THE CONSTITUTION OF MATTER 117 



It will be seen that the number of throws varies from minute to 

 minute. This is to be expected since the chance of an alpha particle 

 entering the opening is governed by the ordinary laws of probability. 

 It will be seen that two throws, marked by asterisks, are much larger 

 than the others. These were due to the passage of two alpha particles 

 through the opening within a short interval. This was readily seen 

 from the motion of the spot of light reflected from the electrometer 

 needle. As the needle was moving slowly near the end of its swing 

 caused by one alpha particle, a second impulse due to the entrance of 

 another was communicated to it. 



By this method, the number of alpha particles expelled from one 

 gram of radium per second was determined. Of course only a minute 

 fraction of the alpha particles was actually counted, but the total num- 

 ber was deduced on the assumption, verified by experiment, that the 

 alpha particles on an average were expelled equally in all directions. 

 In this way, one gram of radium in equilibrium was found to expel the 

 enormous number of 1.36 X 10 11 alpha particles each second. 



Another interesting result followed from these experiments. It 

 has long been known that the alpha particles produce a marked phos- 

 phorescence in crystalline zinc sulphide. When examined by a lens, 

 the light is found not to be uniform but exhibits a very beautiful 

 scintillating effect. By counting the number of scintillations due to 

 the alpha particles, it was found that each scintillation was produced 

 by the impact of a single alpha particle. It is thus seen that two 

 distinct methods, one electrical and the other optical, are available for 

 detecting and counting single alpha particles, i. e., single atoms of 

 matter. This is only possible because the atoms are in swift motion 

 and expend their great energy of motion in ionizing the gas or in 

 producing luminosity in zinc sulphide. 



Still another simple method was devised later. Kinoshita first 

 showed that a single alpha particle produced a detectable effect on a 

 photographic plate which was observable under a microscope. A num- 

 ber of experiments have been made by Eeinganum, Makower, and 

 Kinoshita to examine the effect of single alpha particles on a photo- 

 graphic plate. If a fine needle point coated with a trace of radio- 

 active matter rests on the surface of the film, the plate on develop- 

 ment shows a number of distinct trails radiating from the active point. 

 Each of these trails results from the action of a single alpha particle. 

 A beautiful photograph of this kind (magnification about 300) ob- 

 tained by Kinoshita is shown in Fig. 4. It appears that each alpha 

 particle makes a certain number of the grains, through which it passes, 

 capable of development. 



The use of an ordinary electrometer is not very suitable for counting 

 alpha particles by the electric method, since the time of swing of the 

 electrometer needle is fairly long, and accurate counting can be made 



