Production of Fluorescent Rontgen Radiation. 3 J L 



but deflected, and may have hundreds of encounters before 

 it; so that its chance of meeting a bromine atom is practi- 

 cally as great as ever. If it is argued that the cathode ray 

 is ' absorbed ' by the bromine and C0 2 in proportion to 

 weight, it must be answered that whatever ' absorption ' may 

 mean, there is no clear evidence of the universality of 

 Lenard's law." 



Such a fundamental difference between the action of the 

 C0 2 molecules and the Br atom on the electron, as Professor 

 Bragg has been compelled to ascribe, is purely hypothetical, 

 and until experimental evidence of its truth can be brought 

 forward, it must be treated as an assumption which is merely 

 a convenience when explaining the results of the above 

 experiment from the point of view of the corpuscular theory. 



With regard to Lenard's law, at the time when the paper 

 was written, and in fact even now, the law is not completely 

 established, but recently it has been shown to be an approxi- 

 mate representation of what actually takes place, even for 

 slowly moving corpuscles, and Professor Bragg himself has 

 published results from which he draws important conclusions 

 which necessitate a far more accurate fulfilment of this law 

 than was needed in the experiment referred to. 



However, to settle the point as to whether the fluorescent 

 X-rays are produced directly or indirectly, the following 

 experiment was devised, so that in the case of one X-radiator 

 the corpuscles were certainly absorbed in the radiating sub- 

 stance, while in the case of the other radiator, the corpuscles 

 lost their energy in a substance in which it is impossible to 

 produce measurable secondary fluorescent X-radiation. In 

 constructing an experiment to meet this demand two dif- 

 ficulties have at once to be faced. In the first place, if any 

 appreciable fraction of the total amount of corpuscular 

 radiation produced in a sheet of metal is to escape from the 

 metal, the latter must be exceedingly thin, of the order of 

 10~ 5 cm. Secondly, the method of separating the different 

 parts of the radiator must be such as will readily allow of 

 the whole serving as a convenient secondary radiator. 



Recently, however, I have shown * that gold with other 

 heavy elements is a most efficient secondary X-radiator; and 

 that its type of radiation f , both in regard to the secondary 

 X-radiation and the /3-rays produced, differs in no respect 

 from the type of radiation emitted by an element such as 

 bromine of the more usual group K (elements Cr — I). Now 

 gold in the form of leaf can be obtained in exceedingly thin 



* Chapman, Proc. Roy. Soc. A. vol. lxxxvi. (1912). 

 f Chapman, Proc. Hoy. Soc. A. vol. lxxxviii. (1913). 



