Chemistry and Physics. 561 



namely; — air, sulphur dioxide, ethyl bromide, and methyl iodide. 

 In brief, the symbol x has the same value (3~) for substances 

 whose densities vary from 0'00129 (air) to 10*5 gram/cm 3 (silver). 

 Owen found that the absorption exhibited by gases for charac- 

 teristic radiations is proportional to the fifth power of the atomic 

 weight of the radiating element, and Kauffman has pointed out 

 that this relation is also fulfilled by aluminium as absorber. The 

 diagram of Siegbahn shows conclusively that the above relation 

 is incorrect as stated, and that the atomic number (AT) is the 

 correct parameter and not the atomic weight. Therefore, it fol- 

 lows at once that — = const. T1 = — -—-. Since 2x ~ 6 the fifth 

 <r (JV— ly x 



power must be replaced by the sixth in Owen's law. — Phys. 

 Zeitschr., August 15, 1914, p. 753. h. s. u. 



9. The X-ray Spectrum of Platinum. — The photographic 

 negatives obtained by Bragg, Moseley and Darwin, and deBroglie 

 with platinum as radiator apparently show continuous bands as 

 well as a few fine, discrete lines. By greatly improving the 

 experimental conditions it has been shown, quite recentty, by H. 

 Seemann, not only that the bands consist of a relatively large 

 number of sharp lines but also that some of the single lines found 

 by previous investigators may each be resolved into several finer 

 lines. Furthermore the faint, continuous background present in 

 Seemann's negatives may be accounted for as arising from radia- 

 tion which has been scattered at the crystal surface and elsewhere. 

 Therefore in the case of platinum, at least, reasonable doubt may 

 be entertained as to the existence of any appreciable continuous 

 radiation in the region of extremely short wave-lengths. 



A brief account of Seemann's method of experimenting may 

 not be superfluous. The X-rays were caused to pass out of the 

 bulb through a lithium-glass window. The chief advantage of 

 lithium glass (" Lindemann-glas ") is that it is more transparent 

 to the less refrangible components of the Rontgen ray spectrum 

 than the sodium glass walls of the bulbs ordinarily used. The 

 slit of the spectrograph w r as placed directly in front of the emer- 

 gence window and it was given widths from o-i mm to 0'03 mm . 

 Slit-widths of the order of l mm were used by earlier investigators 

 of this field. A very line rock-salt crystal was used as " grating." 

 It was situated in the axis of a spectrometer table at a distance of 

 6 cm f rom the slit. Seemann found that rock-salt gave incompara- 

 bly better results than crystals of fluorite, gypsum, hematite, Ice- 

 land spar, iron pyrites, mica, sulphur, and zinc blende. A plane 

 photographic plate was held, at a distance of I0 cm from the crys- 

 tal, by an arm which was rigidly fastened to the spectrometer 

 table. This arrangement of apparatus made it possible to photo- 

 graph, at one exposure, a region of the spectrum only ]°-5 

 wide. Successive exposures were taken on different plates, the 

 plate-holder and crystal having been moved together through an 

 angle of 1°*5 each time. The final, scientifically beautiful posi- 



