PREFACE ix 



Council for grants in aid of the work. These grants have dimin- 

 ished my personal expense in publishing this volume and have 

 enabled me to employ a research assistant, Miss Ruth Macrae, 

 M.Sc. (McGill). Miss Macrae's help made it possible for me to 

 carry out the long series of detailed experiments recorded in the 

 last chapter of this volume, and I here desire to express my indebted- 

 ness to her for her valuable services. Once again, Mr. W. B. Grove, 

 M.A., has been kind enough to give me the benefit of his help in 

 reading the proofs. 



A. H. REGINALD BULLER. 



Winnipeg, January 15, 1931. 



