5 i4 THE POPULAR SCIENCE MONTHLY. 



previously referable to any known body, it has been the means of 

 discovering five new metals: caesium 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 be- 

 tween 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, phenom- 

 ena which were previously observable only during moments of total 

 eclipse. Huggins has shown us how to isolate those marvelous pro- 

 tuberances of incandescent gas which burst forth with explosive vio- 

 lence 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 geog- 

 raphy, so to speak, is to-day no longer a closed book to mundane ob- 

 servers. 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 ex- 

 posed 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. Kasmyth'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 knowl- 

 edge of Jupiter and Saturn, those turbulent and volcanic orbs, has 

 helped to impress the general soundness of the evolutionary hypothe- 



