_History of case._ Exposed to ‘mustard gas’ at Ypres on July 12, 1917, when this substance was first employed by the enemy. Wore box respirator for only 30 minutes, so that he was exposed without any protection for nearly four hours. No symptoms were felt until some hours later, when severe vomiting commenced and conjunctivitis developed. Drawing made on the fifth day. The laryngitis and bronchitis were slight, so that the poisonous vapour must have acted only in low concentration. But the reddening of the skin was fairly intense because the man had been sweating freely when exposed to the gas, and he was not washed afterwards nor was his clothing changed. The erythema was succeeded by staining in the same areas of the skin. This reddening, as though the skin had been scorched or deeply sun-burned, is the first cutaneous reaction to mustard gas, though it sometimes may not appear until several days after exposure. It is accompanied by only a slight feeling of warmth and irritation. In addition to the face and arms which are directly exposed to the vapour in the air, the moist surfaces of the axillae, the flexures of the elbows, and the perineum and inner surfaces of the thighs are particularly affected, that is in the places where the skin is often sodden with fatty perspiration. This special distribution of the diffuse erythema characterizes the general dermatitis of mustard gas vapour; but the reaction may be limited to a smaller area in any part of the body, for example where the clothing may have chanced to be splashed by the liquid. The inflammatory reaction is chiefly superficial, and it is not accompanied by much oedema of the subcutaneous tissues except in the eyelids and over the penis and scrotum. Later the dusky red colour deepens, and patches of cyanotic or whitish oedema may arise amid it. Blisters then appear, and the cuticle becomes excoriated; or the skin may be retained while the erythema fades and a brown staining slowly darkens the original area of irritation. [Illustration: PLATE V.] PLATE NO. VI Blistering of buttocks by mustard gas. _History of case._ The man sat down on ground that was contaminated by the poison and the vapour passed through his clothing, causing inflammation of the buttocks and of the scrotum. A diffuse reddening appeared twenty-four hours after exposure, and this was followed by an outcrop of superficial blisters. On the eighth day the erythema began to be replaced by a brown staining, and the drawing was made on the eleventh day during this change of tints. Infection of the raw surface was avoided, and the healing was complete in three weeks.