Colonel Church of the Medical Department had been in France nearly one and a half years prior to the entry of the United States into the war, and had taken sufficient interest in Gas Warfare to collect considerable information and a number of documents from French sources bearing on the defensive side of the subject. Captain Boothby had done the same with the British, including a course in a British Gas Defense School. On this trip they took up the defensive side with the British, while Fries took up the offensive side of the Service. The latter included gases used, gas troops, and ammunition and guns used in Gas Warfare by the Artillery and other branches of the Service. The trip included a brief visit to the headquarters of the First British Army in the vicinity of Lens, where the British Gas Service had a large depot of offensive gas material. =Order Forming Service.= Returning on the 28th of August the order, together with a chart organizing the Service, was completed and submitted to the General Staff. This was published as G. O. 31, September 3, 1917. As a result of a study of the information submitted by Colonel Barber and General Drum, together with his own observations of British organization and work, Fries decided it was advisable to make the Service cover as complete a scope as possible and to make the order very general, leaving details to be worked out as time and experience permitted. This proved to be a very wise decision, because the entire absence of gas knowledge among Americans either in France or the United States made it necessary to build from the bottom up and do it rapidly. At that time, and at all times since, it was found utterly impossible to separate the defensive side from the offensive side. Indeed, many of the worst troubles of the British with their Gas Service throughout nearly the whole war arose from such a division of duties in their Service. Thus, the development of masks must be kept parallel with the development of gases and methods of discharging them. Otherwise a new gas invented may penetrate existing masks and preparations be carried far towards using it before the development of masks are undertaken to care for the new gas. Obviously a gas which our own masks will not take care of cannot be safely used by our own troops until new masks are developed to protect against it.