A.D. 1775.—Cavallo (Tiberius), a distinguished Italian natural philosopher, publishes in London “Extraordinary Electricity of the Atmosphere at Islington,” which volume was reprinted by Sturgeon, and contains his many experiments and important observations upon the line indicated by Franklin. This work was followed in 1777, 1782, 1787, 1795, 1802 by his “Complete Treatise on Electricity,” etc.; by his “Essay on the Theory and Practice of Medical Electricity” (London, 1780, 1781; Leipzig, 1782, 1785; Naples, 1784); and during 1787 was also published in London the first edition of his “Treatise on Magnetism,” a supplement to which appeared eight years later.
He had made many very remarkable observations during the year 1787 on the phenomena of electricity in glass tubes containing mercury, and he had particularly experimented with various substances floating upon mercury in order to test their magnetism.
Before the year 1795 he contrived what he called a multiplier of electricity, a good illustration of which is to be found, more particularly, opposite p. 270, Vol. II. of his “Elements,” etc., published at Philadelphia in 1825. It consisted of two brass plates insulated upon glass pillars, and of a third plate which could be insulated or uninsulated at will, and which, turning on a pivot, or rather a movable arm, could be made to successively convey electricity from one plate to the other until the desired quantity was accumulated. (For the multiplier, see Jean Damel Colladon in “Bibl. Britan.,” Vol. XXIX, N.S. for 1825, p. 316.)
Cavallo also invented a small electroscope and a condenser of electricity. The latter consisted of an insulated tin plate between the sides of a wooden frame lined with gilt paper, one edge of the plate being connected with the body containing the electricity, and the condensation making itself observable at the opposite edge by the electroscope.
In the fourth edition of his “Treatise on Electricity” (1795), which, like the previous editions, was freely translated into other languages, will be found at pp. 285–296 of the third volume mention of the possibility of transmitting intelligence by combinations of sparks and pauses. For his experiments he made use of brass wires 250 English feet in length, and his electric alarm was based upon either the explosion of a mixture of hydrogen and of oxygen, or of gunpowder, phosphorus, phosphuretted hydrogen, etc., fired by the Leyden phial (vide Bozolus at A.D. 1767). It is in Vol. I. p. 358 of the afore-named fourth edition that Cavallo explains the mode of action of the charged Leyden jar. His concluding words deserve reproduction: “Which shows that one side of a charged electric may contain a greater quantity of electricity than that which is sufficient to balance the contrary electricity of the opposite side. This redundant electricity should be carefully considered in performing experiments of a delicate nature.” The same is expressed in other words in the 1825 American edition of his “Natural Philosophy,” Chap. IV. Therein he asserts that glass is impervious to the electric fluid, saying: “If the additional electric fluid penetrates a certain way into the substance of the glass, it follows that a plate may be given so thin as to be permeable to the electric fluid, and, of course, incapable of a charge; yet glass balls blown exceedingly thin, viz. about the six-hundredth part of an inch thick, when coated, etc., were found capable of holding a charge.” (Consult Cavendish’s experiments which produced this remarkable discovery, in Phil. Trans., Vols. LXXV and LXXVIII.)
An electrical machine used by Cavallo in 1777 had a glass cylinder rotated by means of a cord passing around the neck and the wheel, also a cushion (amalgamated with two parts of mercury, one of tinfoil, some powdered chalk and grease) holding a silk flap and freely moving along a groove, and provided with a prime conductor resting on glass legs and with collecting points.
References.—Sturgeon, “Lectures,” London, 1842, p. 12; Young’s “Lectures,” London, 1807, Vol. I. pp. 682, 686, 694, 714; Nicholson’s Journal, 1797, Vol. I. p. 394; Du Moncel, “Exposé,” Vol. III; Aikin’s “General Biography,” Vol. X; Phil. Transactions, 1776, Vol. LXVI. p. 407; 1777, Vol. LXVII. pp. 48, 388; 1780, Vol. LXX. p. 15; 1786, p. 62; 1787, p. 6; 1788, pp. 1 and 255, and 1793, p. 10 (Volta’s letters); likewise Hutton’s abridgments, Vol. XVI. pp. 57, 170, 354, 449; Vol. XIV. pp. 60, 129, 180, 608; see also “Encycl. Britannica,” art. “Magnetism,” Chap. III. s. 1. for Cavallo’s “Observations on the Magnetism of Metals,” etc.
A.D. 1775.—Bolten (Joach. Fred.), a German physician, is the author of “Nachricht von einem mit dem Künstlichen magneten gemachten Versuchein einer Nerven-Krankheit” (Hamburg, 1775), the title of which is here given in full, as the work is not usually found recorded in publications and is considered to be of excessive rarity.
Contrary to the accepted belief of many at the time, Bolten asserts that the application of magnetic plates for the cure of nervous and other affections is not only useless, but has, in many instances, been shown to greatly increase pain. This is proven by M. Fonseca in his Journal, which forms part of the above-named work; by Andry and Thouret (“Obs. et Rech sur ... l’Aimant ...” 8, pp. 599, 661), and by J. David Reuss (“Repertorium,” Vol. XII. p. 18), as well as by observations recorded in another very scarce work, translated into Dutch during 1775 by the celebrated physicist, J. R. Deimann, under the title of “Geneeskundige Proefneeming met den door Koast gemaakten Magneet, door den Heere T. C. Unzer.”
References.—Magnetical cures by different processes are treated of more particularly by Goclenius R., Jr., “Tract. de Mag. Curatione ...” Marp., 1609; J. Robertus, “Curationis Magneticæ ...” Luxemb., 1621, Coloniæ, 1622; Charlton, “A Ternary of Paradoxes ...” London, 1650; G. Mascuelli, “De Medicina Magnetica,” Franckfort, 1613, translated by W. Maxwell (Maxvellus), 1679–1687; Tentzelius, “Medicina Diastatica ...” 1653; A. Van Leuwenhoeck (Phil. Trans., Vol. XIX for 1695–1697, as shown below); J. N. Tetens, “Schreiben ... Magnetcuren,” Bützow and Wismar, 1775; Jacques de Harsu, “Receuil des Effets ...” Geneva, 1783; W. Pigram, “Successful Application ...” (Phil. Mag., Vol. XXXII. p. 154); Kloerich, F. W., “Versuche ...” (“Götting. Anzeigen,” 1765), “Von dem Medicin ...” Göttingen, 1766; M. Mouzin, “De l’emploi ... Maladies,” Paris, 1843. See likewise A.D. 450, and Hell at A.D. 1770.