The second edition of the “Ansichten der Natur” was prepared by me in Paris in 1826; and at the same time two fresh treatises were added,--one an Essay on the Structure and mode of Action of Volcanoes in different regions of the earth; and the other on the “Vital Power,” bearing the title “Lebenskraft, oder der rhodische Genius.” During my long stay at Jena, Schiller, in the recollection of his youthful medical studies, loved to converse with me on physiological subjects; and the considerations in which I was then engaged on the muscular and nervous fibres when excited by contact 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. My 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 development of a physiological idea is the object of the entire treatise; men were fonder at that time than they would now be of such semi-poetic 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 them by well-considered arrangement and comparison; and by opposing the dogmatic half-knowledge and arrogant scepticism which have long too much prevailed in what are called the higher circles of society.