328 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



where the constant B is a function of the temperature, the 

 wave-leu gth A, the atomic weight, and the characteristic 

 temperature 6 of the crystal. 



W. H. Bragg * made a series of measurements of the 

 effect of temperature in reducing the intensity of reflexion 

 by rock-salt. Intensities were compared at 288° K and 

 643° K. The results were, within the errors of experiment, 

 consistent with the ratios given by Debye's formula. The 

 latter gives different values for B according to the assump- 

 tion or otherwise of the existence of the "Nullpunktsenergie." 

 Compton (Joe. cit. p. 47) gives, as the two values for B in 

 NaCl, 4*6 and 36 respectively. 



The mean value for B at 288° C, calculated from W. H. 

 Bragg' s results, is equal to 4'12, and this will be assumed 

 in the calculations which follow. To assume that the effect 

 of temperature on both curves of fig. 2 is the same, is 

 equivalent to supposing that the average amplitude of 

 vibration of sodium and chlorine atoms is the same. This 

 is very probably not the case, and the authors intend to 

 measure the effect of cooling down the crystal in order 

 to obtain an empirical law over a wider range expressing 

 the temperature effect. However, the factor e ~ Bain2 ° does 

 not affect very greatly any but the smallest intensities 

 measured, and will therefore not make much difference to 

 the conclusions to be drawn from the curves. 



IJie Linear Coefficient of Absorption " //." 



15. The coefficient of absorption by rock-salt of the 

 homogeneous radiation was measured in the usual way by 

 interposing plates of rock-salt of various thickness in the 

 path of the direct beam (fig. 3) and measuring the diminu- 

 tion in energy of the beam. Experiments were made 

 w r ith plates from 0*05 cm. to 0'15 cm. thick. The linear 

 coefficient of absorption /ju was found to be 10*7. 



In the theoretical formula it has been assumed that jll is 

 constant. Now, W. H. Bragg has shown that in the case 

 of the diamond, when the crystal is set so as to reflect the 

 radiation, the absorption-coefficient is abnormally large. 

 It was therefore interesting to try whether such an effect 

 is observable in the case of rock-salt. A slip of crystal 

 0*92 mm. in thickness with faces parallel to (100) was set 

 on the spectrometer table at right angles to the homogeneous 

 beam from G± in fig. 3 and the absorption measured. It was 

 then turned through an angle of about 66° until the (100) 

 * W. H. Bragg, Phil. Mag. loc. cit. p. 897. 



