lapse of time measured by the existence of our universe, is of no more weight against the possibility of change in them, in the infinity of antecedent time, than the constancy of species in Egypt, since the days of Rameses or Cheops, is evidence of their immutability during all past epochs of the earth’s history.  It seems safe to prophesy that the hypothesis of the evolution of the elements from a primitive matter will, in future, play no less a part in the history of science than the atomic hypothesis, which, to begin with, had no greater, if so great, an empirical foundation.

[Sidenote:  The old and the new atomic theory.]

It may perhaps occur to the reader that the boasted progress of physical science does not come to much, if our present conceptions of the fundamental nature of matter are expressible in terms employed, more than two thousand years ago, by the old ’master of those that know.’  Such a criticism, however, would involve forgetfulness of the fact, that the connotation of these terms, in the mind of the modern, is almost infinitely different from that which they possessed in the mind of the ancient, philosopher.  In antiquity, they meant little more than vague speculation; at the present day, they indicate definite physical conceptions, susceptible of mathematical treatment, and giving rise to innumerable deductions, the value of which can be experimentally tested.  The old notions produced little more than floods of dialectics; the new are powerful aids towards the increase of solid knowledge.

[Sidenote:  (2) Conservation of energy.]

Everyday observation shows that, of the bodies which compose the material world, some are in motion and some are, or appear to be, at rest.  Of the bodies in motion, some, like the sun and stars, exhibit a constant movement, regular in amount and direction, for which no external cause appears.  Others, as stones and smoke, seem also to move of themselves when external impediments are taken away.  But these appear to tend to move in opposite directions:  the bodies we call heavy, such as stones, downwards, and the bodies we call light, at least such as smoke and steam, upwards.  And, as we further notice that the earth, below our feet, is made up of heavy matter, while the air, above our heads, is extremely light matter, it is easy to regard this fact as evidence that the lower region is the place to which heavy things tend—­their proper place, in short—­while the upper region is the proper place of light things; and to generalise the facts observed by saying that bodies, which are free to move, tend towards their proper places.  All these seem to be natural motions, dependent on the inherent faculties, or tendencies, of bodies themselves.  But there are other motions which are artificial or violent, as when a stone is thrown from the hand, or is knocked by another stone in motion.  In such cases as these, for example, when a stone is cast from the hand, the distance travelled