Suddenly he saw that, just beyond each pearl, was engraved on the ivory rim of the box a number—starting with the large pearl at the top as number one, the circle of numbers ran around the edge of the box until it returned to its starting point, at number twenty-six. In his efforts to see these numbers, which were very small, he gripped the box tightly in his hands to hold it the more steadily toward the rather dim light. In doing so, he suddenly became aware of the fact that the rim or edge of the box, containing the numbers and the circle of pearls, was movable. It fitted so cunningly into the top of the box, that the joint appeared not as a crack or perceptible space, but merely as a fine thin line, apparently a part of the engraving on its surface. Holding the lower part of the box firmly in his left hand, he turned the rim of the top slowly about. At once the purpose of this became apparent. Not only had each pearl, representing a letter of the alphabet, six corresponding values from rim to center, in any one position, but by turning the rim around, twenty-six such positions could be secured, making a total of one hundred and fifty-six different alphabets from which a person desiring to use a cipher might choose.
Again, however, Duvall was conscious of a feeling of disappointment. One hundred and fifty-six different ciphers were no better than a single one, if only one were used. Evidently he had not yet reached the solution of the problem. In employing such a system of ciphers, some combination, precisely similar to the combinations used on the locks of safes, would have to be used. It was absolutely necessary, in order to insure safety, to use not one cipher, but a large number, changing the arrangement of the letters with each line written—even with each word, in order to defy solution. Yet such an arrangement being purely arbitrary, could not well be trusted to memory, for, once forgotten, the translation of the document written, even by the writer himself, would be absolutely impossible. It occurred to him that as there were six different concentric lines of lettering, each constituting in itself a complete cipher, the obvious way to use the box would be to place the pearls in a given position, write six words, using a different alphabet for each word, and then shift the ring of pearls to a new position, and repeat the operation. This, of course, could be done indefinitely, although half a dozen changes would be sufficient to insure a cipher that would absolutely defy solution. Where, however, was the key? That, after all, was the important matter; without it, the snuff box would be as useless to Monsieur de Grissac as it would be to his enemies themselves.
For many minutes Duvall puzzled over the matter, unable to reach any satisfactory conclusion. Then he began to think of the song which had so clearly been repeated, over and over, as a message to him from outside. The words of the refrain began to run aimlessly through his mind, his eyes upon the box. Suddenly he realized that the word cross, in its repetitions, its position as the final word of the song, must have a definite meaning. Before his eyes he saw the cross, so delicately carved as to project scarcely an eighth of an inch above the thin and fragile ivory surface. Instinctively he began to push at it, pressing it this way and that, to discover, if possible, any spring or other means whereby it might be made to turn or lift up. As he did so, his fingers unconsciously pressed upon the large pearl at the top. In a moment the upper surface of the cross slid to one side, disclosing a tiny shallow cavity beneath it, some quarter of an inch in either direction, and no deeper than the thickness of a piece of cardboard. Within this lay a bit of tissue paper, tightly folded.
Duvall drew it carefully out and examined it. Upon it were written six numbers: 12-16-2-8-20-4. There was nothing else upon the paper, but Duvall realized that he held in his hand the key of the cipher.
At once Monsieur de Grissac's agitation, the servant Noël's death, Hartmann's persecution of him, became clear. Evidently there were documents, somewhere, of some nature, which this cipher made intelligible and which, without it, were proof against all attempts to read them. What were these documents? Were they in Hartmann's hands? These questions, he knew, could not be answered now.
Immediately the question rose in his mind: What should he do next? By destroying the tiny slip of paper, he could render the snuff box valueless. Without the key, no one could use it with success. But, the key once destroyed, how could Monsieur de Grissac himself read the documents, for the preparing of which it had been utilized? Possibly, if Hartmann had such documents, they were but copies, obtained through the corruption of some clerk, while the originals remained in De Grissac's possession. For these reasons he dared not destroy the cipher, at least until all other means of escape had been exhausted. Then he realized, in a flash, that if he proposed to utilize the return of the snuff box as a means of obtaining his freedom, he could not hope to do so, if the key was removed. Doubtless Hartmann knew of its existence. In some way he had learned, possibly through the murdered man Noël, that the box contained such a key, and would examine it, and satisfy himself that it had not been removed, before he would allow him to leave the place. This would inevitably result in his being searched, and the key, concealed about his person, found. He stood in an agony of doubt, wondering which alternative he should take.
His reflections were rudely disturbed by the sound of footsteps in the corridor outside the door. In a moment he had replaced the tiny bit of paper in the recess beneath the cross, slid the latter back into place, and thrust the box beneath a mass of straw which lay on top of the packing case against which he had been leaning. Then he turned toward the door and had barely time to hurl the opera hat into a dark corner, when the door opened, and Hartmann appeared on the threshold.
CHAPTER XIX
It was not until early in the afternoon that Grace was able to accomplish anything toward carrying out the instructions which young Lablanche had given her with respect to the phonograph. On her return to Dr. Hartmann's from her expedition to Brussels, she went at once to her room, and locked the record which Lablanche had given her in her trunk. There was nothing to be done now, until after luncheon.