fMn* 







PREFACE. 



POME portions of this work have already 

 appeared at various times in the Edin- 

 burgh Review, in Good Words, and in Addresses 

 to the Royal Society of Edinburgh during the 

 years in which I had the honour of being Pre- 

 sident of that Body. The deep interest of the 

 matter dealt with in those Papers has induced 

 me to expand them, to add new chapters on 

 other aspects of the same subject, and to pub- 

 lish the whole in a connected form. 



Among many other deficiencies which may be 

 observed in this Volume, there is one which 

 demands explanation, lest a serious misunder- 



