It is far from impossible, again, that the planetary system may be invaded by bodies of greater mass than any comets. The sun seems to be placed in so extensive a portion of empty space, that its own proper motion would not bring it to the nearest known star (a Centauri) in less than 139,200 years. But in order to be sure that this long interval of undisturbed life is granted to our globe, we must prove that there are no stars moving so as to meet us, and no dark bodies of considerable size flying through intervening space unknown to us. The intrusion of comets into our system, and the fact that many of them have hyperbolic paths, is sufficient to show that the surrounding parts of space are occupied by multitudes of dark bodies of some size. It is quite probable that small suns might have cooled sufficiently to become nonluminous; for even if we discredit the theory that the variation of brightness of periodic stars is due to the revolution of dark companion stars, yet there is our own globe as an unquestionable example of a smaller body which has cooled below the luminous point.

Altogether, then, it is a mere assumption that the Uniformity of Nature involves the unaltered existence of our own globe. There is no kind of catastrophe which is too great or too sudden to be theoretically consistent with the reign of law. For all that our science can tell, human history may be closed in the next instant of time. The world may be dashed to pieces against some intruding body; it may be involved in a nebulous atmosphere of hydrogen to be exploded a second afterwards; it may be scorched up or dissipated into vapour by some great explosion in the sun; there might even be within the globe itself some secret cause of disruption, which only needs time for its manifestation.

There are even some indications, as already noticed (vol. ii. p. 327), that some violent disturbances have

actually occurred in the history of the solar system. Olbers sought for the minor planets or asteroids, on the supposition that they were fragments of an exploded or fractured planet, and he was rewarded with the discovery of some of them. The retrograde motion of the satellites of the more distant planets, the abnormal position of the poles of Uranus and the excessive distance of Neptune, are other indications of some violent event, of which we have no other evidence. I adduce all these facts and arguments, not to show that there is any appreciable probability, so far as we can judge, of actual interruption within the scope of human history, but to prove that the Uniformity of Nature is theoretically consistent with the most unexpected events of which we can form any conception.

Possible States of the Universe.

When we give the rein to scientific imagination, it becomes apparent that conflict of body with body must not be regarded as the rare exception, but as the general rule and the inevitable fate of each star system. So far as we can trace out the results of the law of gravitation, and the dissipation of energy, the universe must be regarded as undergoing gradual condensation into a single cold solid body of gigantic dimensions. Those who so frequently use the expression Uniformity of Nature, seem to forget that the universe might exist consistently with the laws of nature in the most diverse conditions. It might consist, on the one hand, of a glowing nebulous mass of gaseous substances. The heat might be so intense that all elements, even carbon and silicon, would resemble permanent gases, and all atoms, of whatever nature, would be flying about in chemical independence, diffusing themselves almost uniformly in the neighbouring parts of space. There would then be no life, unless we can

apply that name to the passage through each part of space of similar average trains of atoms, the particular successions of atoms being governed only by the theory of probability, and the law of divergence from a mean exhibited in the Arithmetical Triangle. Such a universe would correspond partially to the Lucretian rain of atoms, and to that nebular hypothesis out of which Laplace proposed philosophically to explain the evolution of the planetary system.

According to another extreme supposition, the intense heat energy of this nebulous mass might have been mostly radiated away into the unknown regions of outer space. The attraction of gravity would then have shown itself between each two particles, and the energy of motion thence arising would, by incessant conflicts, be resolved into heat and dissipated.

Inconceivable ages might be required for the completion of this process, but the dissipation of energy thus proceeding could end only in the production of a cold and motionless stone-like universe. The relation of cause and effect, as we see it manifested in life and growth, would then degenerate into the constant existence of every particle in a fixed position relative to every other particle. Logical and geometrical resemblances would still exist between atoms, and between groups of atoms crystallized in their appropriate forms for ever more. But time, the great variable, would bring no variation, and as to human hopes and troubles, they would have come to eternal rest.

Science is not really adequate to proving that such is the inevitable fate of the universe, for we can seldom trust our best established theories and most careful inferences far from their data. Nevertheless, the most probable speculations which we can form as to the history, especially of our own planetary system, is that it origi

nated in a heated revolving nebulous mass of gas, and is in a state of almost infinitely slow progress towards the cold and stony condition. Other speculative hypotheses might doubtless be entertained. Every hypothesis is pressed by difficulties. If the whole universe be cooling, where does the heat go to? If we are to get rid of it entirely, outer space must be infinite in extent, so that it shall never be stopped and reflected back. But not to speak of metaphysical difficulties, if the medium of heat undulations be infinite in extent, why should not the material bodies placed in it be infinite also in number and quantity. It is quite apparent that we are venturing into speculations which altogether surpass our powers of scientific inference. But then I am arguing negatively; I wish only to show that those who speak of the uniformity of nature, and the reign of law, often misinterpret entirely the meaning involved in those expressions. Law is not inconsistent with extreme diversity, and, so far as we can read the history of this planetary system, it did most probably originate in heated nebulous matter, and man's history forms but a moment in its progress towards the cold and stony condition. It is by very doubtful and speculative hypotheses alone that we can avoid such a conclusion, and I depart least from undoubted facts and well-established laws, when I assert that, whatever uniformities may underlie the phenomena of nature, constant variety and ever-progressing change is the real outcome.

Speculations on the Reconcentration of Energy.

There are unequivocal indications, as I have said, that the material universe, as we at present see it, is progressing from some act of creation, or some discontinuity of existence of which the date may be approxi

mately fixed by scientific inference. It is progressing towards a state in which the available energy of matter will be dissipated through infinite surrounding space, and all matter will become cold and lifeless. This constitutes, as it were, the historical period of physical science, that over which our scientific insight may more or less extend. But in this, as in other cases, we have no right to interpret our experience negatively, so as to infer that because the present state of things began at a particular time, there was no previous existence. It may be that the present period of material existence is but one of an indefinite series of like periods. All that we can see, and feel, and infer, and reason about may be, as it were, but a part of one single pulsation in the existence of the universe.

After Sir W. Thomson had pointed out the preponderating tendency which now seems to exist towards the conversion of all energy into heat-energy, and its equal diffusion by radiation throughout space, the late Professor Rankine put forth a remarkable speculationk. He suggested that the ethereal, or rather, as I have called it, the adamantine medium in which all the stars exist, and all radiation takes place, may have bounds, beyond which only empty space may exist. All heat undulations reaching this boundary will be totally reflected, according to the theory of undulations, and will in all probability be reconcentrated into foci situated in many parts of the medium. Whenever a cold and extinct star happens to pass through one of these foci, it will be instantly ignited and resolved by intense heat into its constituent elements. A discontinuity will occur in the history of that portion of matter, and the star will begin its history afresh with a renewed store of energy.

This is doubtless a mere speculation, incapable of verik 'Report of the British Association' (1852), Report of Sections,

p. 12.