It hit with a dull, hollow plop, and burst into an instant, spreading blackness! In a fraction of a second the room was in utter blackness, a jet night so intense that the powerful glow lamps of the laboratory were utterly lost. There was nothing but a solid, impenetrable wall of blankness.

"Good lord, what is that?" gasped Aarn. "Hey--where in blazes are you? I can't--say, I can't see my hand when it's touching my face. Uh--here's a light now--"

Silence. A chuckle from Spencer. "It won't work--"

"Haw!" Spencer looked at the screen of his heat-eye televisor, and grinned wider.

As though through a slight, bright fog, he could see Aarn, shining brightly, and holding a flash-lamp that was shining equally brightly, but seemed to be curiously affected by the fog. "It's working. It just can't light, can't send a beam. Put it about half an inch from your eye, and you can see it."

Aarn did. "Sweet singing satellites--what a fog that ink makes! What in space is it?"

"Infra-infra-infra fluorescence." Spencer grinned. "And your heat-eye works beautifully. That's what friend Carlisle made for the occasion of our raid. The chemical tanks contain a load of this. It combines with the oxygen of the air to form a chemical dye in particles so tiny they are close to the brownian limits, and won't settle out in less than about three hours under Tell-el's gravity."

"Infra-infra-and so forth. I think I commence to under stand. Will you kindly supply me with one of those heat visors so I can see my way out? What do you do to use it in this?"

"Stick it so close to your eyes, and turn it up so far that you can see it. This fog isn't utterly impenetrable, you know."

"No--but if I am right, it would be darned near it. I take it that this stuff acts the way fluorescence does with ultra violet. It takes ultra-violet, and reduces it to visible light. This takes visible, and reduces it to infra-visible. Right?"