Canton , John , 1718-1772 , natural philosopher

Originally set to the family trade of broadcloth weaving, Canton's learning and mechanical talent, as shown by his creation of an accurate sundial proudly displayed outside the house by his father, brought him to the attention of Dr Henry Miles (Fellow of the Royal Society, 1843). Miles persuaded Canton's father to allow John to reside with him in Tooting, Surrey, until 1738, when John articled himself to Samuel Watkins, master of a school in Spital Square, London, whom he succeeded as master and owner of the school until his death in 1772. Canton's first contributions to science were routine calculations of the times of lunar eclispes, published in the Ladies Diary for 1739 and 1740. Through Miles he met London's best 'experimental philosophers' such as the apothecary William Watson and clockmaker John Ellicott. He rapidly acquired the same reputation, largely for his invention of a new method of making strong artificial magnets. He kept the method secret, hoping to make some income from it, until the publication of John Mitchell's A Treatise of Artificial Magnets (1750). His procedure appeared very similar to Mitchell's, who immediately accused him of plagiarism. This did not prevent the Royal Society from awarding him the Copley Medal for 1751; Canton had a method before Mitchell's publication, and from what is known of his character testifies to his innocence. In 1752 Canton learned of the French experiments confirming Franklin's conjecture about lightning. He was the first in England to repeat the experiments successfully, and in the process discovered independently that clouds came electrified both positively (as theory suggested) and negatively. His work on determining the sign of a cloud's charge led Canton to design the well known experiments on electrostatic induction which have earned him a place in the history of electricity. He also made 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. Other contributions to the subject were a portable pith-ball electroscope (1754), a method for electifying the air by communication (1754), a careful account of that bewildering stone the tourmaline (1759) and an improvement in the electrical machine, coating its cushion with an amalgam of mercury and tin (1762). As a gifted amateur physicist of his time, Canton displayed interest in other topics, such as identifying the cause of the luminosity of seawater (putrefying organic matter); invented a strongly phosphorescent compound 'Canton's phosphor' made of sulphur and calcined oyster shells (CaS); kept a meteorological journal; recorded the diurnal variations of the compass; and demonstrated the compressibility of water, a notable achievement, which depended on measurements so minute he was challenged on his revolutionary interpretation of them, although they stood the scrutiny of a special committee of the Royal Society and earned him a second Copley Medal in 1765. He was a frequenter of the Club of Honest Whigs in the company of Franklin and Dissenting Ministers like Joseph Priestley, whose History and Present State of Electricity owed much to his patient assistance. 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.