PBEFACE TO THE FIFTH ENGLISH EDITION. XI 



references were very largely to German sources, it follows 

 that any who were capable of making use of them would 

 be also capable of referring to them as set put at length 

 in the German original. 



I have to thank my friend and former pupil, Dr. A. H. 

 Reginald Buller, for assistance in simplifying some of the 

 formulae, and testing the resulting reactions. 



The issue of this edition has again been unduly delayed, 

 in this case by an unfortunate breakdown in my health, 

 which necessitated a sojourn abroad, and a complete tem- 

 porary cessation from work. 



W. H. 



UNIVERSITY OF BIRMINGHAM, 

 July, 1900. 



