through a gas. C. T. R. Wilson showed many years ago that the positively and negatively charged ions produced in a gas by the passage of alpha and beta and X rays possessed a remarkable property. When air, for example, saturated with water vapor is suddenly expanded, the air is rapidly cooled and the water tends to deposit on any nuclei present. C. T. R. Wilson showed that in dust-free air, the ions produced by external radiations become nuclei for the condensation of water upon them when the cooling by expansion was sufficiently great. Under such conditions, each ion becomes the center of a visible globule of water, and the number of drops formed is equal to the number of ions present.
C. T. R. Wilson later perfected this method to show the trail of a single alpha or beta particle in passing through the gas; for each of the
ions produced by the flying particle becomes a visible drop of water by the sudden expansion. By suitable arrangements, the trails of the individual particle can be photographed, and the pictures obtained show with remarkable fidelity and detail the ionizing effects produced in the passage of alpha and beta particles or X rays through gases.
Fig. 8 shows the tracks of the alpha particles shot out from a small fragment of radium. The number of ions produced per centimeter in the gas by the alpha particle is so great that the trail of drops shows as a continuous line. The alpha particles are seen to radiate in straight lines from the active point, and have a definite range in air—a characteristic property discovered by Bragg many years ago. The next photograph (Fig. 9) shows a magnified image of these trails. It is seen that the tracks are generally quite straight, but in a few cases there is a sudden bend near the end. The significance and causes of these sudden deviations in the rectilinear paths of the alpha particles will be discussed later.