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 condensed easily to a liquid by cooling it down to 46 degrees Fahrenheit.  A mixture of three-quarters chlorine with one-quarter phosgene has been found most effective.  By itself phosgene has an inoffensive odor somewhat like green corn and so may fail to arouse apprehension until a toxic concentration is reached.  But even small doses have such an effect upon the heart action for days afterward that a slight exertion may prove fatal.

The compound manufactured in largest amount in America was chlorpicrin.  This, like the others, is not so unfamiliar as it seems.  As may be seen from its formula, CCl_{3}NO_{2}, it is formed by joining the nitric acid radical (NO_{2}), found in all explosives, with the main part of chloroform (HCCl_{3}).  This is not quite so poisonous as phosgene, but it has the advantage that it causes nausea and vomiting.  The soldier so affected is forced to take off his gas mask and then may fall victim to more toxic gases sent over simultaneously.