AUTHOR'S PREFACE, 



In compliance with many requests, I beg to offer to 

 the public a series of popular Lectures which I have 

 delivered on various occasions. They are designed for 

 readers who, without being professionally occupied with 

 the study of Natural Science, are yet interested in the 

 scientific results of such studies. The difficulty, felt so 

 strongly in printed scientific lectures, namely, that the 

 reader cannot see the experiments, has in the present 

 case been materially lessened by the numerous illustra- 

 tions which the publishers have liberally furnished. 



The first and second Lectures have already appeared 

 in print; the first in a university programme which, 

 however, was not published. The second appeared in 

 the 'Kieler Monatsschrift ' for May, 1853, but owing to 

 the restricted circulation of that journal, became but 

 little known ; both have, accordingly, been reprinted. 

 The third and fourth Lectures have not previously 

 appeared. 



These Lectures, called forth as they have been by 

 incidental occasions, have not, of pourse, been composed 

 in accordance y^iih. a rigidly u.?\iforin plan. Each of 

 theii^ has b^eiji kep^ perfiectly independent of the others. 



