The Great God Gold by William Le Queux

Frank Farquhar was cleverly working his own game. The Professor had scoffed at the theory put forward by Diamond, therefore he was easily induced to give a written undertaking to regard the knowledge derived from the half-burnt manuscript as strictly confidential, and to make no use of it to his own personal advantage.

"I have to obtain this," the young man explained, "in the interests of Diamond, who, after all, is possessor of the papers. He allowed me to have them only on that understanding."