fied gas in
the trenches, about a yard apart, and running a lead discharge pipe over
the parapet. When the stop cocks are turned the gas streams out and
since it is two and a half times as heavy as air it rolls over the
ground like a noisome mist. It works best when the ground slopes gently
down toward the enemy and when the wind blows in that direction at a
rate between four and twelve miles an hour. But the wind, being strictly
neutral, may change its direction without warning and then the gases
turn back in their flight and attack their own side, something that
rifle bullets have never been known to do.

[Illustration: (C) International Film Service

GERMANS STARTING A GAS ATTACK ON THE RUSSIAN LINES

Behind the cylinders from which the gas streams are seen three lines of
German troops waiting to attack. The photograph was taken from above by
a Russian airman]

[Illustration: (C) Press Illustrating Service

FILLING THE CANNISTERS OF GAS MASKS WITH CHARCOAL MADE FROM FRUIT PITS
IN LONG ISLAND CITY]

Because free chlorine would not stay put and was dependent on the favor
of the wind for its effect, it was later employed, not as an elemental
gas, but in some volatile liquid that could be fired in a shell and so
released at any particular point far back of the front trenches.

The most commonly used of these compounds was phosgene, which, as the
reader can see by inspection of its formula, COCl_{2}, consists of
chlorine (Cl) combined with carbon monoxide (CO), the cause of deaths
from illuminating gas. These two poisonous gases, chlorine and carbon
monoxide, when mixed together, will not readily unite, but if a ray of
sunlight falls upon the mixture they combine at once. For this reason
John Davy, who discovered the compound over a hundred years ago, named
it phosgene, that is, "produced by light." The same roots recur in
hydrogen, so named because it is "produced from water," and phosphorus,
because it is a "light-bearer."

In its modern manufacture the catalyzer or instigator of the combination
is not sunlight but porous carbon. This is packed in iron boxes eight
feet long, through which the mixture of the two gases was forced. Carbon
monoxide may be made by burning coke with a supply of air insufficient
for complete combustion, but in order to get the pure gas necessary for
the phosgene common air was not used, but instead pure oxygen extracted
from it by a liquid air plant.

Phosgene is a gas that may be c