When two substances combine chemically, more or less heat is evolved.  When the combination is effected slowly, as in an animal’s body, heat is slowly evolved.  When the combustion is rapid, as in actual fire, heat is rapidly evolved.  The same phenomenon may reach the eye as light, and the hand as heat, though different senses get two different impressions of the same thing.  So a mechanical disturbance may reach the ear as sound, and be so interpreted, and reach the hand as motion in matter.  In combustion, the oxygen combines rapidly with the carbon, giving out heat and light and carbon dioxide, but why it does so admits of no explanation.  Herein again is where life differs from fire; we can describe combustion in terms of chemistry, but after we have described life in the same terms something—­and this something is the main thing—­remains untouched.

The facts of radio-activity alone demonstrate the truth of the atomic theory.  The beta rays, or emanations from radium, penetrating one foot of solid iron are very convincing.  And this may go on for hundreds of years without any appreciable diminution of size or weight of the radio-active substance.  “A gram of such substance,” says Sir Oliver Lodge, “might lose a few thousand of atoms a second, and yet we could not detect the loss if we continued to weigh it for a century.”  The volatile essences of organic bodies which we detect in odors and flavors, are not potent like the radium emanations.  We can confine them and control them, but we cannot control the rays of radio-active matter any more than we can confine a spirit.  We can separate the three different kinds of rays—­the alpha, the beta, and the gamma—­by magnetic devices, but we cannot cork them up and isolate them, as we can musk and the attar of roses.

And these emanations are taking place more or less continuously all about us and we know it not.  In fact, we are at all times subjected to a molecular bombardment of which we never dream; minute projectiles, indivisible points of matter, are shot out at us in the form of electrons from glowing metals, from lighted candles, and from other noiseless and unsuspected batteries at a speed of tens of thousands of miles a second, and we are none the wiser for it.  Indeed, if we could see or feel or be made aware of it, in what a different world we should find ourselves!  How many million-or billion-fold our sense of sight and touch would have to be increased to bring this about!  We live in a world of collisions, disruptions, and hurtling missiles of which our senses give us not the slightest evidence, and it is well that they do not.  There is a tremendous activity in the air we breathe, in the water we drink, in the food we eat, and in the soil we walk upon, which, if magnified till our senses could take it in, would probably drive us mad.  It is in this interior world of molecular activity, this world of electric vibrations and oscillations, that the many transformations of energy take place.