188 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



In further confirmation Maurice Curie 15 in Paris reported 206.36 

 for a lead from carnotite, and a still lower figure, 206.046, was ob- 

 tained by Honigschmid in A^ienna for a lead from the very pure cry- 

 stallized pitchblende from Morogoro. 16 This is the lowest atomic 

 weight found so far. The highest, 207.9, was also determined by 

 Honigschmid for lead from Norwegian thorite. 



In the absence of the special radioactive evidence which can be 

 used in special cases such as that of lead, the presence of isotopes 

 among the inactive elements can only be detected by the direct 

 measurement of the masses of individual atoms. This can be done 

 by the analysis of positive rays. 



The condition for the development of these rays is briefly ioniza- 

 tion at low pressure in a strong electric field. Ionization, which may 

 be due to collisions or radiation, means in its simplest case the de- 

 tachment of one electron from a neutral atom. The two resulting 

 fragments carry charges of electricity of equal quantity but of op- 

 posite sign. The negatively charged one is the electron, the atomic 

 unit of negative electricity itself, and is the same whatever the atom 

 ionized. It is extremely light and therefore in the strong electric 

 field rapidly attains a high velocity and becomes a cathode ray. The 

 remaining fragment is clearly dependent on the nature of the atom 

 ionized. It is immensely more massive than the electron, for the 

 mass of the lightest atom, that of hydrogen, is about 1850 times that 

 of the electron, and so will attain a much lower velocity under the 

 action of the electric field. However, if the field is strong and the 

 pressure so low that it does not collide with other atoms too fre- 

 quently, it will ultimately attain a high speed in a direction opposite 

 to that of the detached electron, and become a " positive ray." The 

 simplest form of positive ray is therefore an atom of matter carry- 

 ing a positive charge and endowed, as a result of falling through a 

 high potential, with sufficient energy to make its presence detectable. 

 Positive rays can be formed from molecules as well as atoms, so that 

 it will at once be seen that any measurement of their mass will give 

 us direct information as to the masses of atoms of elements and mole- 

 cules of compounds, and that this information will refer to the atoms 

 or molecules individually, not, as in chemistry, to the mean of an 

 immense aggregate. It is on this account that the accurate analysis 

 of positive rays is of such importance. 



In order to investigate and analyze them it is necessary to obtain 

 intense beams of the rays. This can be done in several ways. The 

 one most generally available is by the use of the discharge in gases at 

 low pressure. 



16 M. Curie : Compt. Rend., 158, 1676, 1914. 



18 Honigschmid : Zeit. Elektrochem., 24, 163, 1918 ; 25, 91, 1919. 



