Oct. 4, 1921 dorsey: radioactivte quantity 3S3 



the amount of radium-A that can exist in equilibrium with one gram 

 of radium will contain one r of radium-A, the amount of ionium that 

 can exist in equilibrium with one gram of radium will contain one r 

 of ionium, the amount of uranium-II that can exist in equilibrium 

 with one gram of radium will contain (1/0.96) r of uranium-II, etc. 



As thus defined, an r of any material is that amount of the material 

 that will produce transformed atoms at the same rate as transformed atoms 

 are produced by one gram of radium. 



The value in grams or in atoms of an r of any element depends upon 

 the ratio of the transformation constant and of the atomic weight of 

 the element to those of radium ; denoting the transformation constant 

 of radium by Xo, the number of atoms in one gram of radium by No 

 and the atomic weight of radium by Ao the number of atoms (A^) 

 in one r of any other element is determined by the equation 



N= ^- No 



X 



and the weight in grams of one r of the element is 



Xo .1 

 X -4o 



since NqAq is by definition equal to one gram. 



To the precision with which the transformation constants are known, 

 an r of radium emanation weighs 6.55 X 10~^ gram; pf radium-A, 

 3.54 X 10~^ gram; of uranium-II, 1.51 X 10^ gram; of actinium, 1.29 

 X 10~^gram; of mesothorium-1, 4.29 X 10~^gram. 



The quantity that we have denoted by the letter r plays in radio- 

 activity a part that is analogous to that played by the gram-molecule 

 in physical chemistry; the adoption of some name for it will appreci- 

 ably facilitate the recording, discussion, and presentation of radioac- 

 tive observations and phenomena. 



One illustration of the increased facility that will be secured by the 

 adoption of a name for this quantity is afforded by the frequent use 

 of expressions of the form "radium-C corresponding to one gram of 

 radium" (Rutherford), "von der mit 1 g. Ra in Gleichgewicht 

 stehenden Ra-C Menge" (Hess) ; these would be replaced by the much 

 simpler "one r of radium-C," where the letter r stands for the name 

 adopted. 



Another illustration may be taken from the surgical application of 

 the active deposit of radium. When it is desired to state the amount 

 of, say, radium-C present, a circumlocution like those in the preceding 

 paragraph is required. This long and awkward expression is fre- 



