Two days later the Commandant Marnier came to see him at the Ritz Hotel. They dined together in a corner of the restaurant.
"We have solved the problem of those tubes," said Marnier. "They are nothing more nor less than time-fuses."
"Time-fuses!" Hillyard repeated. "I don't understand."
"Listen!"
Marnier looked around. There was no one near enough to overhear him, if he did not raise his voice; and he was careful to speak in a whisper.
"Two things." He ticked them off upon his fingers. "First, hydrofluoric acid when brought into contact with certain forms of explosive will create a fire. Second, hydrofluoric acid will bite its way through glass. The thicker the glass, the longer the time required to set the acid free. Do you follow?"
"Yes," said Hillyard.
"Good! Make a glass tube of such thickness that it will take hydrofluoric acid four hours and a half to eat its way through. Then fill it with acid and seal it up. You have a time-fuse which will act precisely in four hours and a half."
"If it comes into contact with the necessary explosive," Hillyard added.
"Exactly. Now attend to this! Our workmen in our munition factories work three hours and a half. Then they go to their luncheon."