resinous cakes, which he allowed to cool and harden in the ladles in which they had been melted. For an account of these and others the student is referred to memoirs in the Philosophical Trans actions for 1731, 1735, &c. Desaguliers made many experiments confirming Gray s conclusions, and found that bodies that have the property of being electrically excitable by friction, or electrics per se, have not the power of conduction; whereas conductors are not electrics per se. These terms, introduced by him, were useful in bringing into concise and scientific language the discoveries of Gray. While Gray was pursuing his career of discovery in England, M. Dufay, of the Academy of Sciences, and su- perintendunt of the Royal Botanic Gardens, was actively employed in the same researches. He found that all bodies, whether solid or fluid, could be electrified by an excited tube, by setting them on a glass stand slightly warmed, or only dried; and that those bodies which are in themselves least electrical received the greatest degree of electricity from the approach of the glass tube. He repeated the ex periments of Gray, confirming his results, and found that electricity was transmitted more easily along packthread when it was wetted, and that it might be supported upon glass tubes in place of silk lines. In this way he conveyed it along a string 1256 feet long. He suspended by silken strings and electrified a child as Gray had done ; and hav ing suspended himself in a similar manner, he discovered that an electrical spark, accompanied with a crackling noise, took place when any other person touched him, and he has described the prickling sensation like the burning from a spark of fire, which is at the same time felt either through the clothes or on the skin. The great discovery of Dufay, however, was that of two different kinds of electricity. He fully recognized the importance of this fundamental fact, and gave the name of vitreous electricity to that which is produced by exciting glass, rock-crystal, precious stones, hair of animals, wool, and many other bodies ; and the name of resinous to that which is produced by exciting resinous r>ess tiers Iiufs and

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