This device, which was, I think, first used by Professor Ostwald, is better adapted than the use of fine print to the need of teachers who may wish to make a different selection. Students are earnestly advised to read these paragraphs, even when they are not expected to acquire a full knowledge of them. I wish to express my very sincere thanks to the following members of the Chemical Staff of the University of Illinois, some of whom have met with me almost weekly for three years to read and criticize the successive chapters of the book. 

The criticisms have been very valuable and helpful. C.W. Balke, S.J. Bates, Edward Bartow, G.D. Beal, L. L.Burgess, E.S. Curtiss, C.G. Derick, Lambert Thorp, B.S. Hopkins, Helen Isham (Mattill), Grinnell Jones, C.G. MacArthur, EllenS. McCarthy (Foley), D.F. McFarland, D.A. Maclnnes, C.F. Nelson, S. W.Parr, G.McP. Smith, E.K. Strachan, E.W. Washburn, H.C. P.Weber. I wish also to express my gratitude to Professor Edward W.Morley, Professor Julius Stieglitz of the University of Chicago, Professor J.Bishop Tingle of MacMaster University, and Mr. C.M. Wirick of the Crane Technical High School of Chicago, who have read the proofs and made many useful suggestions. I am also indebted to Professor K.B. Moore of the Bureau of Mines for some valuable criticisms, of the paragraph on Radioactivity.