On April 22nd, 1915, the writer, in company with Major Rankin, saw the Germans launch their first gas attack near St. Julien upon the section of the line held by the French colonial troops and the first Canadian division. This book was written primarily for the purpose of recording this as well as some of the other experiences of the first Canadian division as seen from the unusual angle of a scientist, in the course of 18,000 miles of travel in the front line area. It had the secondary object of giving the average reader some insight into what goes on behind the lines, and the means employed to maintain the health and efficiency of the British and Canadian soldiers in the field.