"That," said Arcot ruefully, "is that! It seems that our visitors don't want to leave any of their secrets lying around for us to investigate. I've an idea that all the other wrecks will go like this one did." He scowled. "You know, we really didn't learn much. Guess we'd better call the headquarters ship and ask for further instructions. Will you attend to it, Lieutenant Greer?" III Swiftly Arcot's sleek cruiser sped toward New York and the Arcot Laboratories. They had halted briefly at the headquarters ship of the Earth-Venus forces to report on their experience; and alone again, the three scientists were on their way home. With their course set, Arcot spoke to the others. "Well, fellows, what are your opinions on--what we've seen? Wade, you're a chemist--tell us what you think of the explosion of the ship, and of the strange color of our molecular ray in their air." Wade shook his head doubtfully. "I've been trying to figure it out, and I can't quite believe my results. Still, I can't see any other explanation. That reddish glow looked like hydrogen ions in the air. The atmosphere was certainly combustible when it met ours, which makes it impossible for me to believe that their air contained any noticeable amount of oxygen, for anything above twenty per cent oxygen and the rest hydrogen would be violently explosive. Apparently the gas had to mix liberally with our air to reach that proportion. That it didn't explode when ionized, showed the absence of hydro-oxygen mixture. "All the observed facts except one seem to point to an atmosphere composed largely of hydrogen. That one--there are beings living in it! I can understand how the Venerians might adapt to a different climate, but I can't see how anything approaching human life can live in an atmosphere like that." Arcot nodded. "I have come to similar conclusions. But I don't see too much objection to the thought of beings living in an atmosphere of hydrogen. It's all a question of organic chemistry. Remember that our bodies are just chemical furnaces. We take in fuel and oxidize it, using the heat as our source of power. The invaders live in an atmosphere of hydrogen. They eat oxidizing fuels, and breathe a reducing atmosphere; they have the two fuel components together again, but in a way different from our method. Evidently, it's just as effective. I'm sure that's the secret of the whole thing." "Sounds fairly logical." Wade agreed. "But now I have a question for you. Where under the sun did these beings come from?"