448 Mr. J. Crosby Chapman on Homogeneous 



The apparatus used to determine the nature of the Secondary 

 Radiation emitted by the vapours when exposed to the rays, 

 consisted o£ an iron box which contained the gas : this was 

 fitted with aluminium windows and was placed as shown in 

 the diagram (p. 44-7), so that the radiations reaching the 

 electroscopes could come only from the vapour inside the 

 chamber. That such is the case with this arrangement is 

 indicated by the dotted lines in the figure which mark the 

 path taken by the extreme scattered rays. 



The process of the experiment was as follows. The radiation 

 from the vapour inside the chamber was allowed to pass into 

 both electroscopes, which were of the ordinary gold-leaf type 

 described by Prof. Barkla. The deflexion in the electroscope 

 M was observed, while there was a certain deflexion in the 

 standardiser S. An aluminium sheet of required thickness 

 was then placed in front of the electroscope M, and the 

 deflexion again read while the standardising electroscope 

 suffered the same deflexion as before. In this way the 

 percentage of the radiation which had been absorbed could 



be determined, and thence a value for — . 



Owing to the difficulties in obtaining and working with 

 gases containing elements of atomic weight greater than 52, 

 in the experiment it has been impossible to determine the 



coefficients — for more than two of the elements. The 



P 

 vapours of ethyl bromide and ethyl iodide were employed on 

 account of their, comparatively speaking, high vapour pressures 

 at the temperature of the experiment. 



Secondary Radiation from the Vapour of Etlnjl Bromide 

 and from Solid Bromine Compounds. 



Air which had previously been bubbled through ethyl 

 bromide and afterwards passed through a glass spiral 

 immersed in water at 4° C, so that it was saturated at a 

 temperature lower than that of the room, was drawn through 

 the chamber by means of a filter-pump and a steady 

 condition was obtained. The X-ray bulb, which was per- 

 manently connected to a pump, was kept at a suitable degree 

 of hardness in order to obtain a maximum intensity of 

 radiation for measuring purposes. The following typical 

 results were obtained with the bromide, proceeding as 

 previously described. 



