CONSTITUTION OF MATTER — EUTHERFORD. 181 



its aid. This is very remarkable when it is remembered that the 

 ionization effect due to a beta particle is on the average not more 

 than one one-hundredth of that due to an alpha particle. 



A photographic record of the entrance of beta j^articles into the 

 detecting vessel is shown in plate 2, figure 3. The upper record is for 

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

 Geiger for this photograph. It is seen that the effect of a beta par- 

 ticle 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 exceed- 

 ingly small compared 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 w^eight of matter with the mini- 

 mum amount of assumption. We have seen that by direct count- 

 ing it has been found that 1.36X10^^ alpha particles are ex- 

 pelled per second from 1 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 

 temperature. Now, the number of alpha particles expelled per 

 year per gram is 4.29X10^*5 giving rise to 156 cubic millimeters of 

 helium, each of these alpha particles is an atom of helium, and con- 

 sequently the number of atoms of helium in 1 cubic centimeter of 

 that gas at normal pressure and temperature is 2.75X10^^. 



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 A^alue 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 

 phenomena has clearly indicated that the atom is not always per- 

 manent and indestructible, it has at the same time supplied the 

 most convincing proof of the actual reality of atoms and has pro- 



