The conclusion was afterwards experimentally verified by Sir William Thomson, and served to explain all the phenomena of regelation. Thus, like the principle of the conservation of energy, the principle of the dissipation of energy serves as a guide in the search after truth. But there is this difference between the two principles--no one can conceive of any method by which to circumvent the conservation of energy; but Clerk Maxwell showed that the principle of dissipation of energy might be overridden by the exercise of intelligence on the part of any creature whose faculties were sufficiently delicate to deal with individual molecules. In the case of gases, the temperature depends on the average energy of motion of the individual particles, and heat consists simply of this motion; but in any mass of gas, whatever the average energy may be, some of the particles will be moving with very great, and some with very small, velocities. By imagining two portions of gas, originally at the same temperature, separated by a partition containing trap-doors which could be opened or closed without expenditure of energy, and supposing a "demon" placed in charge of each door, who would open the door whenever a particle was approaching very rapidly from one side, or very slowly from the other, but keep it shut under other circumstances, he showed that it would be possible to sort the particles, so that those in the one compartment should have a great velocity, and those in the other a small one. Hence, out of a mass of gas at uniform temperature, two portions might be obtained, one at a high temperature and the other at a low, and, by means of a heat-engine, work could be obtained until the two portions were again at equal temperatures, when the services of the "demons" might be again taken advantage of, and the operations repeated until all the heat was used up.