246 Preface 



In a very especial manner I am indebted to the late Professor 

 Guy S. Callender, of the Sheffield Scientific School of Yale Uni- 

 versity, who directed my researches in this field. Without his 

 helpful advice, his illuminating criticism and his stimulating com- 

 panionship this book could not have been written. 



Percy Wells Bidwell. 



New Haven, January 1, 1916. 



