Only two of our senses make us acquainted with matter in a state which may be said to approach the atomic—­smell and taste.  Odors are material emanations, and represent a division of matter into inconceivably small particles.  What are the perfumes we smell but emanations, flying atoms or electrons, radiating in all directions, and continuing for a shorter or longer time without any appreciable diminution in bulk or weight of the substances that give them off?  How many millions or trillions of times does the rose divide its heart in the perfume it sheds so freely upon the air?  The odor of the musk of certain animals lingers under certain conditions for years.  The imagination is baffled in trying to conceive of the number and minuteness of the particles which the fox leaves of itself in the snow where its foot was imprinted—­so palpable that the scent of a hound can seize upon them hours after the fox has passed!  The all but infinite divisibility of matter is proved by every odor that the breeze brings us from field and wood, and by the delicate flavors that the tongue detects in the food we eat and drink.  But these emanations and solutions that affect our senses probably do not represent a chemical division of matter; when we smell an apple or a flower, we probably get a real fragment of the apple, or of the flower, and not one or more of its chemical constituents represented by atoms or electrons.  A chemical analysis of odors, if it were possible, would probably show the elements in the same state of combination as the substances from which the odors emanated.

The physicists herd these ultimate particles of matter about; they have a regular circus with them; they make them go through films and screens; they guide them through openings; they count them as their tiny flash is seen on a sensitized plate; they weigh them; they reckon their velocity.  The alpha-rays from radio-active substances are swarms of tiny meteors flying at the incredible speed of twelve thousand miles a second, while the meteors of the midnight sky fly at the speed of only forty miles a second.  Those alpha particles are helium atoms.  They are much larger than beta particles, and have less penetrative power.  Sir J. J. Thomson has devised a method by which he has been able to photograph the atoms.  The photographic plate upon which their flight is recorded suggests a shower of shooting stars.  Oxygen is found to be made up of atoms of several different forms.

III

The “free path” of molecules, both in liquids and in gases, is so minute as to be beyond the reach of the most powerful microscope.  This free path in liquids is a zigzag course, owing to the perpetual collisions with other molecules.  The molecular behavior of liquids differs from that of gases only in what is called surface tension.  Liquids have a skin, a peculiar stress of the surface molecules; gases do not, but tend to dissipate and fill all space.  A drop of water remains intact till vaporization sets in; then it too becomes more and more diffused.