TEE CONSTITUTION OF MATTER 



121 



through a gas. C. T. E. 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 pro- 

 duced 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 



Fig. 8. Tracks of Alpha Particles from Central Points (C. T. R. Wilson's 



Method). 



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 par- 

 ticles will be discussed later. 



VOL. LXXXVTI. — 9. 



