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Xll PREFACE. 



ful medical studies, loved io converse with me, during my 

 long stay at Jena, on physiological subjects. The inquiries 

 in which I was then engaged, in preparing my work "On the 

 condition of the fibres of muscles and nerves, when irritated 

 by contact with substances chemically opposed," often im- 

 parted a more serious direction to our conversation. It was 

 at this period that I wrote the little allegory on Vital Force, 

 called The Rhodian Genius. The predilection which Schiller 

 entertained for this piece, and which he admitted into his 

 periodical, Die Horen, gave me courage to introduce it here. 

 My brother, in a letter which has recently been published 

 (William von Humboldt's Letters to a Female Friend, vol. 

 ii. p. 39), delicately alludes to the subject, but at the 

 same time very justly adds ; " The development of a physio- 

 logical idea is exclusively the object of the essay. Such 

 semi-poetical clothings of grave truths were more in vogue 

 at the time this was written than they are at present." 



In my eightieth year I have still the gratification of com- 

 pleting a third edition of my work, and entirely remoulding 

 it to meet the demands of the age. Almost all the scientific 

 illustrations are either enlarged or replaced by new and more 

 comprehensive ones. 



I have indulged a hope of stimulating the study of nature, 

 by compressing into the smallest possible compass, the 

 numerous results of careful investigation on a variety of 

 interesting subjects, with a view of shewing the importance 

 of accurate numerical data, and the necessity of comparing 

 them with each other, as well as to check the dogmatic 

 smattering and fashionable scepticism which have too long 

 prevailed in the so-called higher circles of society. 



My expedition into northern Asia (to the Ural, the Altai, 



