142 ABSORPTION OF RUBIDIUM AND CESIUM 



The interval between Na and NH4 is much greater than intervals 

 between the others. Lithium and sodium may be regarded as consti- 

 tuting one group, while ammonium, potassium, rubidium, and cesium 

 fall into another. These are all univalent ions so that their absolute 

 velocities of migration show the same relations as their conductances. 

 If, then, the absorption of potassium by cells in preference to sodium 

 is related to comparative ionic mobilities, lithium should behave like 

 sodium, but rubidium and cesium like potassium. In a number of 

 other physiological processes, just such relationships have been shown. 

 Loeb (2) has pointed out how lithium and sodium stand in one group, 

 physiologically speaking, while ammonium, potassium, rubidium, and 

 cesium clearly constitute a distinctly different group. The members 

 of each group show among themselves a number of similarities in 

 physiological behavior, while the effects of the two groups may be 

 regarded, in some ways, as physiologically divergent. Because of 

 the manner in which hydration affects ionic mobilities the migration 

 velocities of the members of this series seem to provide a basis for 

 grouping them in accordance with their physiological behavior more 

 satisfactorily than do other periodic functions, as for example, atomic 

 volumes. We have sought to show whether or not the contrast of 

 physiological properties among this series holds for their intracellular 

 incorporation. We have not yet made a study of lithium. There is 

 much in the literature on the subject to indicate that lithium is capable 

 of penetrating into cells with no greater facility than sodium. The 

 behavior of ammonium ions is obviously without much bearing on 

 the question of potassium selection, since they freely pass in and out 

 of all cells and are to be regarded, in general, as a waste product and 

 not as constituents of cellular architecture. The experiments herein 

 reported go to prove that rubidium and cesium behave like potassium 

 in processes involving incorporation into the physicochemical structure 

 of muscle and other animal cells. 



Mendel and Closson (6) found that rubidium injected into cats 

 and dogs was largely stored in muscle tissue. Zwaardemaker (7) 

 and his coworkers in researches on aequiradio activity have shown 

 that rubidium and in some degree cesium can replace potassium in 

 maintaining the heart beat, but whether this means penetration into 

 cells or action at their surfaces it is difficult to say. In the light of 



