When their sun began to wane, the Xlarnans at first retreated underground to hoard the heat and life-supporting energies which their nuclear generators could supply. But as their world grew colder, century after century, they devised a means of creating a substitute for the ionosphere—a protective layer of radioactive gases in the upper reaches of the sky which could warm them by means of its slow, controlled reaction, give them eternal light, and yet absorb its own harder radiations. Thus—a pla