"But that day, of course," interposed the girl, "is a long way off. Is this the only physiological danger?" "There is one even _more_ dreadful. The T-radiation." "T-radiation? What is that?" Grossman smiled mirthlessly. "Were I able to tell you, I should be a greater physicist than any who have so far visited Titan. Dozens of the wisest have come, probed, pondered, analyzed ... and left Titan none the wiser for their efforts. Frankly, they do not know! The very name 'T-radiation' is an admission of their failure. It is simply an abbreviation for 'Titan-radiation.' It is an electro-magnetic or radioactive emanation lethal to humans ... that is all they know about it." Young Dr. Roswell wiped his spectacles 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?"