■296 Dr. 0. G. Barkla and Mr. G. H. Martyn on the 



criticism of many chemical points; to my colleague Dr. G. 

 Owen for much practical assistance ; and, as usual, to Prof. 

 Wilberforce for valuable advice and facilities always so 

 readily placed at my disposal. 



Messrs. Johnson, Matthey & Co. went to great trouble 

 to prepare for me specimens of the pure platinum metals in 

 suitable form ; in the cases o£ iridium and ruthenium the 

 specimens were difficult to produce. 



George Holt Physics Laboratory, 

 University of Liverpool. 



XXXII. The Photographic Effect of X-rays and X-ray 

 Spectra. By C. G. Barkla, 21. A., D.Sc, F.R.S., and 

 G. H. Martyn, B.Sc* 



AS the fundamental phenomena accompanying the trans- 

 mission of Rontgen radiation through matter, — those 

 of absorption, secondary radiation, and ionization, — are all 

 intimately connected with the X-ray spectrum charac- 

 teristic of the particular matter traversed, it is to be expected 

 that chemical actions should exhibit similar dependence on 

 .the characteristic radiations. 



In general a very penetrating radiation during trans- 

 mission through a photographic plate produces little photo- 

 graphic effect, while an easily absorbed radiation of the same 

 intensity produces considerable photographic action, increased 

 absorption of energy being accompanied by increased photo- 

 graphic action. Selective effects are, however, not readily 

 observed when heterogeneous beams are used. By the use 

 of homogeneous Rontgen radiations, — the characteristic 

 radiations (series K) for various elements, — the connexion 

 between the photographic action and the absorbability (or 

 wave-length) may be easily studied. 



Primary radiation from an X-ray tube was allowed to fall 

 on a plate of substance the fluorescent radiation of which 

 was to be used. 



A portion of this fluorescent radiation, after passing 

 through a rectangular aperture with aluminium edges 

 (5 cm.X'5 cm.) in a lead screen, was allowed to fall on a 

 rapid photographic plate f enclosed in an envelope of black 

 paper. 



Another pencil of the fluorescent X-radiation passed 

 through a small square aperture of about *5 cm. edge in 

 a lead screen on to the thin paper face of an electroscope 



* Communicated by the Authors. 

 t " Griffin's Professional." 



