412 Prof. W. H. Bragg on the Consequences of 



large number of measurement of absorption and of scattering 

 coefficients f. He has defined two constants which he has 

 called the " reflexion " and the " true absorption " coefficienst. 

 The former really represents roughly the facts of the deflexion 

 oval, the oval being reduced to its axis, and the atom placed 

 at various positions upon it ; the latter represents the expen- 

 diture of energy along the path. His two constants actually 

 stand approximately for the two independent subjects of 

 measurement which we have seen to be important in the 

 case of the /3 ray. It is therefore very interesting to com- 

 pare his calculated values of the true absorption coefficient 

 with the quantity d, which should be approximately in the 

 inverse ratio. To what extent this is due is shown in the 

 following table. The second column gives the values which 

 Schmidt} calculated for the true absorption coefficients of the 

 ft rays of uranium, i. e. the values of his a/D. I do not 

 think the values for radium are available. But it must be 

 quite allowable to use the former instead of the latter, since 

 the /3 rays of radium do not differ much in penetrating 

 power from the /3 rays of uranium; while the values of a/1) 

 for uranium and for actinium are very much the same re- 

 latively to one another, and yet the /3 rays of actinium are 

 much less penetrating than those of uranium. The third 

 column gives the relative values of M, or practically of d, 

 and the last the product of the figures in the two preceding- 

 columns. 



Substance. 



«/D. 



M. 



«/D x kd. 



Lead 



Tin 



1-69 



214 (2-40) 



300 



3-08 



3-26 



3-32* 



100 

 68 

 55 

 54 

 49 

 46 



169 



145(163) 



165 



166 



160 



153 



Zinc 



Iron 



Aluminium 



Card 





* Calculated as for carbon from later figures given bj Schmidt. 



The uniformity of the figures in the last column is only 

 broken seriously by tin. Strange to say, the value 2*14 

 which Schmidt gives for tin is quite out of line with the 

 values he gives for all the other metals ; if these values are 

 plotted and a value for tin obtained from the curve we get 



f See also McClelland and Hackett, Dublin Trans. 1907. ix. p. 37. 

 % Ann. d. Fhys. xxiii. p. 671 (1907); Jahrb. d. Had. 1908, p. 451. 



