A.D. 1757.—Dollond (John), who was at first a silk weaver at Spitalfields, England, which occupation he abandoned in order to give his exclusive attention to scientific experimental studies, discovered the laws of the dispersion of light and constructed the first achromatic telescope as well as several improved instruments for magnetic observations. A full description of the most important of these, accompanied by illustrations, can be found in the articles of the “Encyclopædia Britannica” on magnetic instruments.
References.—Kelly’s “Life of John Dollond,” London, 1808; Phil. Mag., Vol. XVIII. p. 47; Thomas Thomson, “Hist. of Roy. Soc.,” London, 1812, pp. 379–382; “Directions for using the Electric Machine made by P. and J. Dollond,” London, 1761.
A.D. 1757.—Wilcke (Johann Karl), a very distinguished scientist of Stockholm (1732–1796), introduces new phenomena respecting the production of electricity produced by melting electrical substances, which he discovers in continuation of experiments begun by Stephen Grey. He gives the name of spontaneous to the electricity produced by the liquefaction of electrics, observing that the electricity of melted sulphur does not appear until it commences to cool and to contract, its maximum being reached at its point of greatest contraction. Melted sealing wax, he says, becomes negatively electrified when poured into glass, but, when poured into sulphur, it is positively electrified, leaving the sulphur negative (Sir Humphry Davy, “Bakerian Lectures,” London, 1840, p. 36 and notes).
While in Berlin, he and Æpinus investigate the subject of electric atmospheres, and they are led to the discovery that plates of air can be charged in the same manner as plates of glass. (See Canton, A.D. 1753.) This they did by suspending large wooden boards, which were covered with tin and whose flat surfaces were held parallel to and near each other. They found that upon electrifying one of the boards positively the other was always negative, and that with them could be given shocks like those produced by a Leyden jar. They likened the state of the boards to the condition of the clouds and the earth during a thunderstorm, the earth being in one state and the clouds in the opposite, the body of air between them answering the same purpose as the small plate of air between the boards or the plate of glass between the two metallic coatings of the Leyden jar.
In Wilcke’s treatise, alluded to below, he defines the two electricities much more clearly than had previously been done. He distinguishes three causes of excitation, viz. warming, liquefaction and friction; the spontaneous electricity already alluded to, he further says, is the result of the apposition or mutual action of two bodies, in consequence of which one of them is electrified positively and the other negatively; communicated electricity, on the other hand, is that which is superinduced upon the whole or part of a body, electric or non-electric, without the body having been previously heated, melted or rubbed, or without any mutual action between it and any other body. This distinction is, in general, very obvious, but Mr. Wilcke defines it throughout his work in a very clear manner, citing cases wherein they are frequently confounded.
Wilcke and Anton Brugmans (A.D. 1778) first propounded the theory of two magnetic fluids, which was afterward established by Coulomb (A.D. 1785) and perfected by the great mathematician Poisson (A.D. 1811). The hypothesis of the two fluids supposes that a magnet contains minute invisible particles of iron, each of which possesses by itself the properties of a separate magnet. It is assumed that there are two distinct fluids—the austral and the boreal—which reside in each particle of iron. These fluids are inert and neutral when combined, as in ordinary iron, but when they are decomposed the particles of the austral attract those of the boreal, and vice versa, while they each repel one another.
References.—Wilcke, “Disputatio inauguralis physica,” etc., published Rostock, 1757, also his “Herrn Franklin’s briefe von der electricitat,” etc., Leipzig, 1758, his “Jal om Magneten,” 1764, and his “Über den Magneten,” Leipzig, 1758; besides 1794–1795; likewise his different Memoirs in the “Swedisches Musæum,” Vol. I. p. 31, and in both the “Schwedischen Akad. Abhandlungen,” etc. (also Neue Abhand.) and the “Vetensk Acad. Handl.” for 1758, 1759, 1761–1763, 1766–1770, 1772, 1775, 1777, 1780, 1782, 1785, 1786, 1790; “The Electrical Researches of Hon. Hy. Cavendish,” 1879, No. 134.
A.D. 1759.—Hartmann (Johann Friedrich), of Hanover, is the author of three works on electricity, published in that city during 1759, 1764 and 1766, wherein he gives an account of several very curious electrical experiments. One of the most interesting of these demonstrates the progressive motion of the electrical discharge. When he passes the shock through many small cannon balls, sometimes to the number of forty, placed upon small drinking goblets close by one another, all the sparks are seen and all the cracklings are heard at the same moment; but when he substitutes eggs (preferably ten or twelve) for the balls, the progress of the explosion is visible, every two giving a flash and a report separately.
He remarks that upon one occasion, as he re-entered a room which he had just before left, after making therein a number of experiments, he observed a small flame following him as he walked about swiftly while holding a lighted candle in his hand. The flame vanished whenever he stopped to examine it, and he attributed its appearance to the presence of sulphur thrown into the air by continued violent electrification.
References.—Hartmann, “Abhandlung von der verwandschaft,” etc., Hanover, 1759, pp. 58, etc., and 135; also his “Electrische experimente,” etc., Hanover, 1766, and his “Anmerkungen,” etc., 1764, 4to, p. 38; Friedrich Saxtorph, “Elektricitätsläre,” Vol. II; Hamburgisches Magazin (also Neues Hamb. Mag.) for 1759, Vol. XXIV, and for 1761, Vol. XXV; “Nov. Acta Acad. Nat. Curios,” Vol. IV. ss. 76–82, 126; “Göttingischen gemein. Abhand.,” von Jahr 1775.