Mustard gas, besides being highly poisonous, has so many other important qualities as to have given it the designation during the war of the “king of gases.” That broad distinction it still holds. Its introduction at Ypres, on the night of July 12, 1917, changed completely the whole aspect of gas warfare and to a considerable extent the whole aspect of warfare of every kind. It is highly poisonous, being in that respect one of the most useful of all war gases. It produces no immediate discomfort. It has a considerable delay action. It burns the body inside or out, wherever there is moisture. Eyes, lungs and soft parts of the body are readily attacked. It lingers for two or three days in the warmest weather, while in cold, damp weather it is dangerous for a week or ten days, and in still colder weather may be dangerous for a month or longer whenever the weather warms up sufficient to volatilize the liquid. It is only slowly destroyed in the earth, making digging around shell holes dangerous for weeks and months and in some cases possibly a year or more. The Germans first used it simply to get casualties and interfere with or break up the threatened heavy attacks by the British on the Ypres salient. While not stopping the inauguration of these attacks in the fall of 1917, the German use of mustard gas was so effective as to delay the beginning of those attacks for at least two weeks and thus gain valuable time for the Germans, besides causing serious casualties with consequent partial break up of companies, regiments and divisions in the English Army. The German used his mustard gas throughout the fall of 1917 and the winter of 1917 and 1918, as above stated, to produce casualties, to destroy morale, to break up units, and to interfere with operations generally. During that time, however, he developed a more scientific use and when he started his big offensives in March, April, May and June, 1918, he used mustard gas _before_ the battles to cause losses, break up units and destroy morale, and also during the progress of battles to completely neutralize strong points which he felt he did not want to attempt to take by direct assault. Perhaps the most noted case of this was at Armentières in April, when he deluged the city to such an extent that mustard gas is said to have actually run in the streets. So effective was this gassing that not only did the British have to withdraw from the city but the Germans could not enter it for more than two weeks. It, however, enabled the Germans to take the city with practically no loss of life. There were numerous other cases on a smaller scale where mustard gas was used in the same way.