A. C. REDPIELD AND E. M. BRIGHT 47 



The conclusion of Congdon will be seen to be at variance with our 

 experimental result.^ In his experiments it was observed that the 

 addition of enough soft (8-rays to a heterogeneous beam to increase 

 its ionizing power 25 per cent caused an average retardation of 35 

 per cent in the rate of growth of seeds exposed to the radiations. It 

 is to be regretted that Congdon did not determine how greatly the 

 retardation would have been increased by strengthening the primary 

 beam by 25 per cent without changing its character. If this pro- 

 cedure had yielded retardation of less than 35 per cent we could then 

 feel sure that the discrepancy in our results was a real one. 



SUMMARY. 



1 . The physiological effect upon the eggs of Nereis of homogeneous 

 groups of /3-rays of different velocities is proportional to their abiHty 

 to ionize air. 



2. iS-rays of low velocity produce a greater amount of physio- 

 logical change than the same number of rays of high velocity. 



3. These conclusions are consistent with, but do not prove, the 

 view that the physiological effects of radiations from radium and 

 x-rays are due to the production by them of an ionization of some 

 substance in the egg. 



