Absorption and Scattering of X-Rays. 279 



is not negligible in these heavier substances, as it is not even 

 in copper when high-frequency X-rays are employed (see 

 Table I.). 



A point worthy of notice is that as the wave-length of the 

 X-radiation diminishes, after the mass absorption coefficients 

 in various substances become approximately equal — owing 

 to the "fluorescence absorption " becoming very small — the 

 absorptions diminish in different substances at approximately 

 the same rate. This indicates that the scattering diminishes 

 at approximately the same rate in different substances with a 

 diminution of wave-length. 



On the other hand, with longer waves the scattering- 

 increases with the wave-length at very different rates in 

 different substances. This latter variation, as we have ex- 

 plained, is probably due to the electrons acting in groups 

 rather than individually. 



The variations observed — (or rather inferred from ab- 

 sorption experiments, for it has not been observed directly) 

 — when the waves are short, must have another explanation. 

 It may be connected with the fact that when the wave-length 

 is short, the deviation of the observed distribution of X-rays 

 scattered around the substance traversed from that given by 

 the simple theory becomes very marked. It seems certain 

 that scattering loses its simple character. The scattering of 

 7 and X-rays may thus be considered in three stages. With 

 very short waves (7 rays) the scattering increases with the 

 wave-length in all substances at approximately the same rate, 

 until the mass scattering coefficient approaches *2. 



Secondly, there is little or no variation of scattering w r ith 

 wave-length over a long range of wave-lengths in light 

 elements^ over a shorter range of wave-lengths in heavier 

 elements. 



Finally, the scattering increases with w r ave-length, slightly 

 in light elements, more rapidly in heavier elements. 



The second and third stages have already been explained. 

 The first stage needs further investigation. 



Ah sorption associated with X-ray Fluorescence. 



Absorption associated with the phenomenon of X-ray 

 fluorescence may be analysed into several distinct and 

 evidently independent absorptions connected with various 

 sets of electrons within the atom. Thus there is the 

 K absorption which is that associated with the emission of 

 a particular group of electrons which we call the K electrons 

 because their emission is associated also with the emission 



