﻿684 Dr. A. S. Eve and Mr. F. H. Day on the 



slope of the tangent at any point is X. By this graphic 

 method it is easy to see whether X is constant, when the line is 

 straight ; or whether X decreases with the distance, indicating 

 that the more easily absorbed rays are rapidly eliminated. 



This method has been used by one of us * to determine 

 the coefficient of absorption by air of the /3 rays from 

 radium B and radium C, and from the active deposit of 

 thorium. And this method has subsequently been adopted 

 by Hess f in an important and remarkable experiment. A 

 AVulf electrometer was used to measure the y rays from 

 radium C from 1400 mgs. of pure BaCl 2 , over ranges from 

 10 to 90 metres. The results obtained varied from '389 to 

 •493 times 10~ 4 with a mean value /a= '0000447, agreeing 

 well with McClelland's ratio of /"/density for various elements 

 and for water. 



The present paper consists of an account of the application 

 of the method., described above, to the determination of the 

 coefficient of absorption of the Bontgen rays by air. Two 

 electroscopes were used, the one fixed, the other moved to 

 various distances. They were of simple type, with aluminium 

 leaf, a capacity about 3 cm., and of considerable volume, 

 12 litres. Because Bontgen rays are not emitted very 

 steadily from the source, it was necessary to compare the 

 readings of the moved with a fixed electroscope as standard. 

 It is not necessary to know either capacity or volume, only 

 the potentials corresponding to the microscope-scale readings. 

 In every case much scattered radiation reached the electro- 

 scopes from the air, roof, floor, and walls. It is, however, 

 easy to determine the magnitude of this by placing a thick 

 lead screen of size just sufficient to prevent any primary 

 rays reaching the electroscope. The screen was placed at a 

 distance of one to two metres from the electroscope. The 

 scattered radiation was from 25 to 50 per cent, of the total, 

 according to the experimental conditions ; the ionization 

 due to the scattered radiations was in all cases subtracted 

 from the total radiation, so as to obtain the ionization in the 

 electroscope due to rays coming from the direction of the 

 source. 



Short Ranges (4-10 m.). 



At one end of a room 12 metres long was placed the 

 Bontgen-ray bulb connected with a powerful induction-coil, 

 with TVehnelt interrupter. The fixed electroscope was 6 m., 

 and the moved electroscope at one of the stations 4, 6, 8, or 

 10 m., from the source. Measurements were taken both 

 retiring and advancing, in order to eliminate the risk of 

 * Phil. Map-. July 1911. f Phgs. Zeit. 15 Nov. 1911. 



