(b. Stroud, England, 31 July 1718;d. London, England, 22 March 1772),
Canton, who was to earn his living as a school-master, received little formal education himself, for his father removed him from school just as he reached the rudiments of algebra and astronomy in order to set him to the family trade of broadcloth weaving. Young John, however, refused to abandon his studies and continued to labor over his books at night, despite his father’s refusal to allow him a candle for the purpose. His learning and mechanical talent, as expressed in an accurate sundial that the elder Canton proudly set up in front of the house, brought John to the attention of several neighboring gentlemen, including Dr. Henry Miles, who decisively affected his career. Miles, a native of Stroud, occupied a Dissenting pulpit in Tooting, Surrey, on the outskirts of London; and he persuaded the father to allow John to reside with him while he arranged employment more suitable than weaving. Accordingly, in March 1737, Canton proceeded to Tooting, where he remained until the following May, when he articled himself to one Samuel Watkins, master of a school in Spital Square, London. In 1745 he succeeded Watkins and kept the school himself until his death in 1772.
Canton’s first contributions to science were routine calculations of the times of lunar eclipses, published in Ladies’ Diary for 1739 and 1740. There after, probably through the intervention of Miles, who became a fellow of the Royal Society in 1743, Canton met several of London’s best young “experimental philosophers,” men like the apothecary William Watson and the clockmaker John Ellicott, who occupied social positions not much different from his own. By 1747 or 1748 Canton also enjoyed a reputation as an experimentalist, largely for his invention of a new method of making strong artificial magnets. Several French and English philosophers were then occupied with the subject, inspired by the success of Gowin Knight (1746), who refused to disclose his profitable technique. Similarly, Canton, who hoped to derive some income from his invention, kept it secret until 1751, about a year after the publication of John Michell’s A Treatise of Artificial Magnets (1750). It then appeared that Canton’s procedure paralleled Michell’s very closely. Michell cried plagiarism, which did not prevent the Royal Society, which had made Canton a fellow in 1749, from awarding him the Copley Medal for 1751. Doubtlessly Canton had a method before Michell’s book appeared; the question is how closely it coincided with the one he later published as his own. All that is known of his character testifies to his innocence of Michell’s charge.
Canton’s entry into his special field, electricity, was also ill-omened. His first extended appearance in print—two paragraphs signed “A.B.” in the Gentleman’s Magazine for 1747—posed two electrical puzzles. Benjamin Wilson and his friend John Smeaton immediately supplied an answer, a patronizing sneer, and the intelligence that the queries contained nothing new. Canton withdrew to his magnets and to meteorological measurements until, in the early summer of 1752, he learned of the French experiments confirming Franklin’s conjectures about lightning. He was the first in England to repeat the experiments successfully; and in the process he discovered, independently of Lemonnier, Beccaria, and Franklin, that clouds came electrified both positively (as theory suggested) and negatively (which remained a puzzle). The problem of determining the sign of a cloud’s charge apparently led Canton to design the well-known experiments on electrostatic induction that have earned him a permanent place in the history of electricity. In a typical arrangement, an insulated tin tube receives a small positive charge, which causes two cork balls suspended from one end of the tube to repel one another; an electrified glass rod is then brought under the balls, which collapse as the rod approaches and diverge as it recedes. Canton accounted for these phenomena by modifying the old doctrine of atmospheres in much the way Beccaria was to adopt, i.e., by distinguishing what the Italian called “aerial” and “proper” electricity. Franklin later (1755) redesigned Canton’s experiments and so modified the atmospheres as to render them all but superfluous. Their total abolition, an essential step toward modern theories of electrostatics, was first advocated by F. U. T. Aepinus and Johann Karl Wilcke in 1759. Although Canton did not subscribe to their program, his experiments figured importantly in its development.
Canton also enriched the study of electricity with the notable discovery that glass does not always charge positively by friction: the sign of the electricity developed depends upon the nature of the substance rubbed over it and the condition of the surface of the glass. (He found, e.g., that rough glass became negative when rubbed with flannel and positive when rubbed with silk.) Among his many smaller contributions to the subject were a portable pith-ball electroscope (1754); a method for electrifying the air by communication (1754); a careful account of some of the electrical regularities of that bewildering stone, the tourmaline (1759); and a valuable improvement in the electrical machine, coating its cushion with an amalgam of mercury and tin (1762). Throughout, Canton was guided by Franklin’s theories, which he zealously defended against attacks emanating from Wilson’s circle.
Like most gifted amateur physicists of his time, Canton attended to a great many subjects seldom cultivated together today. He examined the luminosity of seawater and identified its cause, the put refaction of organic material; he invented a strongly phosphorescent compound, “Canton’s phosphor,” made of sulfur and calcined oyster shells (CaS); he kept a meteorological journal; he recorded the diurnal variation of the compass; he carefully observed transits, occultations, and eclipses; and he demonstrated the compressibility of water. This last investigation, a notable achievement in eighteenth-century physics, depended upon measurements so minute that several philosophers challenged Canton’s revolutionary interpretation of them. But his results, which compare favorably with modern determinations, stood the scrutiny of a special committee of the Royal Society and earned him a second Copley Medal in 1765.
Judging from the clarity, brevity, and precision of his scientific papers, Canton was an admirable pedagogue. In leisure hours he enjoyed good conversation and a good glass, particularly at the Club of Honest Whigs in the company of Franklin, fellow teachers, and Dissenting ministers like Joseph Priestley, whose History and Present State of Electricity owed much to his patient assistance. Liberal in politics, latitudinarian in religion, devoted to his profession, school-master Canton was one of the most distinguished of the group of self-made, self-educated men who were the best representatives of English physics in the mid-eighteenth century.
I. Original Works. Canton’s most important papers are “A Method of Making Artificial Magnets Without the Use of Natural Ones,” in Philosophical Transactions of the Royal Society, 47 (1751–1752), 31–38; “Electrical Experiments, With an Attempt to Account for Their Several Phaenomena, Together With Observations on Thunder Clouds,” ibid, 48:1 (1753), 350–358; “Some New Electrical Experiments;” ibid, 48:2 (1754), 780–784; [On the Tourmaline], in Gentleman’s Magazine, 29 (1759), 424–425; “Experiments to Prove Water Is Not Incompressible,” in Philosophical Transactions of the Royal Society, 52 (1762), 640–643; “Experiments and Observations on the Compressibility of Water and Some Other Fluids,” ibid., 54 (1764), 261–262; “An Easy Method of Making a Phosphorus That Will Imbibe and Emit Light, Like the Bolognian Stone, With Experiments and Observations,” ibid, 58 (1768), 337–344. Poggendorff lists all Canton’s papers printed in the Philosophical Transactions. A complete bibliography would also include many brief notes in Gentle-man’s Magazine, Ladies’ Diary, and Gazeteer, occasional contributions to books; and scattered astronomical, meteorological, and electrical data in papers published by Henry Miles, William Watson, James Short, and Joseph Priestley in the Philosophical Transactions. The Royal Society of London has preserved Canton’s correspondence, some of which has been published: e. g., by Augustus De Morgan, in The Athenaeum (1849), pp.5–7 162–164, 375; by R. E. Sehofield, in A Scientific Autobiography of Joseph Priestley (Cambridge, Mass., 1966); and by W. C. Walker, “The Detection and Estimation of Electric Charges in the Eighteenth Century,” in Annals of Science, 1 (1936), 66–100.
II. Secondary Literature. The fullest biography of Canton is the notice prepared by his son William for the 2nd ed. of Biographia Britannica, A. Kippis, ed., III, 215–222. See also vol. I of the Canton papers at the Royal Society; The Papers of Benjamin Franklin, L. W. Labaree et al., eds., (New Haven, Conn., 1961–1968), vols. IV-XII; the articles by De Morgan cited above; and V. W. Crane, “The Club of Honest Whigs: Friends of Science and Liberty,” in William and Mary Quarterly, 23 (1966), 210– 233. For Canton’s scientific work consult P. Rivoire, ed. and trans., Traites sur les aimans artificiels (Paris, 1753), preface; J. Priestley, The History and Present State of Electricity, 3rd ed., 2 vols. (London, 1775), I, passim; I. B. Cohen, franklinand Newton (Philadelphia, 1956), pp. 516–543; E. N. Harvey, A History of Luminescence (Philadelphia, 1957), passim; and A. Wolf, A History of Science, Technology and Philosophy in the Eighteenth Century, 2nd ed. (New York, 1952), pp. 251, 272–273.
John L. Heilbron