The Universe Wreckers by Edmond Hamilton — Read

"We had checked the great peril that had hung over us, but we found, as the centuries and ages passed, that we had only checked it, that we had not banished it. For nothing in the universe could halt the cooling of Neptune and Triton. As their interior fires cooled, colder and colder grew their surfaces, despite the roofs that enclosed them. It was then that we had recourse to another means of halting that oncoming cold -- the use of artificial heat. We set up in the giant compartment-city of Neptune, and in that of Triton also, great globes that radiated out unceasing and intense heat. These globes held inside them their own mechanisms, mechanisms that could change etheric vibrations of electricity and light and others into heat-vibrations, by changing their wavelength. And with these radiating their ceaseless heat, and with the great enclosing roofs, the oncoming cold was again checked.

"Yet after a time we were forced to recognize that this check also was but temporary. For we were fighting the most grim and hopeless battle in the universe; we were fighting against the relentless and inevitable changes caused by the immutable physical laws of the universe. So that, aid its failing heat as we might with artificial heat-producers, the interior heat of Neptune was waning still, and more and more globe heat-radiators were required to keep the temperature of Neptune at its usual height. The Neptunians of Triton were faced with the same problem, but their situation was not so desperate as of those upon Neptune, since though Triton had cooled as quickly, its enclosed space was so much smaller than Neptune's, its great roof so close to it also, that it was possible with an effort to keep enough heat-mechanisms going there to maintain the warmth.