ATOMIC WEIGHTS — ASTON. 191 



doubt that this " Primary Dark Space," as it has been called, defines 

 the distance through which an electron, starting- from rest at the sur- 

 face of the cathode, must fall before it acquires sufficient energy to 

 ionize the gas by collision. This explanation is supported in a strik- 

 ing manner by the appearance of the phenomenon when the current 

 density is greatly reduced. Under these conditions alternate dark 

 and bright bands, equally spaced, appear in front of the cathode 

 exactly as would be expected from the production of successive gen- 

 erations of ions. Further support is given by the measurements in 

 hydrogen and helium, which indicate that the actual fall of potential 

 across the primary dark space is twice as great for helium as for 

 hydrogen and, on certain assumptions, is approximately equal to the 

 ionization potential of these gases determined by other means. 



In addition to cathode ray ionization the positive rays traveling 

 towards the cathode themselves are capable of ionizing the gas, and 

 radiation may also play an important part in the same process. The 

 surface of the cathode will therefore be under a continuous hail of 

 positively charged particles. Their masses may be expected to vary 

 from that of the lightest atom to that of the heaviest molecule capa- 

 ble of existence in the discharge tube, and their energies from an 

 indefinitely small value to a maximum expressed by the product of 

 the charge they carry X the total potential applied to the electrodes. 

 The latter is practically the same as the fall of potential across the 

 dark space. If the cathode be pierced, the rays pass through the 

 aperture and form a stream heterogeneous both in mass and velocity 

 which can be subjected to examination and analysis. 24 



A powerful and ingenious method of generating positive rays of 

 metallic elements has been worked out and used with great success 

 by Dempster at Chicago. 25 He employs the element in the metallic 

 state and ionizes its vapor by means of a subsidiary beam of cathode 

 rays. The ions so produced are allowed to fall through a definite 

 potential and being therefore of constant energy can be analyzed by 

 the use of a magnetic field alone. By this arrangement Dempster 

 discovered the three isotopes of magnesium 26 and confirmed those 

 of lithium. A full account of this work has lately appeared. 27 Still 

 more recently he has obtained results with calcium and zinc which 

 indicate that the former consists almost entirely of an isotope 40 

 with probable traces of another 44, and that the latter has four 

 isotopes — 64, 66, 68, and 70. Since the vast majority of the elements 



24 Certain pages of the Franklin Institute article describing the method and apparatus 

 used in Dr. Aston's so-called " mass-speetograph " investigations are here omitted, as 

 they were given in a previous article on Dr. Aston's work in the Smithsonian Report for 

 1920, page 223. 



25 Dempster : Science, Dec. 10, 1920. 

 28 Dempster: Science, April 15, 1921. 

 "Dempster: Phys. Rev., 18, 415, 1921. 



