120 THE POPULAR SCIENCE MONTHLY 



tides and the lower for alpha particles. I am indebted to Dr. Geiger 

 for this photograph. It is seen that the effect of a beta particle is 

 just as marked and as definite as for an alpha particle with the old 

 form of detector. We are thus in a position not only to count single 

 atoms of matter but also to detect the presence of a single electron in 

 swift motion, although the mass of the latter is exceedingly small com- 

 pared with that of the lightest atom. 



I would now very briefly direct your attention to some results, which 

 to my mind not only completely prove the hypothesis of the atomic 

 structure of matter but allow us at once to calculate the number of 

 atoms in a given weight of matter with the minimum amount of as- 

 sumption. We have seen that by direct counting it has been found 

 that 1.36 X 10 11 alpha particles are expelled per second from one gram 

 of radium in equilibrium with its rapidly changing products. Now it 

 has been definitely shown, by methods I need not discuss here, that each 

 alpha particle consists of a helium atom carrying two unit positive 

 charges. Since the alpha particle, when it has lost its charge, becomes 

 a neutral helium atom, we should expect to find that helium would be 

 produced by radium at a definite rate. This is found to be the case, 

 and it is not difficult to determine by actual measurement the volume 

 of helium formed by a known quantity of radium in a given time. It 

 has been found that one gram of radium in equilibrium produces each 

 year 156 cubic millimeters of helium at standard pressure and tempera- 

 ture. Now the number of alpha particles expelled per year per gram 

 is 4.29 X 10 18 , giving rise to 156 cubic millimeters of helium ; each of 

 these alpha particles is an atom of helium, and consequently the num- 

 ber of atoms of helium in one cubic centimeter of that gas at normal 

 pressure and temperature is 2.75 X 10 19 . 



It appears to me that no more direct and convincing proof could 

 be obtained of the atomic structure of matter or of the number of 

 atoms forming a given weight or volume of helium ; for the number of 

 separate constituents are counted and the volume of the resulting gas is 

 measured. The value so obtained is in good accord with measurements 

 based on entirely different data of various kinds. 



It is somewhat remarkable that while the study of radioactive phe- 

 nomena has clearly indicated that the atom is not always permanent 

 and indestructible, it has at the same time supplied the most convinc- 

 ing proof of the actual reality of atoms, and has provided 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 particles 

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



