pried up the strip of wood, and removed from the cavity behind it a heavy bar of iron.
"Now," he said, "if you will follow me to the basement I will show you the mechanism which acted on this bar of iron, causing the door to open or close."
WE filed down into the basement behind him, and he led the way to a point directly beneath the living room door. The ceiling was covered with plasterboard, a block of which he removed. Fastened to the floor in a semicircle was a string of large electro-magnets.
"All of these magnets," he said, "were caused to act in their turns by impulses of varying wave lengths which closed and opened their circuits. Naturally they pulled the bar of iron although separated by two heavy layers of wood, as there is no insulation which will stop magnetic waves, thus closing or opening the door at the will of the operator. The poker and the heavy overstuffed chair were caused to travel about the room in the same manner, the latter probably having iron bars inserted in the legs, by utilizing other electro-magnets fastened beneath the floor and concealed by this plasterboard.
"While we are here we may as well clear up the mystery of the luminous footprints, for I see the removal of this square of ceiling has already disclosed a part of the mechanism. You will observe here, a glass tube, above which there are two lead plates. The top plate is movable, and is connected with an electro-magnetic device for moving it. In the bottom plate is cut in miniature, the shape of a human footprint. The glass tube is what is known as a Crookes Tube, and the rays which emanate from it when an electric connection is established are known as X-rays. Although these rays are, in themselves, invisible, some of them have the property of making certain substances phosphorescent. The rays which have this property can be cut off by a lead screen of of the correct thickness. One of the substances which can be rendered luminous is sulfid of zinc, and is probably the one used, although I have not yet had an opportunity to verify this. The substance, whatever it may be, has probably been ground into exceedingly minute particles and rubbed into the rug above our heads. A luminous footprint can thus be made to appear on the rug by the simple expedient of turning on the current in the Crookes Tube and sliding back the upper plate in such a manner that the toe prints will first be visible, then the ball of the foot, and finally the heel. I'm sure that if we remove more squares of plasterboard we will find a row of these contrivances about two feet apart, leading to a point beneath the center of the room, where two of them will be found side by side. For the present, however, we will go upstairs to continue our investigation in other directions."
When we were once more in the living room, the doctor asked for a step ladder, and Riggs was sent to bring one. When he brought it, the doctor placed it in the center of the room and climbed up to where the central lighting fixture projected from the wall.
"In this fixture," he said, "are concealed one of the sources of the icy breath, and also the source of the ghastly and foul smelling spectre which rose from the center of the floor on two succeeding evenings. You will observe that the entire fixture, central hemisphere and surrounding collar, appears to be made from frosted glass. The central hemisphere from which the light emanates is glass, but the surrounding collar is of metal covered with a white substance. That white substance is common frost."
So saying, he scraped off a quantity of the frost and handed it down to us for our inspection.
"Please take special notice of the designs on this collar," he said, "for they are particularly well suited for the purposes for which our friend Hegel intended them—a series of circles, each about an inch from the other, reaching entirely around the collar. I will now do by force what the builder of this device previously did by mechanical means, controlled by radio."
He took the screw driver 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 plantwhich is just above my head, and 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
