Letters to Mrs R. H. Caldwell, from her son Robert Douglas Caldwell and other soldiers, 1917-1919 - Page 61
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[Page 61]
The respirator is carried in a haversack at the side & troops at the front are never without it. When in use it is slung round the neck in front of the body, it consists of a mask that fits over the face in an airtight manner with a mouth piece & nose clip inside; we breathe in & out through the mouth piece & the air coming in has to come through a tin of different chemicals which purifies it. The air that is breathed out gets away through another channel by way of a rubber safety valve which will not let any air come in. One other thing used for gas is what is known as the P.H. helmet which is rearly only a flannel bag with eye pieces in & a mouth piece through which you breathe out & the air that is breathed in has to come through the flannel which is chemicalised, this helmet is also absolute proof against all gases