CONSTITUTION OF MATTER RUTHERFORD. 179 



alpha particles through the opening within a short interval. This 

 w as 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 Avas actually counted, but 

 the total number was deduced on the assumption, verified by experi- 

 ment, that the alpha particles on an average were expelled equally 

 in all directions. In this way, 1 gram of radium in equilibrium was 

 found to expel the enormous number of 1.36X10^^ alpha particles 

 each second. 



Another interesting result followed from these experiments. It 

 has long been known that the alpha particles produce a marked 

 phosphorescence 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 scintilla- 

 tions 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 

 number of experiments have been made by Eeinganum, Makower, 

 and Kinoshita to examine the effect of single alpha particles on a 

 photographic plate. If a fine needle point coated with a trace of 

 radioactive matter rests on the surface of the film, the plate on 

 development 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) obtained by Kinoshita is show^n in plate 2, figure 1. 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 count- 

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

 of the electrometer needle is fairly long, and accurate counting can 

 be made when only a few alpha particles enter the detecting vessel 

 per minute. This difficulty can be got over by the use of a string 

 electrometer in which the moving system consists of a fine silvered 

 quartz fiber suspended between two charged parallel i)lates and 



