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 

 historical 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 pos- 

 sible, reference has been made to 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 neces- 

 sary. Most of the illustrations of apparatus are new. The 

 photomicrographs 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. 



BUFFALO, NEW YORK, August, 1903. 



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