The search-light went out, and Carnes and the lieutenant could hear the slide of the ultra-violet light which Dr. Bird was manipulating open. For two or three minutes the doctor worked with his apparatus. "All right!" he cried suddenly. "Lights on and get out of here!" Carnes snapped on the search-light and Lieutenant Leffingwell swung the tank around and headed for the cave mouth. For a few feet their progress was unhindered and then the tank ceased its forward motion, although the motor still roared and the track slid on the cave floor. Carnes watched with horror as one side of the tank bent slowly in toward him. There was a rending sound, and a portion of the heavy steel fabric was torn away. Dr. Bird bent over something on the floor of the tank. Presently he straightened up and threw a small object into the darkness. There was a flash of light, and bits of flaming phosphorus flew in every direction. The anchor which held the tank was suddenly loosed and the machine crawled forward at full speed, while a roar as of escaping air mingled with a bellowing shriek burdened the smoke-laden air. "Faster!" cried the doctor, as he threw another grenade. * * * * * Lieutenant Leffingwell got the last bit of speed possible out of the tank and they reached the cave mouth without further molestation. "I had an idea that our friend wouldn't care to pass through a phosphorus screen," said Dr. Bird with a chuckle as he climbed out of the tank. "He must have been rather severely burned the other day, and once burned is usually twice shy. Where is Major Brown?" The commanding officer stepped forward. "Drive a couple of cattle into the cave, Major," directed Dr. Bird. "I want to fill that brute up and keep him quiet for a while. I'm going to develop my films." Lieutenant Leffingwell and Carnes peered over the doctor's shoulders as he manipulated his films in a developing bath. Gradually vague lines and blotches made their appearance on one of the films, but the form was indistinct. Dr. Bird dropped the films in a fixing tank and straightened up. "We have something, gentlemen," he announced, "but I can't tell yet how clear it is. It will take those films fifteen minutes to fix, and then we'll know." In a quarter of an hour he lifted the first film from the tank and held it to the light. The film showed a blank. With an exclamation of disappointment he lifted a second and third film from the tank, with the same result He raised the fourth one. "Good Lord!" gasped Carnes. * * * * *