previously referable to any known body, it has been the means of discovering five new metals: cæsium and rubidium (detected by Bunsen), thallium (by Crookes), indium (by Richter), and gallium (by Lecoq).
Our knowledge of the sun's constitution, in particular, has advanced with extraordinary rapidity during the period here under review. Even thirty years ago we knew little of the central orb of our system save a few naked mathematical facts as to his diameter, his density, his attractive power, and the spots on his surface. Thirty years of constant investigation have now enabled us to picture to ourselves, with tolerable accuracy, the actual state of the sun's fiery exterior. The new era began with Schwabe's discovery of the periodicity of the sun's spots in 1851. The development of spectroscopic analysis between 1854 and 1870 followed hard on this first impulse. Since 1860 eclipses have yielded us valuable results. Observations on transits of Venus have largely corrected a serious error in our calculations of our primary's distance from the earth. Janssen and Lockyer have taught us how to observe at any time, by means of the spectroscope, phenomena which were previously observable only during moments of total eclipse. Huggins has shown us how to isolate those marvelous protuberances of incandescent gas which burst forth with explosive violence from time to time from the edge of the photosphere. Tacchini, Secchi, Young, and others have carried out these interesting researches to a still higher pitch of certainty and accuracy; and the sun's geography, so to speak, is to-day no longer a closed book to mundane observers. We know our central luminary now as a mass of intensely heated gas, surrounded by a shell of luminous cloud, the photosphere, formed by the cooling of condensable vapors at the surface where exposed to the cold of outer space; and floating in a chromosphere of incondensable gases (notably hydrogen) left behind by the formation of the photospheric clouds. The mysterious corona alone as yet evades our methods of research.
In the solar system at large, great advances have been made in the details of planetary astronomy. The differences in kind between the older group of interior planets, now in their cold and solid age, and the younger group of exterior planets, still in their boisterous and fiery youth, have been well ascertained. This truth—of so much interest from the evolutionary point of view—has been especially worked out by R. A. Proctor. Nasmyth's observations on our own dead satellite, the moon, have given us a graphic and appalling picture of a worn-out world in its last stage of lifeless, waterless, and airless decrepitude. New moons have been added to Mars, and several tedious additions have been made by minutely obstetrical astronomers to the already inconveniently large family of the minor planets. All our fresh knowledge of Jupiter and Saturn, those turbulent and volcanic orbs, has helped to impress the general soundness of the evolutionary hypothe-
