﻿Absorption of the ft Rays from Uranium. 383 



in the chamber B, and this was in turn a measure of the 

 intensity of the radiation entering the chamber, and thus the 

 ratio of the leak of the electroscope when the plate A was 

 uncovered (except for the thin aluminium plate used to cut oft* 

 the a. rays) to the leak when the plate A was covered with a 

 thin sheet or layer of the substance under investigation, 

 enabled the coefficient of absorption, X, to be calculated, 

 when the thickness of the absorbing material was known. 



In almost every case experiments were made with several 

 different thicknesses of the substance under observation, in 

 order to ascertain whether the absorption of the substance 

 was a simple exponential function of the thickness. The 

 logarithms of the intensities of radiation were plotted against 

 the thicknesses of material through which the radiation had 

 passed, and specimen curves so obtained are shown in fig. 2. 

 In the large majority of cases the curve was a straight line, 

 as illustrated by the curves for aluminium and potassium ; 

 thus showing that for these substances, within the limits of 

 experimental error, the law of absorption was expressed by 

 the relation 



I/I = 6-A 



In the case of a few of the heavier metals, namely, lead, 

 gold, platinum, tin, silver, and palladium, there was a more 

 or less marked departure from the exponential law, for very 

 small thicknesses of material, as illustrated by the curve for 

 tinfoil (fig. 2, p. 384). McClelland * has shown that this would 

 be the case if an appreciable amount of secondary radiation 

 were produced by the action of the incident /3 rays; and experi- 

 ments made later upon the above metals showed that there was 

 a considerable amount of return radiation from them. We 

 shall return to this question later. For the present we may 

 state that the vnlue of X employed is that calculated from the 

 straight portion of the curve after X has become constant. 



Whenever possible, the element was procured in the form 

 of thin foil. Many, however, were only obtainable as 

 powders. The lighter powders were formed into layers by 

 spreading them out evenly on shallow cardboard trays of 

 different depths ; the heavier ones were finely powdered, 

 suspended in chloroform, and allowed "to deposit on thin mica 

 sheets. The absorption due to the tray or sheet was of course 

 measured and allowed for. ■ ~ 



The thickness of the layer was in every case measured by 

 finding the weight of a known area. (In the case of the 



* Scientific Trans. Koy. Soc. Dublin, 1906. 



