On the Development of Cold Bodies and the Cooling of Bodies.

I have already shown it to be probable that in the partial impact of very large bodies a proportion of the gas will be dissipated into space, sometimes in very considerable proportion. page 15 Suppose the gas to have escaped the effective attraction of our system, it has still the attraction of the galaxy, and it is probable that a very large ratio of the particles will have their molecular motion almost completely stopped, before beginning their return journey. We have thus particles of gas almost at rest, and I need not say that all speculations on the dynamical theory of gases suppose that a gas at rest is at absolute zero. It is certain that all radiation from gas that we are acquainted with, is not produced by the molecular velocity. The definite rate of radiant vibration proves it to be due to an internal pendular motion of the parts of the molecule itself. It is, therefore, almost certain that these cold molecules would not be heated by radiation. In fact, without fresh impacts it appears that the radiant vibrations are of very short duration. Thus, suppose a body of small mass were travelling slowly through an atmosphere of such a gas, it might collect it up, and produce a body much cooler than the original body. Again, suppose such slowly-moving molecules were to impinge upon a small body at a moderate temperature, the particle would acquire the temperature of the body, it would thus have a considerable velocity given it. The increased velocity would take the particle away again from the body, and would carry off some of its heat. This particle may then pass away to positions of still higher potential, and again come to rest, or it may travel until it comes into impact with another body. The velocity it acquired from the comparatively cool body will tell in its final impact, and exalt the temperature. I have already shown that hydrogen probably fills space, and it is doubtless this element and the lighter element helium that do this important work of keeping the smaller cosmical dust of the universe cool, and permitting it to act as an absorbing curtain to pick up lost radiant heat. This method of preventing the escape of radiant energy may appear very hypothetical, but my next letter, which will be devoted to the development of potential energy, will, I hope, show it to be much more effectual than would appear on a superficial view of the case.