Nearly all great modern scientific discoveries have been made by teachers of science and others, who spend a large portion of their lives in experimental investigation, searching for new truths, and not by persons who have hit upon them by accident. The greatest discoveries in physics and chemistry in modern times, were made chiefly by such men as Newton, Cavendish, Scheele, Priestley, Oersted, Volta, Davy and Faraday: all great workers in science. It is either by observing matter and its forces under new conditions or from a new aspect, that nearly all discoveries are made; thus Priestley placed some oxide of mercury in an inverted glass vessel, and heated it by means of the Sun's rays and a lens, and discovered Oxygen. This substance was nearly discovered by Eck de Sulsbach three hundred years before; he heated six pounds of an amalgam {4} of silver and mercury, and converted the latter metal into a red oxide like cinnabar, and he remarked, "a spirit is united with the metal, and what proves it is this, that this artificial cinnabar submitted to distillation, disengages that spirit." The "spirit" was evidently oxygen. Some discoveries are made by observing the phenomena of bodies placed under special conditions by those operations of nature over which we have little or no control. All our knowledge of Astronomy, and much of that of geology and physiology, was acquired in this way. Nearly all modern discoveries of importance in physics or chemistry require long and difficult investigations to be made in order to completely establish their truth. When Crookes discovered Thallium, he saw the first sign of its existence in a momentary flash of green light in a spectroscope, but he had to expend upon the subject several years of most difficult labour, and a considerable sum of money, in order to prove the correctness of his suspicion that he had discovered a new metal. M. Lecocq de Boisbaudran discovered the metal Gallium and Bunsen discovered Rubidium and Caesium in a similar manner. Discoveries in science, are usually made, not by trying to obtain some valuable commercial or technical result, but by making new, reliable, and systematic investigations. By investigating the chemical action of electricity upon saline bodies, {5} Sir Humphrey Davy isolated sodium and magnesium, which has led to the establishment at Patricroft near Manchester, of the manufactures of those metals. By the abstract researches of Hofmann and others upon Coal-tar, many new compounds were discovered, and the extremely profitable manufacture of the splendid coal-tar dyes was originated.