182 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



vided some of the most direct methods of determining the values 

 of atomic magnitudes. 



TRACKS OF SWIFT ATOMS AND ELECTRONS. 



We have seen how it is possible to detect single alpha and beta 

 particles and to count their number. We will next consider a most 

 remarkable experimental method, not only for detecting such par- 

 ticles but of following in detail the effects produced by them in their 

 passage 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 remark- 

 able 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 radiation 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 pic- 

 tures obtained show with remarkable fidelity and detail the ionizing 

 effects produced in the passage of alpha and beta particles or X 

 rays through gases. 



Plate 3, figure 1, 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 (pi. 3, fig. 2) shows a mag- 

 nified 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 recti- 

 linear paths of the alpha particles will be discussed later. 



A radioactive substance like radium emits not only alpha particles 

 but beta particles which are electrons in very swift motion. These 

 beta particles are generally far more penetrating than the alpha 

 rays, but produce a much smaller number of ions per centimeter 

 of their path through a gas. In plate 4, figure 1, is seen the track 

 of a swift beta particle crossing the expansion chamber. It will be 

 observed that the path is not straight but tortuous, due to the marked 



