(iv) Cyanosis next appears, in association either with a full venous congestion or with the pallid face of circulatory failure. The development of these dangerous symptoms may occur after many hours’ delay, and sometimes with unexpected rapidity in an apparently slight case as the result of muscular effort. (v) Death, which may or may not be preceded by mild delirium or unconsciousness, rarely occurs after the first or second day. =Di-chlor-ethyl-sulphide= is spoken of as being a _vesicant_. It may exert its irritant action either as a vapour in low concentration in the air or by direct contact from splashes of the liquid. The liquid or vapour clings to the clothing of men exposed to Yellow Cross shells, and thus slowly exerts its continuously irritant action on their bodies. No irritant effect at all is felt on first exposure, whatever the concentration may be, but after a delay of about two to six hours the skin and mucous membranes begin to react with a progressive inflammation that may result in local necrosis and desquamation of these covering membranes. There is intense conjunctivitis; the skin turns an angry red, and this erythema is soon followed by skin blistering here and there over the face and body. The passage of the vapour down the respiratory tract may cause such severe injury to the lining mucous membranes of the trachea and bronchioles that they are eventually destroyed and sloughed away. Bacterial infection then seizes upon these raw surfaces, and the patient may die from secondary septic broncho-pneumonia. Death is never the direct result of the action of the poisonous vapour. From the 2nd day onward through the first and second week severely affected men may die, but only as the result of secondary bacterial infection. This poison therefore differs entirely from the lung irritants such as Phosgene, which kill directly and speedily by flooding the lungs with oedema fluid. The main features of poisoning from Mustard Gas may be resumed as follows: (i) _Delay_ of the irritant effect for at least two to three hours, and then a comparatively slow development of the various inflammatory reactions. (ii) Vomiting, and a sense of burning in the eyes, with discomfort in the throat, hoarse cough, and some retro-sternal pain. (iii) Intense conjunctivitis that temporarily ‘blinds’ the man. (iv) Burning of the exposed skin surfaces and of the moist areas in the axillae and groin, followed by blistering, excoriation, and brown staining. (v) Inflammatory necrosis of the mucous membrane of the trachea and bronchi, with the secondary development of infective bronchitis or septic broncho-pneumonia. (vi) Death is relatively uncommon: it occurs later than the first day and only as the result of septic complications. No. I. Microscopic section of human lung from phosgene shell poisoning. Death at the nineteenth hour after gassing.