Knowing as he did the deadly nature of the gas from his own experience and that of his comrades, some of whom had been killed by it, Private Drew lost no time in sounding his warning to the moving picture boys.  He had taken part in the raid on the Germans, had seen and engaged in some hard fighting, and had been sent to the rear with an order from his officer.  And it was as he started that he saw, from one section of the Hun lines, the deadly gas rolling out.

He knew from the direction and strength of the wind just where it would reach to, and, seeing the moving picture boys in its path, he called to them.

“Put on your masks!  Put on your masks!” cried the soldier.  At the same time, as he ran, he loosed his from where it hung at his belt and began to don it.

The gas masks used in the trenches are simple affairs.  They consist of a cloth helmet which is saturated with a chemical that neutralizes the action of the chlorine.  There are two celluloid eye holes and a rubber tube, which is taken into the mouth and through which the air breathed is expelled.  All air breathed, mixed as it is with the deadly chlorine, passes through the chemical-saturated cloth of the helmet and is thus rendered harmless.  But it is a great strain on those who wear the masks, for nothing like the right kind of breathing can be done.  In fact, a diver at the bottom of the sea has better and more pure air to breathe than a soldier in the open wearing a gas mask.

It was the first experience of Blake and his chums with the German gas, though they had heard much about it, and it needed but the first whiff to make them realize their danger.

Even as Private Drew called to them, and as they saw him running toward them and trying to adjust his own mask, they were overcome.  As though shot, they fell to the ground, their eyes smarting and burning, their throats and nostrils seeming to be pinched in giant fingers, and their hearts laboring.

One moment they had been operating their cameras.  The next they were bowled over.

“Put on your——­” began Blake; and then he could say no more.  He tried not to breathe as he fumbled at his belt to loosen his mask.  He buried his nose deep in the cool earth, but such is the nature of this gas that it seeks the lowest level.  There is no getting away from it save by going up.

In a smoke-filled room a fireman may find a stratum of cool, and comparatively fresh, air at the bottom near the floor.  This is because cold air is heavier than the hot and smoke-filled atmosphere.  But this does not hold with the German gas.

And so, before Blake could slip over his head the chemical-impregnated cloth, he lost consciousness.  In another moment his two companions were also unconscious.  Private Drew, struggling against the terrible pressure on his lungs, managed to get his helmet over his head, and then he gave his attention to his friends.