He took the screwdriver and, reaching up, inserted the end and pried at one of the circles. It came open, revealing the fact that it was a small hinged trap door. What surprised us the most, however, was the fact that a small white globe fell out of it and broke on the rug. “Switch off the lights for a moment,” he said. Someone pressed the light switch, and all of us saw the now familiar vision of a spectre materializing from the floor. “Turn them on,” he ordered. They were turned on once more. “The ghost,” he said, “is nothing more than a mixture of foul-smelling gases, one of which is slightly phosphorescent. This mixture, as you will observe, is visible in the dark but invisible in the light. The gas is imprisoned in small, thin globes of ice which shatter when they strike the rug, and melt in a few seconds, leaving no trace other than a few drops of water which quickly evaporate or are absorbed by the rug fibres. These globes are kept in a small refrigeration plant which is just above my head, and which is probably quite thoroughly insulated against heat. The intense cold in this plant is produced by a substance which is not new to science, but the use of which for this particular purpose is quite new. The substance is frozen CO₂ or carbon dioxide, and when expanded into a gas it is identical with the substance that gives zest to soda water and bottled beverages. It has a temperature of 114° below zero, Fahrenheit, and evaporates to a dry gas without going through the intermediate liquid state with which we are familiar in most substances. “The cold air and gas from this refrigerating chamber, when propelled into the room by small, noiseless fans through others of these hinged openings which do not contain the gas balls, creates the phenomenon of the icy breath. It can also create the illusion of a light touch from a cold hand, as I have proved experimentally. The slight breeze moving the small hairs on one’s hand or arm gives the sensation of one having been lightly touched while the coldness of the breeze makes it appear that one has been touched by something cold. The closet, in which I came so near being asphyxiated and frozen to death, is equipped with a similar refrigeration plant, and it is probable that we shall find more of them which have not been used, in other rooms. * * * * * “The matter of the lights going out and again being turned on will be settled as soon as we can find the radio-controlled rheostat and switch which operates them. Is everything clear?” “You have not explained what it was which drove my dog mad,” Miss Van Loan reminded him.