1 66 CHARLES PACKARD. 



I will not here review the results obtained by other investi- 

 gators who have tested the effects of radium radiations and X- 

 rays on Protozoa. Different methods of exposing the cells lead 

 to such wide variations in results that comparisons cannot safely 

 be made. In general, however, cells which are sensitive to the one 

 are also sensitive to the other type of radiation. Great differ- 

 ences in the susceptibility of cells of the same species have been 

 reported. Thus Zuelzer (3) states that Pelomyxa palustris when 

 exposed to 6 mg. of radium element dies sometimes within one 

 hour and sometimes only after four hours of continuous exposure. 

 Such differences make any conclusion as to the length of the 

 lethal dose out of the question. But with appropriate methods 

 the lethal dose is found to be constant. 



METHODS. 



The method used in the following experiments is this. The 

 radium was enclosed in a glass capsule which prevented the alpha 

 rays from escaping. In the experiments on the relation of tem- 

 perature and permeability to susceptibility, the strength equalled 

 13.4 mg. of element. In the third series, on the change in per- 

 meability induced by the radiations, the strength was 25 mg. of 

 element. The radium tube was used unscreened and was sup- 

 ported above the drop of culture medium at a distance of 2 mm. 

 Thus all of the rays which emerged from the lower side of the 

 tube could reach the cells. The whole preparation was kept in a 

 moist chamber at the desired temperature. 



In order to determine what type of rays produced the effects 

 which are to be described, I interposed between the radium tube 

 and the Paramcecia lead sheets of various thicknesses, thus filter- 

 ing out the more penetrating rays. All of the beta rays are 

 stopped by 2 mm. of lead: the gamma rays are not affected. It 

 became apparent at once that the changes produced in the 

 Paramcecia were due to the action of the slowest beta rays, for 

 when a lead screen of 0.12 mm. was interposed the Paramcecia 

 were affected hardly at all. This is to be expected, for the sur- 

 face layer of the cells is very thin and can absorb only those rays 

 which have a low velocity: it offers almost no resistance to rays 

 having considerable powers of penetration. 



In conducting these experiments it was found necessary to use 



