from Ordinary Material*. 



54 1 



Tinfoil and tin-plate gave notably different values, but 

 tinfoil is. I believe, an alloy : specimens of foil and plate 

 agreed amongst themselves. Only in the case of wood were 

 large differences found, but so porous a material, after lying 

 some time in a laboratory where considerable quantities of 

 pure radium salts have been used, might be expected to 

 acquire a spurious radioactivity. 



§11. An attempt was made to measure the coefficient of 

 absorption of the rays. Three methods were employed : — 



(1) By moving the plate away from the window so as to 

 interpose a layer of air of known thickness. 



(2) By interposing a layer of aluminium-foil. 



(3) By cutting a second window in the other side of the 

 zinc vessel, covering it with a layer of aluminium-foil of 

 different thickness, and comparing the increase caused by the 

 plate first before one window and then before the other. 



In these ways we can show that the coefficients of absorp- 

 tion of the rays from different materials are not the same, 

 and obtain some estimate of their value. But the attempt to 

 ascertain the value accurately is frustrated by the difficulty 

 we have met before : we do not know the relation between 

 the intensity of the rays from the surface and their inclination 

 to the surface normal. Thus, if we place a layer of thick- 

 ness (1 and absorption V on the face of the material, the 

 effect of the ravs will be cut down in the ratio e~ x ' d if the 



rays are projected normally, and in the ratio 



_ In 



dy if 



they are equally projected in all directions. In Table II. 



Table II. 



XV. 



Ratio. 







A. 



B. 



0-25 



05 



078 

 061 

 047 

 0.T7 

 0136 





0-52 

 034 

 0-26 



015 

 0038 



o-T.j 



1-00 



200 



tin; ratios of reduction are given for different values of XV, 

 (Aj when the ray- are projected normally, (B) when they are 

 emitted equally in all direction-. 



