206 CHARLES PACKARD 



layer of living tissue be of considerable thickness. When this 

 stopping occurs, due to the violent encounters of the rays with 

 the protoplasmic molecules, the gamma rays disappear and their 

 energy is converted into that of beta particles which travel 

 on in the direction of the original gamma ray. These secondary 

 beta rays produce effects similar to those produced by the pri- 

 mary beta rays. The great penetrating power of the gamma 

 radiations accounts for the fact that few of them are thus changed 

 into beta rays in protoplasm and hence produce few changes 

 in living matter unless an enormous number is allowed to act 

 on it. X rays which are similar to these radiations produce 

 their characteristic effects by means of the secondary beta 

 rays which they generate. The effects produced by these two 

 types of radiations are therefore comparable. 



The amount of radiation which is stopped by matter depends 

 on the density of the elements composing it. Roughly, the 

 absorptive power varies with the square root of the atomic 

 weight of the elements, and therefore follows closely the group- 

 ing of the elements in the periodic table. From this it is clear 

 that the Hght elements composing protoplasm, such as • carbon, 

 oxygen, hydrogen, and nitrogen have relatively low powers of 

 absorption. 



The .absorption of radiations by compounds is equal to the 

 coefficients of absorption of the elements composing them. That 

 is, the stopping power of a compound depends on the number 

 and atomic weight of its constituent atoms. This rule appUes 

 to all of the compounds studied thus far. Borodowsky showed 

 that the absorption of beta rays by liquids follows accurately 

 an additive law, and does not depend on the concentration or 

 chemical dissociation of the liquids. The amount of radiation 

 which an organic liquid can absorb can be deduced from a 

 knowledge of its constituent elements. From this it follows 

 that the amount of absorption is not influenced by the molec- 

 ular structure of substances. Applying these facts to the 

 protoplasm of the Arbacia egg, it is evident that the protoplasm 

 will always stop a definite amount of the radiation provided 

 that the chemical make up of the eggs remains unchanged. It 



