Rolling down upon the American and French battlelines, coming out of the German trenches, where it had been generated as soon as it was noted that the wind was right, drifted a cloud of greenish yellow, choking chlorine gas.

Chlorine gas is made by the action of sulphuric acid and manganese dioxide on common salt. It has a peculiar corrosive effect on the nose, throat and lungs, and is most deadly in its effect. It is a heavy gas, and instead of rising, as does hydrogen, one of the lightest of gases, it falls to the ground, thus making it dangerously effective for the Huns. They can depend on the wind to blow it to the enemy's trenches and fill them as would a stream of water.

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.

He knew that to save their lives he must get their helmets on; for a few breaths of the gas will not kill. But they will disable a person for some time, and a little longer breathing of it means a horrible death.

And so, working at top speed, the soldier, now himself protected from the fumes, though he had breathed more of them than he liked, labored to save his friends.

Suddenly a new terror developed, for, wearing their own helmets which made them look like horrible monsters out of a nightmare, the Germans charged against the French and Americans, whom they hoped to find disabled by the gas.

"Here they come with blood in their eyes if I could only see it!" mused Private Drew, as he finished fastening the helmet on Charles Anderson, having already thus protected Joe and Blake. All three boys were now unconscious, and what the outcome would be the soldier could only guess.

"But there won't be any guesswork if I leave 'em here for the Huns," he reasoned. "I've got to help 'em back—but how?"

The Germans, in a counter-offensive, were striving to regain some of the lost ground, and, for the moment, were driving before them the French and American forces. Back rushed the advance lines to their supporting columns, and Drew, seeing some of his own messmates, signaled to them, for he could not talk with the helmet on.

Fortunately his chums of the trenches understood, and while some of them caught up the unconscious boys and started with them to the rear, others saved the moving picture machines.

And then, just as it seemed that the Germans would overtake them and dispose of the whole party, there came a rush of helmet-protected Americans who speedily dispersed those making the counter-attack, pursuing them back to the very trenches which they had left not long before.

The fight went on in that gas-infested territory, a grim fight, desperate and bloody, but in which the Allies were at last successful, though Blake and his two chums saw nothing of it.

"They're in a bad way," the surgeon said, when he examined them soon after Drew and his friends brought them in. "I don't know whether we can save them."

But prompt action, coupled with American ingenuity and the knowledge that had been gained from the experience of French and British surgeons in treating cases of gas poisoning, eventually brought the moving picture boys back to the life they had so nearly left.

It was several days, though, before they were out of danger, and by that time the French and Americans had consolidated the gains it cost them so much to make, so that the place where the three boys had been overcome was now well within the Allied lines.

"Well, what happened to us?" asked Joe, when he and his chums were able to leave the hospital.

"You were gassed," explained Private Drew, who had had a slight attack himself. "Didn't you hear me yelling at you to put on your helmets?"

"Yes, and we started to do it," said Blake. "But that stuff works like lightning."

"Glad you found that out, anyhow," grimly observed the soldier. "The next time you hear the warning, 'Gas!' don't stop to think, just grab your helmet. And don't wait longer than to feel a funny tickling in your nose, as if you wanted to sneeze but couldn't. Most likely that'll be gas, too. Cover your head when you feel that."

"Thanks!" murmured Blake, for he and his chums understood that the soldier and his mates had saved their lives.

Now that the moving picture boys were out of danger and could take some stock of themselves and their surroundings, their first thoughts, naturally, were of their apparatus.

"Did they get our machines?" asked Joe.

"No; we saved the cameras for you," answered Drew.

"What about the boxes of exposed film—the ones the War Office is so anxious to get?" asked Blake.

"I didn't see anything of them," said the soldier. "We were too anxious to get you out of the gas and save the cameras to think of anything else. I didn't see any boxes of films, but I'll ask some of the boys who helped me."

Blake and his chums waited for this information anxiously, and when it came it was a disappointment, for no one knew anything of the valuable reels.

"Though they may be there yet," said Drew. "There was some fierce fighting around that shell crater where we carried you from, but it's within our lines now, and maybe the boxes are there yet. Better go and take a look."

This Blake, Joe and Charlie lost no time in doing. After a little search, for the character of the ground had so changed by reason of the shell fire they hardly knew it, the boys located the place where they had so nearly succumbed. They found the spot where their cameras had been set up, for they were marked by little piles of stones to steady the tripods. But there were no boxes of films.

"Gone!" exclaimed Blake disconsolately, as he looked about. "And we'll perhaps never get another chance to make such pictures again!"

"It surely is tough luck!" exclaimed Joe.

They saw a sentry on guard, for this place was far enough from the lines of both forces to obviate the use of trenches.

"What are you looking for, Buddies?" asked the soldier, who knew the moving picture boys.

"Some valuable army films," explained Blake, giving the details. "They're very rare, and we'll probably never get any others like them."

"Did you leave them here?"

"Right around here," answered Joe. "I think just near this pile of rocks," and he indicated the spot he meant.

"Say, now," exclaimed the American private, "I wouldn't be surprised but what those two fellows took 'em!"

"What two fellows?" cried Blake.

"Why, just as I was coming on duty here I saw two fellows, one dressed as a German soldier and the other in a blue uniform, walking around here. I thought they were up to no good, so I took a couple of shots at 'em. I don't believe I hit either of 'em, but I came so near that I made 'em jump. And then, just before they ran away, across No Man's Land, I saw them stoop down and pick up something that looked like boxes. I thought they might be something they had lost in the fight the other day, for the scrap went back and forth over this section. But now, come to think of it, they might have been boxes of your films."

"I believe they were!" cried Blake.

"What two fellows were they you saw?" asked Joe.

The soldier explained, giving as many details as he could remember, and Charlie cried:

"Lieutenant Secor for one—the chap in the blue. A French traitor!"

"He did have a uniform something like the French," admitted the private. "The other was a Fritz, though."

"Labenstein!" murmured Joe. "I wonder if it is possible that they are with the Hun army and have learned through spies that we are on this front. If they have, they would know at once that those were boxes of films, and that's why they stole them! Do you think it possible, Blake?"