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celluloid was inserted in the helmet for vision. This was easily cracked and the mask was otherwise defective in having no outlet valve to prevent the harmful accumulation of carbon dioxide inside the helmet. This mask was issued to all troops in the field by July 6, 1915.

3. Phosgene, on Dec. 11, 1915. Phosgene was ten times more poisonous than chlorine. By July, 1915, it was learned that phosgene would l>e employed by the Germans during the following December. The British Intelligence Service ascertained not only this important fact but also the exact area within which the attack would take place. With five months to prepare, the British developed the P. Helmet.

This was similar in shape to the Hypo "^r^Sr^SK Helmet but was made of flannelette and * ¿."¿V was provided with two glass eyepieces.

It also had an expiratory valve made of rubber, very similar to the outlet vaive •¿^grfji on present-day masks (see Figs. 109 and tvES 110). The helmet was dipped in a soJu-4551 tion of caustic soda, phenol, and glycerine. The first two of these substances react to form sodium phenolate which neutralized phosgene, hence the name P. or Phenolate Helmet. It was used by the British during the large phosgene attack Fxo. lio.—British Hyp©. P., or near Pilckum on Dec. 19,1915. It saved many lives though it was not fully satisfactory against high concentrations. Meanwhile the Russians had discovered that a substance known as urotropine or hexamethylenetetramine readily neutralized phosgene. With this information, the British now discarded the P. Helmet for the P.H. (phenate-hexamine) Hcbnct, similar except for the protective solution in which it was dipped (see Figs. 109 and 110). The new solution was urotropine, caustic soda, phenol, and glycerine. The P.H. Helmet

P.H. Helmet In ute, skirt buttoned under tunic«

gave much better protection than the P. Helmet, was effective for about 24 hours' continuous use, and would withstand a high concentration of phosgene.

By the latter part of 1915, the Germans had commenced the extensive employment of lacrimators either alone or in conjunction with lethal gas. The P.H. Helmet offered little protection against lacrimators. Accordingly in September, 1915, goggles made of rubber with mica eyepieces were issued for use in connection with the P.H. Helmet. This involved difficulties of adjustment which led to the development of the P.H.G. Helmet, having tight-fitting goggles attached to the mask. This helmet, however, was also difficult to adjust and was soon discarded. With the

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