EDITOE'S INTEODUCTION 



The works to be comprised in this Series are 

 intended to give on each subject the informa- 

 tion which an intelligent layman might wish to 

 possess. They are not primarily intended for 

 the young, nor for the specialist, though even 

 to him they will doubtless be often useful in 

 supplying references, or suggesting lines of re- 

 search. 



Each book will be complete in itself, care, 

 however, being taken that while the books do 

 not overlap, they supplement each other ; and 

 while scientific in treatment, they will be, as 

 far as possible, presented in simple language, 

 divested of needless technicalities. 



The rapid progress of science has made it 

 more and more difficult, and renders it now 



