and therefore inconceivable; but they have been shown to be diverse, but interchangeable modes of molecular motion, omnipresent, ceaselessly active.  The wondrous phenomena of light, heat, and electricity are seen to be due to the rhythmical vibration of atoms.  There is thus no such thing as rest:  from the planet to the ultimate particle, all things are endlessly moving:  and the mystic song of the Earth-Spirit in “Faust” is recognized as the expression of the sublimest truth of science:—­

    “In Lebensfluthen, im Thatensturm,
  Wall’ ich auf und ab, webe hin und her,
      Geburt und Grab,
      Ein ewiges Meer,
      Ein wechselnd Weben,
      Ein gluehend Leben,
  So schaff’ ich am sausenden Webstuhl der Zeit,
  Und wirke der Gottheit lebendiges Kleid.”

In a discussion containing so much that is noble, however, we are sorry to observe that Dr. Youmans is betrayed into using the current expressions concerning an “ether” which is supposed to be the universal vehicle for the transmission of molecular vibrations.  We are told, that, while “the vibrations of a sonorous body produce undulations in the air,” on the other hand, “the vibrations of atoms in a flame produce undulations in the ether.”  We would by no means charge Dr. Youmans with all the consequences naturally deducible from such a statement.  We believe that he uses the term “ether” simply to render himself more intelligible to those who have been wont to make use of it to facilitate their thinking.  Such an object is highly praiseworthy, and is too often left out of sight by those who write elementary works.  But the good service thus rendered is far more than counterbalanced by the host of erroneous conceptions which at once arise at the introduction of this luckless term.  This notion of an “imaginary ether” should be at once and forever discarded by every writer on physics.  The very word should be remorselessly expunged from every discussion of the subject.  It is one of the most baneful words in the whole dictionary of scientific terminology.  It stands for a fiction as useless as it is without foundation.  It is useless because superfluous, and not needed in order to account for the phenomena.  An ether is no more necessary in the case of light than it is in the case of sound.  Thermal vibrations are the oscillations of atoms, not the undulations of an ether.  If it be urged that rays of light and heat will traverse a vacuum, we reply, that the much-derided aphorism, “Nature abhors a vacuum,” is as true at this day as it was before Torricelli’s experiment.  A perfect vacuum has never been produced; and if it were to be produced, the ether must be excluded, else it would be no vacuum, after all.  For, if there were such a thing as an ether, it must of course be some form of matter; nobody ever claimed for it the character of motion or force.  If it be considered as matter, then, we are confronted with new difficulties; for all matter must exert gravitation.  Weight