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Show viii PREFACE TO THE SECOND AND THIRD EDITIONS. tact with chemically different substances, often gave a more specific and graver turn to our discourse. The "Rhodian Genius" was written at this time: it appeared first in Schiller's "Horen," a periodical journal; and it was his partiality for this little work which encouraged me to allow it to be reprinted. 1\ly brother, in a letter forming part of a collection which has recently been given to the public (Wilhelm von Humboldt's Briefe an eine Freundin, th. ii. s. 39), touches tenderly on the subject of the memoir in question, but adds at the same time a very just remark: "The deYelopment of a physiological idea is the object of the entire treatise; men were fonder at that time than they would now be of such semipoetic clothing of severe scientific truths." In my eightieth year, I am still enabled to enjoy the satisfaction of completing a third edition of my work, remoulding it entirely afresh to meet the requirements of the present time. Almost all the scientific Elucidations or Annotations have been either enlarged or replaced by new and more comprehensive ones. I have hoped that these volumes might tend to inspire and cherish a love for the study of Nature, by bringing together in a small space the results of careful observation on the most varied subjects; by showing the importance of exact numerical data, and the use to be made of thent by well-considered arrangement and comparison; and by opposing the dogmatic half-knowledge and arrogant skepticism which have long too much prevailed in what are called the higher circles of society. The expedition made by Ehrenberg, Gustav Rose, and myself, by the command of the Emperor of Russia, in 1829, to Northern Asia (in the Ural and Altai 1\'lountains, and on the shores of the Caspian Sea), falls between the period of publication of the second and third editions. This expedition has contributed materially to the enlargement of my views in all that regards the form of the surface of the earth, the direction of mountain-chains, the connection of Steppes and Deserts with each other, and the geographical distribution of plants in relation to ascertained conditions of temperature. The long subsisting want of any accurate knowledge on the subject of the great snow-covered mountain-chains which are situated between the Altai and the Himalaya ( i. e. the Thian-schan and the |