And now, as we view the radiometer, the scene shifts by yet another century, and we come out of cloud-land and into our own proper age. We are at the close of the nineteenth century--no, I forget, we are fairly entering upon the twentieth. Need I say that these again are troublous times? Man still wages warfare on his fellow-man as he has done time out of mind; as he will do--who shall say how long? But meantime, as of yore, the men of science have kept steadily on their course. But recently here at the Royal Society were seen the familiar figures of Darwin and Lyell and Huxley and Tyndall. Nor need we shun any comparison with the past while the present lists can show such names as Wallace, Kelvin, Lister, Crookes, Foster, Evans, Rayleigh, Ramsay, and Lock-yer. What revolutionary advances these names connote! How little did those great men of the closing decades of the seventeenth and eighteenth centuries know of the momentous truths of organic evolution for which the names of Darwin and Wallace and Huxley stand! How little did they know a century ago, despite Hutton's clear prevision, of these marvellous slow revolutions through which, as Lyell taught us, the earth's crust had been built up! Not even Jen-ner could foresee a century ago the revolution in surgery which has been effected in our generation through the teachings of Lister. And what did Rumford and Davy know of energy in its various manifestations as compared with the knowledge of to-day, of Crookes and Rayleigh and Ramsay and Kelvin? What would Joseph Priestley, the discoverer of oxygen, and Cavendish, the discoverer of nitrogen, think could they step into the laboratory of Professor Ramsay and see test-tubes containing argon and helium and krypton and neon and zenon? Could they more than vaguely understand the papers contributed in recent years to the Royal Society, in which Professor Ramsay explains how these new constituents of the atmosphere are obtained by experiments on liquid air. "Here," says Professor Ramsay, in effect, in a late paper to the society, "is the apparatus with which we liquefy hydrogen in order to separate neon from helium by liquefying the former while the helium still remains gaseous." Neon, helium, liquid air, liquid hydrogen--these would seem strange terms to the men who on discovering oxygen and nitrogen named them "dephlogisticated air" and "phlogisti-cated air" respectively.