“I suppose it would be a good idea,” agreed Blake.  “Go to it, Mac, and we’ll be ready when you are.”

Four days of anxious waiting followed, with the men keyed up to concert pitch, so to speak, and eager for the word to come that would send them out of the trenches and against the ranks of the Germans.

But for a long time no word came from the higher command to prepare for the assault, though many knew it was pending.  Perhaps the Germans knew it, too, and that was what caused the delay.  None could say.

Blake, Joe and Charlie were in readiness.  They had their cameras adjusted, had plenty of fresh film, and but awaited the word that would send them from their comparatively comfortable house with the French family into the deadly trenches.

Finally the word came.  Once more in the gray dawn the boys took their places with their cameras in the communicating trench, while ahead of them crouched the soldiers eager to be unleashed at the Germans.

And then they went through it all over again.  There was the curtain of fire, the artillery opening up along a five-mile front with a din the boys had never heard equalled.

Waiting for the light to improve a little, the boys set up their cameras in a little grove of trees where they would be somewhat protected and began to make the pictures.

The battle was one of the worst of the war.  There were many killed and wounded, and through it all—­through the storm of firing—­the moving picture boys took reel after reel of film.

“Some fight!” cried Blake, as a screaming shell burst over their heads, some scattering fragments falling uncomfortably close to them.

“I should say yes!” agreed Joe.  “But look, here comes Drew on the run.  I wonder what’s happened.”

They saw their friend the private rushing toward them, and waving his hands.  He was shouting, but what he said they could not hear.

And then, so suddenly that it was like a burst of fire, Blake, Joe and Charles experienced a strange feeling!  Some powerful odor overpowered them!  Gasping and choking, they fell to the ground, dimly hearing Drew shouting: 

“Gassed!  Gassed!  Put on your masks!”

CHAPTER XX

“GONE!”

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.