In electricity great strides were made. Between the years 1705 and 1711 Francis Hawksbee published in the Transactions of the Royal Society several experiments, in which he had, for the first time, discovered the production of the electric spark by friction, and electrical attraction and repulsion. In 1720 Stephen Gray, a pensioner of the Charterhouse, published the result of his experiments on this subject, with a list of the substances which showed electricity under friction; and in 1732 he discovered the conducting property of non-electrical bodies. Before 1739, Dufray, keeper of the King's Garden at Paris, discovered the repellent power of two similarly-electrified bodies, and the attraction of these positively and negatively electrified--or, as he termed it, possessing the vitreous and the resinous electricity. Cuneus and Lallemand discovered the mode of accumulating the electric fluid in what was called the Leyden jar in 1745. This discovery gave a new impetus to inquiry, and Nollet, in France, and Watson, in England, conceived the hypothesis of the jar being overcharged on one side and undercharged on the other. This growing perception of the positive and negative conditions of the electric fluid received confirmation from the experiments of Benjamin Franklin, in America. Franklin soon improved the Leyden jar into an electrical battery; and, in 1752, he proved the identity of electricity and lightning by his grand experiment of the kite. On this he recommended lightning conductors, which, however, were not used in England till ten years afterwards. On the laws of heat and cold, and atmospheric changes under their influence, many interesting facts were ascertained by the aid of the thermometers of Fahrenheit and Réaumur. Dr. Martin, of St. Andrews, distinguished himself in these inquiries, and published his discoveries and deductions in 1739 and 1740. In 1750 Dr. Cullen drew attention to some curious facts connected with the production of cold by evaporation. Dr. Joseph Black discovered what he called latent heat, and continued his researches on this subject beyond the present period.