332 



Prof. Bragg and Mr. Kleeman 



on 



thin tinfoil. When a film is not uniform, the fact is made 

 evident by a distortion of the curve, e. g. in the case of 

 copper. It will be seen that the top of it slopes too much, 

 implying that the a rays do not come into the ionization- 

 chamber as suddenly as they ought to do. Some have, in 

 fact, been less checked than others because they have passed 

 through thinner portions of the somewhat uneven film. We 

 have measured, in such cases, the drop of the curve as shown 

 by its amount at the middle point of the top of the Ra C 

 portion. E. g., in the case of platinum we have measured 

 the depth of the middle point of the top of the curve below 

 that of the middle point of the top of the normal curve. 



The results of these experiments are shown in the following 

 table. The first column of figures gives the product of the 

 thickness of the metal film and its density, the second the 

 corresponding drop of the curve, multiplied by the density 

 of air, and the third the ratio of these two products. 





I. 



II. III. 



i 

 IV. Eatio 1V./III. 



Gold 



Platinum 



Tin* 



Silver 



•0121 



•00633 



•0051 



•00967 



•00873 



•00258 



•00396 3-05 

 •00192 S-29 

 •00212 2-41 

 •00402 2-41 

 •00492 1-78 

 •00209 1-23 



11-2 

 14-0 



10-85 

 10-4 

 7-96 

 5-15 



4-65 

 4-25 

 4-50 

 4-30 

 4-45 

 4-20 



Copper 



Aluminium ... 



* Tinfoil contains a certain proportion of lead ; but this could not affect 

 the result very appreciably. 



It is remarkable that the numbers in the third column are 

 nearly proportional to the square root of the atomic weights 

 of the metals. To bring this out more clearly, the square 

 roots are shown in a fourth column, and in a fifth the ratios of 

 the numbers in the two previous columns. 



Moreover, air itself falls approximately into line with the 

 metals. Its ionization ratio should be entered in the third 

 column as unity, and the average square root of its atomic 

 weight as (4\/14+ a/16)/5 = 3'79. The corresponding entry 

 in the last column should be also 3*79. 



Also it is easily seen that hydrogen is not far away from 

 the others. For it has been shown by Strutt that its con- 

 ductivity under ionization as compared with air is '226. As- 

 suming for the present that this means that a 1 cm. layer of 

 hydrogen has the same effect as a '226 cm. layer of air, then 

 the ratio of the product of the thickness and the density in 

 the case of the hydrogen to the similar product in the case 

 of the air is l/ll'l X '226 = "31. The square root of its atomic 

 weight is 1, and lj'?>l — ?>'2. 



