AUTHOR'S PBEFACE. 



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 illustrations which the publishers 

 have liberally furnished. 



The first and second Lectures have already appeared in 

 print ; the first in a university programme, which, how- 

 ever, 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 course, been composed 

 in accordance with a rigidly uniform plan. Each of them 

 has been kept perfectly independent of the others. Hence 



