254 
PHYSICS: L. W. MCKEEHAN 
Proc. N. a. S. 
investigation to other metals. Targets of the various metals can be in- 
serted and the characteristics of each studied under the same conditions 
by moving them successively into the path of the primary electron stream. 
There is no reason to believe that the secondary electron characteristics 
should be the same for all metals, and a knowledge of their characteristics 
in this respect should be of value in connection with the electron theory of 
metals. 
A more detailed account of this work will be published in the Physical 
Review in the near future. 
iGehrts, Ann. Physik, 36, 1911, p. 995. 
0. von Baeyer, Physik. Zs., 10, 1909, p. 176; Deut. Phys. Ges., Verh., 10,24,1908, 
p. 953; Ber. D. Physik Ges., 10, 1910, p. 96 and p. 953. 
N. R. Campbell, Phil. Mag., 22, 1911, p. 276; 24, 1912, p. 527; 25, 1913, p. 803; 
28, 1914, p. 286; 29, 1915, p. 369. 
A. W. Hull, Physic. Rev. 7, 1916, p. 1. 
F. Horton and Miss A. C. Davies, Roy. Soc. Proc. London, 97A, 1920, p. 23. 
1. G. Barber, Physic. Rev., 17, 1921, p. 322. 
Davisson and Kunsman, Science, 54, 1921, p. 522. 
THE CRYSTAL STRUCTURE OF POTASSLUM 
By Iv. W. McKeehan 
Research Laboratories of the American Telephone and Telegraph Company, 
AND THE Western Electric Company, Incorporated. 
Communicated June 13, 1922 
Potassium, when examined by X-rays at room temperature appears to 
be amorphous, as previously reported by Hull.^ But if potassium, cast 
in a thin- walled cylindrical glass tube, is cooled to — 150° C. in a current 
of air by the method recently employed in getting the crystal structure of 
mercury, 2 large crystals, giving well marked reflections of the K-radiation 
of molybdenum, are formed. These crystals are, in fact, so large that it 
is a matter of some difficulty to obtain enough different orientations in a 
single sample, and the data are not of great accuracy. Continuous rota- 
tion of the sample about the axis of the cylinder so weakens the intensity 
of the occasional reflections that they can no longer be located with cer- 
tainty. The best results are obtained by rotating the sample through small 
angles at such long time-intervals that individual crystals in suitable 
orientations have long enough to affect the film. 
The best fit vvith the data is furnished by assuming the atoms to be 
arranged in a body-centered cubic lattice with a = 5.20 X 10 ~^ cm. 
and therefore with a density 0.92 gm./cm.^ This density is higher than 
