42 Journal of the Mitchell Society [September 



unit of electricity. In view of the comparative masses of it and the 

 electron the atomic weight of an element is considered numerically 

 equal to the number of protons in its nucleus. The atomic weight 

 of helium is 4. Since its nucleus has been shown by Rutherford to 

 be the alpha (a) particle it contains 4 protons. While it has been 

 proven by Aston that the mass of the helium atom is not exactly 

 four times that of the In'drogen atom, this loss of mass, due to the 

 packing effect, is in harmony with the electromagnetic theory of mass. 

 That the electromagnetic theory is true, and that the mass of elec- 

 tron is wholly electrical, seem established by the experimental find- 

 ings of various workers with the beta (/?) particles of radium, whose 

 velocities range from 0.3 the velocity of light to approximate equality. 

 While it has been impossible to apply the same test, variation of 

 mass with velocity, to the sloAver moving positive charges, it seems 

 illogical to assume for them another explanation of mass. 



The atomic member, however, is determined by the excess of pro- 

 tons over electrons in the nucleus. Hence we see that atomic struc- 

 tures may have equal atomic numbers and unequal atomic masses. 

 Such bodies were discovered by Rutherford and Soddy in their 

 study of the radioactive substances and were named by Soddy isotopes. 



Since the chemical and practically all the physical properties of 

 the atom are determined by its atomic number, isotopes cannot be 

 separated by chemical methods. The most effective means of sep- 

 arating them has been the positive ray method, first devised by Thom- 

 son and afterwards modified and improved by Aston. In view of 

 the recent lectures here by the last named it is needless for me to 

 speak of his method further than to say the determinations from his 

 mass-spectra have a very higli degree of accuracy. However, I hope 

 I may })e pardoned for a few remarks about these recently discov- 

 ered structures dosi)ite the leetiires of the distinguished visitor and 

 notwithstanding the illuminating paper recently presented before the 

 Elisha Mitchell Society by a distinguished authority in chemistry. 



INIany, at least, of the fractional atomic weights can be explained 

 by the presence of isotopes. Neon, with atomic weight 20.2. has 

 two isotopes of mass 20 and 22. The atomic weight can be explained 

 by the assumption of its having these isotopes in the ratio of 9 to 1 

 respectively. IMagnesium with its two isotopes, 24 and 25. and the 

 atomic weight 24.32 can be explained by assuming a ratio of 17 to 8 

 for the isotopes. 



