PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 773d meeting was held on April 29, 1916, at the Cosmos Club. 

 President Briggs in the chair, 43 persons present. The minutes of 

 the 771st and 772d meetings were read in abstract and approved. 



Mr. C. W. Kanolt presented a paper entitled X-ray spectra. The 

 speaker presented a resume of the subject, including a review of the 

 state of the theories of the X-rays previous to the use by Laue of a 

 crystal as a diffraction grating, an explanation of the theory of the 

 three-dimensional grating presented by a crystal, and an account of 

 the experimental methods employed and the results obtained. These 

 results give information relative to the nature of the X-rays, permit 

 the determination of the arrangement of the atoms in the simpler 

 crystals, and give some information relative to the structure of the 

 atoms. It was pointed out that it is also possible to determine atomic 

 masses by the measurement of the angles of reflection of X-ray beams 

 from suitable crystals, and the determination of the densities of the 

 crystals. A knowledge of the wave-length of the rays is not required. 

 To obtain by this method an accuracy greater than that of the results 

 that have been obtained by chemical methods it would be necessary 

 to make more accurate measurements of spectra than most of those 

 made hitherto. However, in most of the work that has been done 

 great accuracy has not been required or attempted, and it appears 

 that the methods could be improved sufficiently to give atomic-mass 

 determinations of greater accuracy than most of those that have been 

 made by chemical methods. 



The paper was discussed by Messrs. Crittenden, Bauer, and L. 

 J. Briggs with reference to the highest atomic numbers and the excep- 

 tions found by Moseley in the order of the elements. 



Mr. F. E. Wright then presented a communication on The analysis 

 of crystal structure by X-rays. In recent years Laue and, especially, 

 W. H. and W. L. Bragg have developed effective methods for using 

 X-rays in the analysis of crystal structure. Laue's method is to send 

 general X-ray radiation through a thin crystal plate and thus to obtain 

 on a photographic plate a diffraction pattern which is an expression 

 of the symmetiy relations of the crystal plate. In the Bragg reflec- 

 tion method characteristic X-rays of definite wave-lengths are used 

 and the intensity of the reflected rays is measured by means of an 

 ionization chamber and an electroscope. The crystal plate is mounted 

 in a spectrometer and for a given wave-length, X, and a fixed distance, 



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