Some Contemporary Advances in Physics - VII Waves and Quanta

HE invaluable agent of our best knowledge of the environing world, and yet itself unknown except by inference; the intermediary between matter and the finest of our senses, and yet itself not material; intangible, and yet able to press, to strike blows, and to recoil; impalpable, and yet the vehicle of the energies that flow to the earth from the sun--light in all times has been a recognized and conspicuous feature of the physical world, a perpetual reminder that the material, the tangible, the palpable substances are not the only real ones. Yet its apparent importance, to our forerunners who knew only the rays to which the eye responds and suspected no others, was as nothing beside its real importance, which was realized very gradually during the nineteenth century, as new families of rays were discovered one after the other with new detecting instruments and with new sources. Radiation is not absent from the places where there is no eye-stimulating light; radiation is omnipresent; there is no region of space enclosed or boundless, vacuous or occupied by matter, which is not pervaded by rays; there is no substance which is not perpetually absorbing rays and giving others out, in a continual interchange of energy, which either is an equilibrium of equal and opposite exchanges, or is striving towards such an equilibrium. Radiation is one of the great general entities of the physical world; if we could still use the word "element," not to mean one of the eighty or ninety kinds of material atoms, but in a deeper sense and somewhat as the ancients used it, we might describe radiation and matter, or possibly radiation and electricity, as coequal elements.