ing knowledge of the problems presented. Some of these have been made at the suggestion and with the counsel of Prof. L. H. McFadden, formerly Professor of Chemistry and Physics in Otterbein University, who has also prepared most of the section on "Rust" —a subject to which much attention has recently been given.

Increasing knowledge and experience have served to strengthen the author's faith in the hypothesis under which he has been working.

Dayton, 0., Sept. 1, 1909.