26 PHYSIOLOGICAL EFFECTS OF /3- AND 7-RAYS 



ated according to the formula of Wood and Prime. ^ A curve was then 

 plotted relating the intensity of radiation to the membrane volumes re- 

 sulting. This curve was used to determine the intensities of radiation 

 reaching the first series of eggs through the various thicknesses of 

 aluminium. 



From data obtained in this way a coefficient could be obtained 

 expressing the absorption of the physiologically effective radiation 

 by aluminium. In our experiments the radioactive source has con- 

 sisted of radium emanation, in equihbrium with its products, con- 

 tained in a slender glass tube, the walls of which were sufficiently 

 thick to absorb all the a-rays.^ Such a preparation emits /3- and 

 7-rays. The former are much less penetrating than the latter and 

 possess as a result a much higher coefficient of absorption. If the 

 physiological effect is due wholly or in large part to one of these 

 types of radiation to the exclusion of the other, the absorption coeffi- 

 cient of the physiologically effective radiation should be of the same 

 magnitude as the absorption coefficient of that type of radiation. 

 If, on the other hand, both sorts of radiation contribute considerably 

 to the physiological reaction, the absorption coefficient determined 

 by it should have an intermediate value. Fig. 1 shows the results 

 of three experiments. Against the thickness of the aluminium filters 

 are plotted the logarithms of the per cent of the radiation passing 

 through the filters, estimated from the effect upon Nereis eggs. 

 Through these points we have drawn a line which corresponds to 

 an absorption coefficient, ^u, of 23.9 cm.~^ estimated according to 

 the equation 



- = e-M-i 



lo 



where / is the intensity of radiation passing through a thickness of 

 aluminium d, and h is the intensity of the original beam. 



The jS-radiation from preparations such as we have employed is 

 due chiefly to the disintegration of Radium B and C. The velocity 



* Wood, F. C, and Prime, F., Jr., Ann. Surg., 1915, Ixii, 751. 



^ We are greatly indebted to Dr. William Duane and to Dr. R. B. Greenough, 

 the Director of the Cancer Commission of Harvard University, for placing a 

 supply of radium emanation at our disposal. 



