After the maid had left the room, the chemist rose from his chair and walked toward the door leading to the library. "If you will excuse me for a few minutes, Agnes, I shall see what these men want. Wait for me here, if you will. I shall be back directly." So saying, he noiselessly opened the folding-doors and passed into the library, closing the doors carefully behind him. Freed from the presence of her father, Agnes almost instantly regained her composure. She had not, however, had much time to collect her thoughts, when she was suddenly startled by a loud shrill whistle, which brought her to her feet in alarm. It is a well-known fact that there is, in the ring of a door bell, a complex range of expression, which differentiates to an observant ear the characteristics of the ringer. No one is likely to mistake the postman's ring for that of the beggar; and no young girl is liable to confound her father's ring with that of her lover; but, to a careful observer, the gradations of quality, of intensity, of duration, in a ringing door bell, are almost as great as in the voices of the ringers themselves. Perhaps the range of expression in the whistle of a speaking-tube is less extended; but in the whistle which reached Agnes Murdock's ears there was something that struck a chill of terror to her heart, like a wild despairing cry of anguish, and which caused her to spring without hesitation to the tube, the mouthpiece of which protruded from the wall of Murdock's study. "Well?" She asked the question in anxious tones, as if realizing that life and death were in the balance. Then she placed her ear to the mouthpiece. At first, she could not make out the words spoken by her invisible interlocutor. Then, gradually, they fell upon her ear with terrible distinctness; and she stood spellbound, as in a horrible nightmare, with sudden terror in her staring eyes, and with the fearful sense of impotence in her trembling limbs. CHAPTER XXIII. THE SPEAKING-TUBE. Nature has implanted in every one of its living creatures, from the top to the bottom of the scale, the strongest of all instincts--that of self-preservation. As Sturgis fell forward and clutched wildly at the air, his hands struck the stone wall of the square chamber. No conscious impression was made upon his brain by the contact; but, automatically, his fingers tightened as they slipped over the smooth surface. His right hand struck an obstacle and closed upon it, in the convulsive grip of a dying man. Then a sudden gleam of consciousness swept across his sluggish brain. It was the speaking-tube! He clung to it with the remnant of his strength and eagerly placed his lips to the mouthpiece. For a few minutes he drank in with avidity the revivifying draughts of air which gradually brought him back from the brink of death.