PREFACE. IX 



I may with complete truthfulness say that this work is 

 based upon a study of original memoirs rather than upon a 

 study of text books. In the interest of the student nearly 

 all papers are quoted by their full titles and few have been 

 quoted which have not been read throughout and studied. 

 Whenever quotations have been made at second hand the 

 fact is stated. 



Another feature which I have desired to render promi- 

 nent in this work is the description of the methods which 

 have been followed in important and, to borrow a con- 

 venient Germanism, 'epoch-making 7 researches. It seemed 

 the more important to do this as I desired to write in the 

 interest of the truly scientific student, anxious not merely 

 to learn what has been already acquired to science, but 

 wishful himself to extend her boundaries. 



I have, so far as possible, tried all the experimental 

 processes mentioned in this work, and throughout it I have 

 incorporated the results of my own independent researches 

 which in many cases have not yet been published else- 

 where. 



Thus much as to the plan of this book : 1 know only too 

 well its deficiencies. I trust, however, that notwithstanding 



', ' o 



these it may assist the progress of science, and whilst I 

 plead for it the indulgence of my scientific brethren, I would 

 beg of them to aid me by communicating to me any errors 

 which they may discover, or any suggestions for a better 

 exposition of the subjects discussed. 



In the discharge of my very arduous work I have been 

 greatly helped by many friends. In the first place I have to 

 express my unbounded acknowledgments to my friend and 

 former pupil Mr John Priestley, who has, with the exception 

 of some comparatively unimportant sections, written the 



