Out of all the various substances used by the Enemy in Gas Warfare only two have been chosen for illustration of their effects in this Atlas. They are Phosgene (COCl_{2}), and Di-chlorethyl-sulphide, (C_{2}H_{4}Cl)_{2}S, or ‘Mustard Gas’. =Phosgene= is the chief of all the many gasses and liquids that are used for their effects as _pulmonary irritants_. Chlorine belongs to this group and was the first Poison Gas used by the Germans in April 1915, but it has long since been superseded by more effective chemical substances. The pulmonary irritants are inhaled as gasses or vapours. They may cause some watering of the eyes, but the chief effect noticed at once is a catching of the breath or a choking sensation so that the chest feels gripped and incapable of free respiration. Coughing and vomiting may follow, and then after a delay of time varying from a few minutes to several hours an inflammatory reaction appears in the lungs themselves, with the development of an acute oedema that may commence insidiously and yet progress so rapidly as soon to be an immediate menace to life itself. The alveoli fill with oedema fluid, which then rises into the bronchial tubes and may appear in a most abundant expectoration of thin frothy fluid. Aeration of the blood is seriously interfered with, because the air sacs are either drowned with oedema fluid or burst by the efforts of coughing. Moreover the actual circulation through the lungs is embarrassed, both by the pressure of the oedema fluid on the capillary vessels and by the local thrombosis that occurs in many places in the smaller lung vessels. The blood itself is concentrated by the loss of serum so that the count may rise to even eight or nine million red corpuscles to the cu. mm. and this change probably adds to the difficulties of the circulation. The gassed man can no longer get the oxygen that he wants, and he either dies in obvious asphyxia with progressive circulatory failure, or he collapses as the result of some muscular effort that suddenly makes a greater call for oxygen and so reveals the deficiency of the supply. Death is the result simply of this inflammatory oedema of the lung, and it occurs chiefly in the first and second day after exposure to Phosgene. A few cases may chance to develop secondary bacterial infections of the lungs and to succumb to a later broncho-pneumonia, but they are relatively rare. The main clinical features of acute Phosgene poisoning may therefore be summarized as follows: (i) Catching of the breath, choking, and coughing _immediately_ on exposure to the gas. (ii) Inability to expand the chest in a full breath after removal from the poisoned air. (iii) Vomiting, hurried shallow respiration, and sometimes coughing with an abundant expectoration, follow. Pain is felt behind the sternum and across the lower part of the chest. Fine râles are heard in the axillae and over the back.