6 Prof. Rutherford and Miss Brooks : Comparison of 



the plate was applied. The absorption of the radiation in a 

 layer of air is negligible compared with that of an equal 

 thickness of solid matter. The maximum current* between 

 the plates is proportional to the intensity of the radiation. 



Preliminary experiments showed that the current dimi- 

 nished very approximately in G.P. with the distance of 

 material traversed, so that the value of X determined was 

 independent of the thickness of the plate. 



This shows that most of the rays emitted have approxi- 

 mately the same penetrating power. The rays of radium, 

 examined in a similar manner, did not fall off regularly, 

 showing that the rays emitted consist of particles having a 

 wide range of velocities, and consequently a wide range of 

 penetrating power. This is clearly shown by Becquerel, who 

 examined (by the photographic method) the amount of de- 

 flexion of the rays in a magnetic field after passing through 

 different thicknesses of various metals. 



The following table represents the results obtained. 



Substance. 



X. 



Density. 



X 



T)ensi'cy 



Glass 



14-0 

 14-2 



65 

 216 



37 

 44 

 140 

 60 

 75 

 122 

 96 



245 

 2-78 

 1-14 

 •40 

 •70 

 7-8 

 260 

 8-6 

 105 

 11-5 

 7-3 



5-7 

 51 

 5*7 



5-4 



5-3 

 5-6 

 5-4 





Ebonite 



Wood 



Cardboard 



Iron 



Aluminium 



Copper 



7-0 



Silver 



7'1 



Lead 



10-8 

 132 



Tin 





It will be observed that the value of the coefficient of 

 absorption divided by the density is very approximately the 

 same for such different substances as glass, mica, ebonite, 

 wood, iron, and aluminium. The divergences from the law 

 are, however, great for the other metals examined, viz. 

 copper, silver, lead, and tin. In tin the value of X divided 

 by the density is 2*5 times its value for iron and aluminium. 

 These differences show that the law of the absorption of 

 cathode-rays depending only on the density, is not true for 

 all substances. 



Experiments are at present in progress to see whether there 

 is any simple numerical connexion between the values of X 

 divided by density for different metals, and to extend the 



* Kutherford, Phil. Mag. Jan. 1899. 



