the Fluorescent JRontgen Radiations, 397 



order to distinguish it from the fluorescent radiation. Ob- 

 servation of radiation of this type has been possible owing 

 to the fact that the lightest elements — from hydrogen to 

 sulphur — when subject to a beam of X-rays of ordinary 

 penetrating power, emit X-radiation which at a distance of 

 several centimetres in air is almost entirely, if not entirely, 

 a scattered radiation. If these elements emit fluorescent 

 X-radiations under such conditions, they are either absorbed 

 in a very thin layer of air and do not reach the electroscope, or 

 are so penetrating as to produce no appreciable ionization in 

 the electroscope. The former appears probable, the latter 

 highly improbable, as will subsequently be seen. 



The scattered radiation is indistinguishable in most of its 

 properties from the primary radiation producing it. It has 

 the same penetrating power as the primary. When slight 

 differences are observed these differences are most marked 

 when the experimental conditions are such as to introduce a 

 quantity of the fluorescent radiation, that is when more 

 penetrating primary beams are used. In experiments with 

 only moderately penetrating rays, the primary and secondary 

 beams are indistinguishable as regards penetrating power. 

 It has yet to be proved that a secondary X-radiation which 

 does not contain some fluorescent X-radiation differs in 

 penetrating power from the primary radiation producing it. 



As the ordinary primary beam is extremely heterogeneous, 

 as shown by successive absorptions by similar plates of any 

 material, the similarity in character of the scattered radiation 

 is evidence of the equality of scattering of the various con- 

 stituents, that is the various constituents appear in the 

 secondary beam in the same proportion as in the primary. 

 It is also seen by direct experiment that X-rays varying 

 considerably in penetrating power are scattered to the same 

 extent in passing through a given quantity of matter. A 

 limit to this law has not been found, though the range of 

 penetrating power might be considerably extended. 



The scattered radiation proceeding in a direction perpen- 

 dicular to that of propagation of the original primary radia- 

 tion is very fully polarized, and in the same manner as light 

 scattered from small particles. 



The scattered radiation is not equally intense in all direc- 

 tions around the primary beam, but exhibits a slight polarity 

 of the primary such as would be expected on the electro- 

 magnetic theory of the production of X-rays. 



The writer found the polarization to diminish with an 

 increase in the penetrating power of the radiation from the 

 bulb, whereas with the bulb in a mven state the more 



