A little over thirty years ago, at the Royal Institution of Great Britain, James Clerk Maxwell (1831-1879) delivered a lecture on "Action at a Distance." It was no new subject, but rather one of the oldest and most often discussed subjects from the days of the ancient Greeks down to the present. We talk of gravitation as an attraction or pull between the various bodies of the universe, but how can they pull one another without some material bond between? This was Sir Isaac Newton's great puzzle which he never solved, though he expended upon it the greatest efforts of his great intellect. The sun appears to repel the tail of the comet, yet how can there be a push without intervening material with which to push? When we speak of light pouring or streaming in, do we think of it as a substance? When we speak of warm bodies losing heat, or when we cover them to keep the heat in, are we thinking of heat as a substance? What are heat, light, electricity, magnetism, and gravitation? These are no new questions. They are certainly older than history. Various ideas have prevailed at different times. It is much easier to change our ideas than to change our language. You occasionally see and hear the words calorie and caloric used in connection with heat. They stand for an idea, abandoned for three generations, that heat is a substance called caloric, which saturates warm bodies and drains out of them when they cool off. I hardly think these ideas either arise or fall without good and sufficient reason. Each theory has been the natural conclusion from our observations of nature as far as we have gone with them. To be sure, it is difficult for us to see how men acquired, from any observations of nature, the idea of light which seems to have prevailed previous to the time of Aristotle, three and a half centuries B.C. This idea was that objects were made visible by something projected from the eye itself. Still, the questions which I have indicated regarding heat, light, and electricity have impelled men for many centuries to observe nature for hints as to the answers. The doctrine of the universal ether as a medium for transmitting wave motions, and of light, heat, and electricity as being motions of different wave length, is the natural conclusion of the present time. It may give place to another theory when we have further facts to reason upon. Imagine your never having seen a harp or other musical instrument. Would it require a long time, do you think, for you to find out its use, at least to this extent, that it will produce tones whenever the strings are made to vibrate? That the short strings vibrate more rapidly than the long ones, and at the same time produce tones of a higher pitch? Imagine that having become familiar with the harp you should successively come upon scores of other musical instruments of very differing types. You would soon become adept at divining their uses. Now, a study of the microscopic structure of the