INTRODUCTION

TO FIFTH EDITION

During the ten years that have passed since the first edition of this book, the author has continued his observations, experiments, and tests on a more elaborate scale than ever before and with careful attention to every point needing consideration.

In these experiments he has been assisted by his colleagues, Mr. John R. Dempsey (painter) and Mr. Donald A. Kohr (chemist and paint-maker), to whom he hereby makes grateful acknowledgment.

In this edition of the book many changes have been made, due largely to the increas-

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