EFFECT OF RADIUM ON CELL DIVISION 205 



suits to vary in these ratios, but under the conditions of the 

 experiment it is difficult to estimate in these terms. It was 

 found impossible to expose eggs at 2j inches for thirty-six times 

 as long as at f inch for during that prolonged period the eggs 

 pass through all of their phases of division at least twice. 



A further difficulty is found in the fact that the beta rays, 

 which are very active in producing changes in protoplasm, as 

 I have shown (Packard '15), are easily stopped by air so that at 

 a distance of two inches or more very few of them reach the eggs. 



A few experiments were made in which the eggs were cooled 

 in a freezing mixture so as to retard their rate of development, 

 and the radium was applied for a longer time. But the results 

 were not satisfactory as it was found difficult to hold the con- 

 trol eggs and the radiated eggs at exactty the same stage of 

 development. It was also impossible to keep both lots at 

 exactly the same temperatures throughout the experiment. 

 Although some of the trials tended to confirm the expectations, 

 the general results were not conclusive or reliable. 



• DISCUSSION 



The radiations of radium produce effects in matter only when 

 they are absorbed wholly or in part by it. The beta raj-s, be- 

 cause of their relatively low velocity are quickly stopped by 

 many substances, the particles colliding with the molecules of 

 the substance, and in many instances producing a marked ionisa- 

 tion of the molecules. As these rays are not homogeneous but 

 are made up of particles projected with varying velocities, it 

 follows that the slower particles are stopped more quickh' than 

 the more rapid ones and produce greater chemical effects. 

 The fact that they affect protoplasm more vigorously than the 

 rapid rays was shown in a recent paper (Packard '15). Whether 

 all of the effects noted in those experiments were due to an ionisa- 

 tion of protoplasm is a point yet to be determined. 



The gamma rays are exceedingly penetrating and are stopped 

 completely only by thick sheets of the heavj' metals. Some of 

 them however are stopped by protoplasm, especially if the 



