In considering the results of our new and old observations, some points of striking interest emerge. In the first place, all the elements from fluorine to potassium inclusive suffer disintegration under _α_ ray bombardment. As far as our observations have gone, there seems little doubt that the particles ejected from all these elements are H nuclei. The odd elements, B, N, F, Na, Al, P, all give long-range particles varying in range from 40 cms. to 90 cms. in the forward direction, the even elements, C, O, Ne, Mg, Si, S, either give few particles or none at all as in the case of C and O, or give particles of much less range than the adjacent odd numbered elements. The differences between the ranges of even-odd elements become much less marked for elements heavier than phosphorus. This obvious difference in velocity of expulsion of the H nuclei from even and odd elements is a matter of great interest. Such a distinction can be paralleled by other observations of an entirely different character. Harkins has shown that elements of even atomic number are much more abundant in the earth's crust than elements of odd atomic number. In his study of Isotopes, Aston has shown that in general odd numbered elements have only two isotopes differing in mass by two units, while even numbered elements in some cases contain a large number of isotopes. This remarkable distinction between even and odd elements cannot but excite a lively curiosity, but we can at present only speculate on its underlying cause. VELOCITY OF ESCAPE OF HYDROGEN NUCLEI