This mode of experimenting extended to helium, however, has yielded the most interesting result[67] that every sixth one on the average of all the passages, or “shots,” which detached any electrons at all from the helium atom detached both of the two electrons which the neutral helium atom possesses. Since some of the ionization produced along the path of an -ray is probably due to slow-speed secondary -rays produced by the -ray, it is probable that the fraction of the actual passages through helium atoms of -rays themselves which detach both electrons is greater than the foregoing one in six. It has been estimated by Fowler at as high as three in four. The foregoing experimental result of one in six was obtained only at the very end of the range of the -rays where they have their maximum ionizing power. When these rays were near the beginning of their range, and therefore were moving much more rapidly, the fraction of the number of double catches to total catches was only about half as much, i.e., the chance of getting both electrons at a single [Pg 143] shot is much smaller with a high-speed bullet than with a slow-speed one. This is to be expected if the two electrons are independent of each other, i.e., if the removal of one does not carry the other out with it. The foregoing is, I think, the only experiment which has yet been devised in which the act of ionization is isolated and studied as an individual thing. Since 1913, however, very definite evidence has come in from two different sources that multiply-valent. ions are often produced in discharge tubes. The most unambiguous proof of this result has been furnished by the spectroscope. Indeed, Mr. Bowen and the author have recently found with great definiteness that high-voltage vacuum sparks give rise to spectral lines which are due to singly-, doubly-, trebly-, quadruply-, and quintuply- charged atoms of the elements from lithium to nitrogen, and even to sextuply-charged ones in the case of sulphur.[68] In view of the foregoing studies with X-rays, -rays, and -rays, it is probable that these spectroscopically discovered multiply-charged ions are produced by successive ionizations such as might be expected to take place in a region carrying a very dense electron current, such as must exist in our “hot-sparks.”