The blisters in this case were probably aggravated by pressure, for the inflamed skin becomes very fragile, so that the surface layer is readily loosened by pressure or careless rubbing. The blisters may be very tiny bullae, as on the eyelids, or they may coalesce into areas many inches across, covering a collection of serous fluid which perhaps itself contains enough of the irritant substance to injure other skin if it is allowed to flow over it. The blisters are usually quite superficial and almost painless in their development. But the raw surface that is left after the blister has burst becomes most acutely sensitive to all forms of mechanical irritation. Deeper destruction of the dermis may be caused by spreading necrosis where the substance attacks the skin locally in high concentration, or when secondary infections are implanted on the raw surface. Chronic and painful sores then result, and in this event the skin does not regenerate completely, so that thinly covered scars for a long time will mark the site of the burn. [Illustration: PLATE VI.] PLATE NO. VII Burning of scrotum and penis by mustard gas. _History of case._ From the same incident as that described under Plate No. VI. Inflammation commenced at the close of the first day after exposure. Drawing made on the eleventh day when the red erythema had almost faded from the inner aspects of the thighs. The scrotum is oedematous and the raw surfaces have become the seat of a mild secondary eczematisation. The injuries were soon and completely healed. The perineum is peculiarly liable to be inflamed after exposure to the vapour of mustard gas, and the penis and scrotum become oedematous as well as reddened. Balanitis and pain with micturition may be troublesome. When the skin is excoriated, secondary infections of the raw surface are very likely to develop unless adequate precautions are taken to prevent sepsis. But with careful cleansing of the skin and clothes of a casualty after exposure to the vapour, inflammation of the perineum can be reduced to a comparatively trifling incidence. [Illustration: PLATE VII.] PLATE NO. VIII Brown staining from mustard gas. This purplish-brown, or brown, or brownish-black tint usually appears in areas that were first inflamed and red, but it may arise without such preceding erythema. Its distribution is in the same areas as those in which erythema occurs, that is over the exposed skin surfaces of the neck and hands, or on the sheltered moist flexures of the body. It may appear at any time from the fifth or sixth day onwards, and it persists for several weeks, until the stained cuticle desquamates. There is no deep pigmentation. The drawing was made from a case on the eighteenth day after exposure to gas, and the brown tint was present on the sixth day. [Illustration: PLATE VIII.]