The professor said, “That’s it! There’s one now! You can see the ceiling waver through it.”

There was a sudden motion of the air. The unseeable something which had deflected the smoke ring moved. The tendrils of smoke wavered and curled through the space from which they had previously been barred.

“It’s one of them!” exulted the professor. “Right in here! But why doesn’t the Monster react? Fetch him out.”

Lane dragged the dog, cowering, from underneath a stool. He held the dog up. The brute panted and wriggled. He gave no sign of fright. His tongue lolled.

“If there is something here,” said Lane, “he doesn’t smell it. And it can’t be seen or he’d see it. It—”

There were now flat layers of tobacco smoke in the air, made visible by sunlight striking into the room through closed glass windows. There was no air movement except the extremely slow general turnover of air in a closed room, but something passed swiftly through those tranquil layers of vapor, disturbing them. It was startling. It was appalling. Lane did not see any wavering of the background behind it.

“Item!” said the professor with satisfaction. “We have a good observation indicating that there are sometimes dynamic systems in air which can move through smoke layers and disturb them. Perhaps we should provide ourselves with sheets to pull over our heads.”

She beamed at Lane, who looked warily at Carol.

“It got in, probably when the dog did,” he said grimly.

The professor rubbed her hands. “Of course!” she said zestfully. “But we know how to keep it from harming any of us! I’m going to catch this specimen and find out a few things about it!”

Lane’s eyes went back to Carol. She was watching all the interior of the trailer with steady, intent eyes—beautiful eyes, Lane thought, but troubled now.

“If it’s what we think, it’s dangerous,” Lane pointed out. “The first thing should be to get her away from this place. I feel responsible. I let the thing in here.”