"It's explosive, and about doubles the force of your explosions. It eats your engines right up, though. They used to use it in motor-boat races until a rule was made against it. You see, an engine is ruined after twenty minutes or so, and it made the racing unfair for people who couldn't buy a new engine for every race." Teddy's face grew thoughtful. "Picric acid," he said meditatively. "Suppose we used it in the gas of your plane. Would we have a chance of catching Varrhus?" "I don't know," Davis said thoughtfully. "I hardly think so. It would make our speed better, but if it were anything of a chase our motors would be ruined before we'd gone far." "The acid attacks the steel of the cylinders and makes the bore too large?" Teddy seemed to be thinking rapidly. "Yes. You lose all your compression." Teddy looked at Evelyn. "Suppose the pistons and the interiors of your cylinders were plated with platinum? Platinum is one of the hardest metals, and should stand up under a great deal of wear." "Would platinum resist the attack of the acid?" Davis grew excited. "Surely." Davis jumped to his feet. "Then we've got him! New piston rings will let you plate the cylinders without reboring them unless you're going to plate them heavily. Can you do the plating?" "Try," said Teddy. "We make a hundred and eighty with straight gasoline," said Davis excitedly. "With doped gas----How long will it take to fix my motors?" "Four or five hours. We'll borrow the acid vats of some electro-plating concern. Evelyn will mix the solution of platinum salts. I'll go arrange to borrow the vats while you get your motors disassembled and brought here on a motor truck." Teddy hastily began to put on his coat. "You're going to try to fight Varrhus again?" asked Evelyn anxiously. "Are we?" asked Davis cheerfully. "Just ask me! We are." "You hit him several times in the last fight," said Evelyn faintly, "and it didn't do any good." "We'll use armor-piercing bullets this time," said Davis exuberantly. "Or we may be able to mount a one-pounder automatic. I think the plane will stand it. And at worst we can ram him." Evelyn turned a trifle pale. "That means you'll both be killed." Davis smiled. "Maybe not. We'll take a chance anyway, won't we, Gerrod?" Teddy nodded shortly. "I'm going to get Varrhus or he's going to get me," he said succinctly. They started for the front door. The commissioner of police was just getting out of his car. "News, most likely," said Teddy, and they waited. The commissioner of police looked worried when he shook hands with Teddy.