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 possible, reference has been 

 made to articles in American and English journals likely to be 

 easy of access. Besides the ones just named, numerous addi- 

 tions have been made which the recent advances in our knowledge 

 have rendered necessary. 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 facil- 

 ities in making these photographs. 



Buffalo, New York, August, 1903. 



Vll 



