the X Radiation from a Coolidge Tube. 157 



than fi = 22 must be present in some quantity before its pre- 

 sence can be detected by absorption methods. The minimum 

 value found by Hull, ^ = 17*5, is somewhat less than the 

 value, /uu = 22, found in these experiments, but the difference 

 is no doubt to be ascribed to the difficulties of accurate 

 measurement of fi in both cases. 



From the dotted portion of the curve, the minimum value 

 of n for lead at 196,000 volts (X = -063 A.U.) should be 

 about 5. The observed value is 8*5. Taking into account 

 that the minimum value of ix for 196,000 volts must be 

 somewhat less in any case than 8'5, and that the actual curve 

 of absorption is probably somewhat steeper than the dotted 

 portion of the curve, there is not a marked divergence between 

 the observed and the calculated results. Taking these factors 

 into consideration, the absorption measurements are not in 

 themselves inconsistent with the view that the maximum 

 frequency of the radiation from a Coolidge tube is given by 

 the quantum relation, E = 7n>, over the range of voltage 

 examined. Hull and others have already shown by crystal 

 methods that this relation certainly holds up to 100,000 volts 

 and probably up to 150,000 volts. 



The peculiarities of the absorption by lead of X rays of 

 different frequencies affords a simple explanation of the 

 results obtained by Rutherford, Barnes, and Richardson *. 

 In their experiments the absorption of the end rays by 

 aluminium was found unchanged between 142,000 and 

 175,000 volts after the rays had passed through 2*19 mm. 

 of lead as absorber. A reference to the table shows that 

 under these conditions the issuing radiation consisted mainly 

 of the characteristic radiation of lead with a value of /jl = 22, 

 and no observable change in the absorption by aluminium is 

 to be expected under the experimental conditions. 



Absorption by Aluminium. 



A few isolated and approximate measurements were made 

 of the absorption of the rays by aluminium under different 

 conditions. In order to avoid complications due to the 

 characteristic radiations of heavy elements like lead, the 

 greater part of the radiation was first absorbed by its 

 passage through an element of low atomic weight like iron. 

 Under such conditions, the absorption results should not be 

 seriously influenced for frequencies much higher than that 

 of the K radiation of iron. The following results were 



* Loc. cit. 



