408 Prof. C. G. Barkla on the Spectra of 



the softest known fluorescent radiation characteristic of R 

 is approximately proportional to the absorption in aluminium. 

 With rays of higher penetrating power than that of the 

 fluorescent radiation of R, there is a regular deviation from 

 this proportionality, and when the penetrating power is far 

 removed from that of the fluorescent radiation there is 

 again approximation to proportionality to the absorption in 

 aluminium. 



This recurrence of the proportionality suggests the possi- 

 bility, indeed the probability, of similar relations with rays 

 of penetrating power in the neighbourhood of that of a softer 

 fluorescent radiation not yet discovered. This argument 

 might have been applied to indicate the probability of a 

 radiation belonging to series L, when only that of series K 

 was known. It is equally applicable in indicating the pro- 

 bability of a radiation belonging to an undiscovered series M, 

 consisting of still softer radiations, for the same features as 

 those described are shown by the absorption curves for 

 gold and platinum over a range of penetrating power in- 

 cluding that of the fluorescent radiation of series L. These 

 absorption curves thus suggest the existence of a more 

 absorbable fluorescent radiation from gold and platinum 

 than that of series L. From the similarity of the behaviour 

 of all the elements we must then admit the probability of the 

 existence of a series M, and the possibility of further series 

 M, N, &c. 



Again, the writer has shown the connexion which exists 

 between absorption, secondary radiation, and ionization. 

 The emission of fluorescent radiation from a substance tra- 

 versed by a primary beam of X-rays is invariably accompanied 

 by a special absorption of the primary rays, and when the 

 substance is in the gaseous state by a special ionization of 

 that substance. It is natural to connect the absorption of 

 soft primary beams and the ionization produced by these in 

 passing through matter with the emission of an undetected 

 softer fluorescent X-radiation of series M. 



The extreme variation in the penetrating power of the 

 fluorescent radiations thus far observed is enormous, the K 

 radiation from cerium being more than a thousand times as 

 penetrating as the L radiation from silver. The determi- 

 nation of more penetrating radiation than that from cerium 

 and of more absorbable radiation than that from silver has, 

 however, been limited solely by the unsuitability of the 

 methods hitherto employed. It will probably be an easy 

 matter by modification of these methods to considerably 

 extend the number of radiations in these two series. 



