hour after the gas was turned on 80 per cent. of the opposing troops were knocked out.  The Canadians, with wet handkerchiefs over their faces, closed in to stop the gap, but if the Germans had been prepared for such success they could have cleared the way to the coast.  But after such trials the Germans stopped the use of free chlorine and began the preparation of more poisonous gases.  In some way that may not be revealed till the secret history of the war is published, the British Intelligence Department obtained a copy of the lecture notes of the instructions to the German staff giving details of the new system of gas warfare to be started in December.  Among the compounds named was phosgene, a gas so lethal that one part in ten thousand of air may be fatal.  The antidote for it is hexamethylene tetramine.  This is not something the soldier—­or anybody else—­is accustomed to carry around with him, but the British having had a chance to cram up in advance on the stolen lecture notes were ready with gas helmets soaked in the reagent with the long name.

The Germans rejoiced when gas bombs took the place of bayonets because this was a field in which intelligence counted for more than brute force and in which therefore they expected to be supreme.  As usual they were right in their major premise but wrong in their conclusion, owing to the egoism of their implicit minor premise.  It does indeed give the advantage to skill and science, but the Germans were beaten at their own game, for by the end of the war the United States was able to turn out toxic gases at a rate of 200 tons a day, while the output of Germany or England was only about 30 tons.  A gas plant was started at Edgewood, Maryland, in November, 1917.  By March it was filling shell and before the war put a stop to its activities in the fall it was producing 1,300,000 pounds of chlorine, 1,000,000 pounds of chlorpicrin, 1,300,000 pounds of phosgene and 700,000 pounds of mustard gas a month.

Chlorine, the first gas used, is unpleasantly familiar to every one who has entered a chemical laboratory or who has smelled the breath of bleaching powder.  It is a greenish-yellow gas made from common salt.  The Germans employed it at Ypres by laying cylinders of the liquefied 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