Co-operative tests were carried out with the Gas Defense Division to determine the value of given masks under field conditions. Companies of infantry, fully equipped for the field, would wear masks for hours at a time digging trenches, cutting timber, drilling, etc., and imitating in every way, as far as possible, actual field conditions. During these activities tons of gas in cylinders were released in such a way that the men were enveloped in a far higher concentration than would probably ever be the case in actual battle. These tests gave valuable data for criticizing gas mask construction. Another line of activity consisted of a study of the persistency and relative effectiveness of various samples of mustard gas, in which the liquid was distributed uniformly upon the surface of grassy zones one to three feet in width, which formed the periphery of circular areas 14 to 21 feet in diameter, the central part of each circle being occupied by animals. The work of the Proving Division was brought to an end (by the Armistice) just at the time when it had reached its greatest usefulness. Not only were the physical properties and personnel of the Division developed to the maximum degree, but the production of gas shell in this country for shipment to France had just reached the stage where the Proving Ground could have been used to its fullest extent in their proving. TRAINING DIVISION From the standpoint of the man at the front the Training Division is one of the most important. To him gas warfare is an ever present titanic struggle between poisonous vapors that kill on one side, and the gas mask and a knowledge of how and when to wear it, on the other. Because of this it is rather surprising that we did not hear more about this branch of the Service. It did exist, however, and credit must be given to those camp gas officers who remained in the United States performing an inconspicuous and arduous duty in the face of many local obstacles. [Illustration: FIG. 8.]