The extreme mobility of steel is seen when the red-hot metal is plunged into water. Instantly every particle takes a new position, making it a hundredfold more hard than before it was heated. But these particles of transferred steel are still mobile. A man’s razor does not cut smoothly. It is dull, or has a ragged edge that is more inclined to draw tears than cut hairs. He draws the razor over the tender palm of his hand a few times, rearranges the particles of the edge and builds them out into a sharper form. Then the razor returns to the lip with the dainty touch of a kiss instead of a saw. Or the tearful man dips the razor in hot water and the particles run out to make a wider blade and, of course, a thinner, sharper edge. Drop the tire of a wagon wheel into a circular fire. As the heat increases each particle says to its neighbor, “Please stand a little further off; this more than July heat is uncomfortable.” So the close friends stand a little further apart, lengthening the tire an inch or two. Then, being taken out of the fire and put on the wheel and cooled, the particles snuggle up together again, holding the wheel with a grip of cold iron. Mobile and loose, with plenty of room to play, as the particles have, neither wire nor tire loses its tensile strength. They hold together, whether arms are locked around each other’s waist, or hand clasps hand in farther reach. What change has come to iron when it has been made red or white hot? Its particles have simply been mobilized. It differs from cold iron as an army in barracks and forts differs from an army mobilized. Nothing has been added but movement. There is no caloric substance. Heat is a mode of motion. The particles of iron have been made to vibrate among themselves. When the rapidity of movement reaches four hundred and sixty millions of millions of vibrations per second it so affects the eye that we say it is red-hot. When other systems of vibration have been added for yellow, etc., up to seven hundred and thirty millions of millions for the violet, and all continue in full play, the eye perceives what we call white heat. It is a simple illustration of the readiness of seeming solids to vibrate with almost infinite swiftness.