Hoping that this little book will, to a considerable extent, fill the gaps in my previous works, I present these treatises in remodelled form. SVANTE ARRHENIUS. STOCKHOLM, _November, 1915_. CONTENTS CHAPTER I PAGE ORIGIN OF STAR-WORSHIP 1 Position and practical value of astronomy. Worship of stars. Chronology. The Australian negro’s conception of the stars. Day and night, summer and winter. Solar year. Sun-worship. Changing phases of the moon in chronology. The Mexican “Tonalamatl.” Moon-worship in Mesopotamia. Significance of the moon in astrology. The sun and the heat. Agriculture’s demand on chronology. Worship of the planet Venus by the Mexicans and the Babylonians. The Church Calendar. The Zodiac. The seven planets. The week. Correspondence and sympathetic magic. The Platonic-Aristotelian philosophy. Astrology and alchemy. Tycho Brahe. Occult sciences. Aristarchos from Samos. Kopernicus. The progress of Astronomy. CHAPTER II THE MYSTERY OF THE MILKY WAY 41 Primitive conceptions of the Milky Way. Anaxagoras and Demokritos. Ptolemaios. Galilei. Cosmogenic speculations. Wm. Herschel’s statistical researches regarding the distribution of the stars. The Milky Way as the foundation of the stellar system. The Milky Way as a nebula. Classification according to age of the stars, their distribution and velocity. Motion in the Orion nebula. The planetary nebulæ. Kapteyn’s “star-drifts.” The origin of the Milky Way. Comparison between the Milky Way and a spiral nebula in “The Dogs of Orion.” A few details from the Milky Way. The infinitely great and the infinitely small. The magnitude and destiny of the Milky Way. CHAPTER III THE CLIMATIC IMPORTANCE OF WATER VAPOUR 84 The four elements of Aristotle. Humid-warm climates. The Congo and Amazon basins. The carboniferous age. The effect of cloudiness. Desert climate. Steppes. “Kevirs” and “Bayirs.” Sand dunes. The great Kevir. Climatic changes. Khanikoff’s description. Salt lakes. Deposits of salt through evaporation. Huntington about the arefaction of the earth. Humid period during the ice-age. Climatic changes during historic time. Africa, Asia, Greece, Italy, Sicily. West-Europe’s climate has grown more marine. Present conditions. CHAPTER IV ATMOSPHERE AND PHYSICS OF THE STELLAR BODIES 119