associated with these things through our experience in a three-dimension world.  We see, or contemplate, bodies which are indivisible; if we divide them, their nature changes; if we divide a molecule of water, we get atoms of hydrogen and oxygen gas; if we divide a molecule of salt, we get atoms of chlorine gas and atoms of the metal sodium, which means that we have reached a point where matter is no longer divisible in a mechanical sense, but only in a chemical sense; which again means that great and small, place and time, inside and outside, dimensions and spatial relations, have lost their ordinary meanings.  Two bodies get inside of each other.  To the physicist, heat and motion are one; light is only a mechanical vibration in the ether; sound is only a vibration in the air, which the ear interprets as sound.  The world is as still as death till the living ear comes to receive the vibrations in the air; motion, or the energy which it implies, is the life of the universe.

Physics proves to us the impossibility of perpetual motion among visible, tangible bodies, at the same time that it reveals to us a world where perpetual motion is the rule—­the world of molecules and atoms.  In the world of gross matter, or of ponderable bodies, perpetual motion is impossible because here it takes energy, or its equivalent, to beget energy.  Friction very soon turns the kinetic energy of motion into the potential energy of heat, which quickly disappears in that great sea of energy, the low uniform temperature of the earth.  But when we reach the interior world of matter, the world of molecules, atoms, and electrons, we have reached a world where perpetual motion is the rule; we have reached the fountain-head of energy, and the motion of one body is not at the expense of the motion of some other body, but is a part of the spontaneous struggling and jostling and vibration that go on forever in all the matter of the universe.  What is called the Brunonian movement (first discovered by the botanist Robert Brown in 1827) is within reach of the eye armed with a high-power microscope.  Look into any liquid that holds in suspension very small particles of solid matter, such as dust particles in the air, or the granules of ordinary water-color paints dissolved in water:  not a single one of the particles is at rest; they are all mysteriously agitated; they jump hither and thither; it is a wild chaotic whirl and dance of minute particles.  Brown at first thought they were alive, but they were only non-living particles dancing to the same tune which probably sets suns and systems whirling in the heavens.  Ramsay says that tobacco smoke confined in the small flat chamber formed in the slide of a microscope, shows this movement, in appearance like the flight of minute butterflies.  The Brunonian movement is now believed to be due to the bombardment of the particles by the molecules of the liquid or gas in which they are suspended.  The smaller the particles, the livelier they are.  These particles themselves are made up of a vast number of molecules, among which the same movement or agitation, much more intense, is supposed to be taking place; the atoms which compose the molecules are dancing and frisking about like gnats in the air, and the electrons inside the atoms are still more rapidly changing places.