hen they had taken off and were well under way, Van opened his black case and set a vacuum-tube apparatus in operation. They were nearing the fringe of the glowing sea of red that was the vast blanket of moon weed. It now extended to within a few miles of Baltimore and stretched northward as far as the eye could see.
"It was a cinch," Van was explaining. "When I first saw that the growth slowed up under the arc-lights at Tomkins Cove it gave me the glimmering of an idea. Then, on the following day, when we learned that the weed spread more slowly in sunlight, I was convinced. The stuff is dormant on the moon, you know."
"Why?" Bart asked breathlessly.
"Because there is no atmosphere surrounding the moon, and the sun's rays are not filtered before they reach its surface as they are here. The invisible rays, ultra-violet and such, are present in full proportion. And the moon weed can not flourish when subjected to light of the higher frequencies. It died out when the moon lost its atmosphere, and only revived on being brought to earth—probably a million times more prolific in our dense and damp atmosphere and rich soil. The thing's a cinch to dope out."
"Yeah!" Bart commented drily. Van was now talking and he could have bitten off his tongue for interrupting him.
This machine of Van's was a generator of invisible light in the ultra-indigo range, Van explained. You couldn't see its powerful beam, but they had proved in the laboratory that it was certain doom to the moon weed. They had grown the stuff from seed in steel cages, and played with it until they were all satisfied. Now would come the final test. Ten thousand planes were being equipped with the new generator, which was merely an adaptation of standard directional television transmitters, and to-night these would start out to fight the weed. It was a cinch!
eneath them the red cauldron seethed and tossed as they sped northward; the crimson blanket of death that was steadily covering the country.