carefully and interrupted, "But ... er ... but surely, Factor, these physicists were able to determine the wave-length of the radiation? Did that not tell them—?"

Grossman said bluntly, almost rudely, "The radiation lies in the Hertzian range, Doctor Roswell. Does that knowledge help you any? Perhaps now you can tell us why these rays are deadly?"

Roswell flushed and faltered into silence. The girl glanced curiously at Grossman.

"Hertzian range, Factor?"

"Electrical waves ranging between 1 m. and 1/10 c.m. in length, Miss Graham. Their place is between the so-called 'short waves' of radio transmission and the infra-red or heat waves. Their existence has been known, theoretically, for at least two hundred years. But man has never been able to find a reason, a place, or use for them. Nor have they been found to occur freely in nature elsewhere than on Titan."

"And," asked Captain Burke, "you say these waves are deadly to humans? But how, then, have our colonists managed to win and maintain a foothold—"

"I should have said," admitted Grossman, "the waves are deadly to unshielded humans. Lead sheathing protects the wearer from harm; consequently men in bulgers are quite safe. And one of the first acts of the Solar Space Patrolmen, upon reaching Titan, was to project a series of leaden highways or avenues between the cities of the satellite. Upon these, and only upon these, may Earthmen travel unprotected by bulgers. To stray from one of these roadbeds means exposure to the T-radiation. And that, in turn, means death!"

Rockingham Roswell shuddered delicately. "Beastly!" he murmured. "Deuced unpleasant sort of place, what? But, I say ... how about the natives? How did they manage to survive before our countrymen built those jolly old lead roadways?"

Grossman pursed his lips impatiently at the affected young scholar.

"They, Doctor Roswell," he said scornfully, "are immune to the T-radiation. Certainly you are acquainted with the principles of selective breeding?"

"Selective—oh, yes! Survival of the fittest ... all that fiddle-di-diddle? You mean the present Titanians are the present Titanians simply because they adapted their physiques to the surroundings, eh? Why, rather! That's clear enough. Still, if they can stand the radiation, I don't see why other humans—"

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