Page:Life in Motion.djvu/24

movements of matter, such as the wheeling of the planets in their elliptical orbits round the sun, the spinning of the earth on its axis, or the still grander movements of the firmament, as revealed by the proper motions of the stars. Such movements impress the imagination with a sense of vastness and of irresistible power. Then there are the movements on the surface of the world itself—the tides, the flow of rivers, the hurricane, the clouds travelling athwart the sky, and many others. These are movements of great masses of matter; and when they are studied by the natural philosopher, he finds that they are regulated by well-known dynamical laws. All such movements are evident to the senses; but there are other movements that are not so, and which can be detected only by special methods of research. Such are those that occur, as it were, below the surfaces of things. These are called molecular, because the bodies that move are minute particles or molecules of matter, far too small to be seen with the eye or even with the aid of the most powerful microscope. Still, the natural philosopher tells us that the movements of these little particles are controlled by dynamic laws