Absorption of (3 Rays by Liquids. 181 



or the solvent. Accordingly it seemed desirable to discover 

 whether the abnormality arose from the rays or from the 

 solutions. The following pages contain a brief description 

 of measurements of the value of \/p for the /3 rays of 

 uranium passing through various solutions and other liquids. 



2. The apparatus used was identical in principle with 

 that of Crowther and needs no lengthy description, except in 

 regard to one detail. It was thought that some difficulty 

 might be found in obtaining layers of liquids which should 

 be thin enough to be penetrated by a measurable proportion 

 of the rays and yet be uniform in thickness. Such layers 

 must not be much more than 0*2 cm. thick and, if the liquid 

 were merely poured into a tray covering the active material, 

 a slight deviation of the tray from the horizontal and a 

 distortion of the surface by capillary action might cause 

 variations in the thickness sufficient to produce notable errors 

 in the measurements. Accordingly, the device was adopted 

 of absorbing the liquid in a sheet of thick filter-paper placed 

 in a carefully levelled tray. 



In the final form of the apparatus this tray was formed 

 out of two zinc rings, 16*5 cm. in external diameter, 8*5 cm. 

 in internal diameter, and 0*35 cm. thick, screwed tightly 

 together and enclosing between them a sheet of mica about 

 0*02 cm. thick. The upper surface of the mica and the 

 internal edge of the upper zinc ring formed the tray in 

 which the filter-paper was placed. In the tray formed by 

 the lower surface of the mica and the internal edge of the 

 lower zinc ring, was placed about 50 grammes of uranium 

 oxide held in place by paraffin-wax melted over its surface. 

 The rings rested upon 3 V-ways, along which they could be 

 pushed into a definite position under the ionization-vessel : 

 the ways were levelled so that the upper surface of the mica 

 was accurately horizontal when the rings were in the fixed 

 position. A thin sheet of mica rested on the upper ring and 

 formed a cover to the tray, preventing evaporation of the 

 liquid. 



The ionization-vessol was a circular cylinder, 1G'5 cm. in 

 diameter and 8*0 cm. high. The top and the sides were of 

 zinc sheet, the bottom, through which the rays passed, was 

 of tinfoil. An electrode, passing through ebonite insulation 

 and the usual guard-ring, led to a Wilson tilted electroscope. 

 The natural ionization of the vessel was measured every hour 

 and, of course, subtracted from the observed ionization when 

 the uranium rays were acting. 



3. In measuring the absorption coefficient of the solid 

 solutes, measurements were made of the value of I for about 



