678 Mr. Elis Hjalmar on 



up to and including Ca. At K it was not to be found, 

 though the exposure was extended a long time. 



Before I could advance in the next domain S — Na, it was 

 necessary to get a precision-measured value of the lattice 

 constant of gypsum. The oldest value fixed by E. Friman* 

 was not satisfactory, so W. Stenstrom f set about a new 

 investigation. His working may here be a little recapitu- 

 lated, because there is a very interesting fact in it. From 

 six spectrograms of Cu f} x in the first three orders, he calcu- 

 lated a mean value of the lattice constant. But when using 

 this value on another line with longer wave-length, he found 

 that Bragg' s formula n\ = 2d suk)) (n is the order of the 

 spectrum) jjave decreasing X for increasing n. The deviations 

 were much too great to be explained through experimental 

 errors. Stenstrom is of the opinion that these deviations are 

 dependent on refraction of the rays in the crystal. However 

 this may be, the matter is of such an importance that it is 

 here scrutinized anew. Another circumstance is here con- 

 sidered. The temperature coefficient k is for gypsum rather 

 high, k =0-000025. It is easily calculated that this fact will 

 sometimes cause a deviation in A, which is of the same 

 magnitude as the experimental errors. But corrections due 

 to the temperature are easily carried out. The temperature 

 has here been exactly determined in the spectrograph. 

 18°"0 C is taken as standard temperature. 



At first the lattice constant was determined from Cuft in 

 three orders on 16 plates. The mean values are given in 

 Table III. 



Table III. 

 CuftX= 1388-87 X.U. 



n. <p iS . log 2d, 



1 5° 15' 34"-5 1-18044-8 



2 10 S3 39 -8 57 



3 15 57 12 G8 



$ 18 is the corrected angle of reflexion. The total mean 

 value becomes 



log 2d = 1-18056-8, 



and is here used in Tables IV.- VI. 



The deviations from Bragg's formula can now be examined. 



* E. Friman, " Untersuchungen liber die Hochfrequendspektra der 

 Elem.," Diss., Lund, 1916. 

 t W. Stenstrom, loc. cit. 



