Biological Effects of Ionising Radiations 115 



of a cell may result from the generation within a certain 

 small region of an amount of energy which, if spread over the 

 whole cell, would not raise its temperature by more than one 

 hundred-millionth of a degree Centigrade. With the advent of 

 the high-voltage X-ray tube, the betatron, and the cyclotron, the 

 study of the influence of radiation type or quality upon biological 

 response has assumed a practical importance, for with the help 

 of these machines, it is possible to generate almost any type of 

 ionizing radiation under conditions which are suitable for the 

 treatment of a deep-seated tumor. 



Linear Ion Density, the Distinguishing Feature of an 

 Ionizing Radiation, from the Biological Standpoint 



The discovery of radium followed quickly upon the discovery 

 of X-rays, and some of the earliest biological experiments with 

 ionizing radiations were carried out with "naked" and "screened" 

 radium. As the screens used were of just sufficient thickness 

 to absorb all the beta rays, the experiments were, in effect, com- 

 parative studies of the effects of the beta and alpha rays as they 

 are generated by a small quantity of radium. Striking differences 

 were at once noticed. ^^' ^^ Hardy ^^ observed that an alkaline 

 solution of serum globulin, i.e., on the negative side of the iso- 

 electric point, was coagulated, and that an acid solution became 

 clearer when exposed to naked radium. When screens were 

 introduced to absorb all the alpha rays, so that the drop of 

 solution was exposed only to the beta rays, no effect was ob- 

 served even after twenty times the exposure. Chambers and 

 Russ ^ observed that erythrocytes were hemolyzed when exposed 

 to both alpha and beta rays, but not when the alpha rays were 

 eliminated. Colwell and Russ ^ found that, when emulsions of 

 bacteria were exposed to both alpha and beta rays, marked 

 agglutination occurred before the lethal point was reached. When 

 the alpha rays were eliminated, there was no agglutination, al- 

 though a lethal condition was reached. 



A consideration of the physical differences which obtained 

 in these experiments will serve to illustrate important points in 



