592 
PHYSICS: MILLIKAN, GOTTSCHALK AND KELLY 
passing a sharply limited beam of X-rays, beta rays or 7 rays imme- 
diately underneath the drop, catching upon the drop the positive ion, 
formed by the ionization of a neutral molecule by the rays under inves- 
tigation, and finally measuring the charge communicated to the drop 
by the advent of the ion upon it through observing the speed imparted 
to the drop by its new increment in charge. 
Just before the war, Millikan attacked the more difficult and the more 
interesting problem of catching by the same general method, the ions 
formed by the passage of an alpha particle through an atom, expecting 
in this case to find that this relatively huge and powerful ionizing agent 
would often detach more than one negative electron from a single atom. 
When he was called to other duties by the war, the experimental work 
already begun was continued and completed by Gottschalk and Kelly. 
The results are as follows: 
1. Alpha rays have been shot through atoms of the most diverse sorts 
(H, C, O, N, CI, I, Hg) and of atomic weights from 1 to 200, without 
bringing to light in any case evidence of the formation of multiply- valent 
ions. 
2. Twenty-nine hundred ions formed by the passage of a rays through 
neutral molecules have been caught on oil drops at the instant of 
ionization and the charges carried by each of these ions individually 
measured. Of these 2900 captures, 5 might possibly have corresponded 
to double charges, though even these were in all probability due to the 
nearly simultaneous advent upon the drop of two singly charged ions. 
3. In no single case has an a particle been observed to form an ion 
carrying three or more charges, even though mercury, from which octi- 
valent ions had been expected, was one of the substances tested. 
4. Alpha ray ionization consists ^ then at least 99 times out of a 100, in 
the case of all the gases and vapors studied, in the detachment of a single 
negative electron from a neutral molecule, 
* A detailed report of these experiments will shortly be published in the Physical Review, 
2 Millikan, R. A., and Fletcher, H., London Phil. Mag., (Ser. 6) 21, 1911, (753). 
