creep in, age after age, toward the sun." Not one can escape its fiery end. And, finally, the heat of the sun itself--that is, its molecular energy--must be transformed into radiant energy, and diffused and lost as a working force in infinite space. "Thus do the inexorable laws of mechanics indicate that the sun's store of heat, which can only suffer loss and not gain, must _be finally exhausted_."[163] There are thus special geological and astronomical facts which have long been regarded as indicative of the principle that the existing order of the material universe is _temporal_--it had a beginning, and must have an end. But the modern Theory of Energy,[164] with its three great laws of Conservation, Transformation, and Dissipation, must be regarded as a comprehensive, complete, and final settlement of the question. It has been shown, first, that no system of machinery can _create_ force any more than it can create matter; and that the amount of energy in the universe, or in any limited system which does not receive energy from without, or part with it to external matter, is a constant or invariable quantity. This is the _Law of the Conservation of Energy_. It has been proved, secondly, as an experimental fact that, in general, one form of energy may, by suitable processes, be transformed wholly or in part to an equivalent amount of another form; and the sole and only function of all possible machines is the conversion or _transformation_ of energy. This is the _Law of the Transformation of Energy_. This law of Transformation is, however, subject to the limitations which are imposed by the _Law of the Dissipation of Energy_, the discovery of which is mainly due to Sir William Thomson. He has shown that every machine does its work against friction. "A material system can never be brought through any returning cycle of motions without spending more work against the mutual forces of its parts than it gained from these parts, because no relative motions can take place without meeting with frictional or other forms of resistance." No known process of transformation is exactly reversible. Whenever an attempt is made to transform and retransform energy by an imperfect process, part of the energy is converted into heat, and the heat is _dissipated_, so as to become useless because incapable of further transformation. It therefore follows that, as energy is constantly in a state of transformation, there is a constant degradation of energy to that final unavailable form of uniformly diffused heat; and this will go on as long as transformations occur, until the whole energy of the universe has taken this form.[165] The reader will find an extended discussion of this great question in Thomson and Tait's "Natural Philosophy," vol. i. pp. 188-304, in which it is shown that the present material system is not a _dynamically conservative_ but a _dissipative_ system, and therefore that in such a system "perpetual motion" is an impossibility.