"How? Gas of some sort?" "The windows were all closed and locked and I have already ascertained that the gas and water lines have not been tampered with. Gas won't penetrate through a solid roof in sufficient concentration to knock out a man like that. It was something more subtle than gas." "What was it?" "I don't know yet. The clue to what it was lies, as I told you, in those windows." Carnes moved over and surveyed the windows closely. "I see nothing unusual about them except that they need washing rather badly." "They were washed last Friday, but they do look rather dirty, don't they? Suppose you take a rag and some scouring soap and clean up a pane." The detective took the proffered articles and started his task. He wet a pane of glass, rubbed up a thick lather of scouring soap and applied it and rubbed vigorously. With clear water he washed the glass and then gave an exclamation of astonishment and examined it more closely. "That isn't dirt, Doctor," he cried. "The glass seems to be fogged." Dr. Bird chuckled. "So it seems," he admitted. "Now look at the rest of the glass around the laboratory." Carnes looked around and then walked to a table littered with apparatus and examined a dozen pieces carefully. "It's all fogged in exactly the same way, Doctor," he said. "The only piece of clear glass in the room is that piece of plate glass on your desk." * * * * * Dr. Bird picked up a hammer and struck the plate on his desk a sharp blow. Carnes ducked instinctively, but the hammer rebounded harmlessly from the plate. "That isn't glass, Carnes," said the doctor. "That plate is made of vitrilene, a new product which I have developed. It looks like glass, but it has entirely different properties. It is of enormous strength and is quite insensitive to shock. It has one most peculiar property. While ultra-violet and longer rays will penetrate it quite readily, it is a perfect screen for X-rays and other rays of shorter wave length. It appears to be the only piece of transparent substance in my laboratory which has not been fogged, as you call it." "Do short waves fog glass, Doctor?" "Not so far as I know at present, but you must remember that very little work has been done with the short wave-lengths. In the vast range of waves whose lengths lie between zero and that of the X-ray, only a few points have been investigated and definitely plotted. There may be in that range a wave-length which will fog glass." "Then your theory is that some sort of a ray machine was put in operation before the helicopter landed?"