The sound of the door-bell at this minute prevented Coucou from replying. In front of the door stood Carmen.
"Thank Heaven you have come, mademoiselle."
"You haven't found Gontram yet?"
"No."
"Monsieur Gontram and his comrades are in subterranean chambers in this house."
"Knock at the walls, Coucou," said Madame Caraman, "and then we can wait for an answer."
Coucou knocked three times with a hammer against the wall. At the end of the second knock came back in answer twenty-five.
"What does that mean?" asked Coucou, in affright.
"I know," cried Carmen; "twenty-five knocks signify the letters of the alphabet!"
"Then we must answer to show that we understand the language," said Madame Caraman. "Coucou—quick—twenty-five knocks."