PREFACE TO THE THIRD EDITION. 



The plan used in the preceding editions of this manual has 

 been followed in the preparation of the present one. The 

 only departures have been in the insertion of a short histor- 

 ical sketch and the freer use of references to original articles 

 and reviews. It is hoped that these features will assist in 

 arousing the interest of students. As far as possible, refer- 

 ence has been made to x articles in American and English 

 journals likely to be easy of access. Besides the ones just 

 named, numerous additions have been made which the recent 

 advances in our knowledge have rendered necessary. Most 

 of the illustrations of apparatus are new. The photomicro- 

 graphs also are new and original, with a few exceptions noted 

 in the text. It is probably needless to say that none of them 

 were retouched. The writer is indebted to the Gratwick 

 Laboratory, of Buffalo, for the use of its facilities in making 

 these photographs. H. U. W. 



BUFFALO, NEW YORK, August, 1903. 



VI 1 



