to the trenches
compressed in steel cylinders. These are dug in at the bottom of the
trench and connected with pipes leading out over the parapet. When the
valves of the cylinders are opened, the gas escapes with a hissing
sound, which, on a still night, can frequently be heard at a
considerable distance. It mixes with the air and is carried by the
wind towards the opposing trenches, spreading out as it goes forward.
A continuous wave of gas and air is thus formed, the color of which
may vary:

(a) Because of the weather conditions. In very dry air it may be
almost transparent and slightly greenish in color, while in
damp weather it forms a white cloud.
(b) Because it may be mixed with smoke of any color.

A cloud attack can only take place when there is a steady but not too
strong wind blowing from the enemy's lines towards our own. A wind
between 4 and 8 miles an hour is the most likely condition. An 8-mile
wind will carry the gas cloud twice as quickly as a man walks rapidly.

Gas attacks may occur at any time of the day, but are most likely to
be made during the night or in the early morning.

Gentle rain is without appreciable effect on a gas attack, but strong
rain washes down the gas. Fogs have hardly any effect and may, in
fact, be taken advantage of to make an attack unexpectedly. Water
courses and ponds are no obstruction to a gas cloud.

The gas used by the enemy is generally a mixture of chlorine and
phosgene, both of which are strongly asphyxiating. The gases are
heavier than air, and therefore, tend to flow along the ground and
into trenches, shelters, craters and hollows. The gas cloud may flow
round slight eminences, thus leaving patches of country which remain
free from gas.

Chlorine and phosgene strongly attack the mucous membranes of the
respiratory organs, causing bad coughing. In strong concentrations of
gas, or by longer exposure to low concentrations, the lungs are
injured and breathing becomes more and more difficult and eventually
impossible, so that the unprotected man dies of suffocation. Death is
sometimes caused by two or three breaths of the gas. Even when very
dilute, chlorine can be recognized by its peculiar smell, which is
like chloride of lime, but stronger and more irritating.

Both chlorine and phosgene also exert a strongly corrosive action on
metals, so that the metal parts of arms must be carefully protected by
greasing them.

The speed with which