Page:Popular Science Monthly Volume 45.djvu/496

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SHALL ask your attention this evening to the scientific principles which are involved in the condensation of atmospheric vapor, and to some of the attempts which have been made to produce this condensation by artificial means.

Since the change from atmospheric vapor to water involves a change of the physical state of the same substance from a gas to a liquid, it is important that we understand clearly the difference between these two physical states.

Both liquids and gases are undoubtedly made of very small particles called molecules. In a gas these molecules are not held together by any force, but each molecule is a perfectly independent body, free to move in any direction without reference to any other molecule, except as its motion may be interfered with by colliding with another. Under all known conditions these gaseous molecules are actually in rapid motion, each one moving at its own rate and in its own path, unaffected by any known force except gravitation. Each molecule will, accordingly, move in a straight line until it collides with another molecule. When two molecules collide, their direction of motion will be changed according to the angle of collision, but on account of their high elasticity they rebound with the same force with which they collide, and the sum of their motions will be practically the same as before. Hence, no number of collisions between the molecules themselves will ever bring them to rest.

If confined within solid walls, they strike against these walls and rebound from them just as they do from each other. In doing so each molecule exerts a pressure upon the wall during its time of contact, and the sum of these pressures is the whole pressure of the gas upon the walls of its containing vessel.

These walls are likewise composed of similar molecules, but held together by some unknown force, and it is the surface layer of these molecules which must bear the shock of the molecular bombardment of the gas. Accordingly, the molecules of the solid walls, while not free to be driven about from one place to another, like the gaseous molecules, are nevertheless set in vibration; and since they can not lie as close together while in vibration as they could at rest, the solid mass of the walls is made to expand. By measuring the amount of this expansion we can