PHYSICS: P. W. BRIDGMAN 
351 
1 Shapley, Harlow, Mt. Wilson Contr., No. 152, 1917 (1-28), pp. 22, 23 and footnotes; 
No. 157, 1918 (1-26), p. 10; No. 161, 1918 (1-35), sections I, II, and III. 
2 No. 157, 1918 (1-26), p. 12; No. 161, 1918 (1-35), section IV. 
^Ibid., No. 157, 1918 (1-26); No. 161, 1918 (1-35), sections VII and VIII. 
*Ibid., No. 161, 1918 (1-35), section VIII; No. 157, 1918 (1-26), pp. 12-14. 
^ But whether the absence of clusters is real or only apparent, we must remember that 
the assignment of a definite thickness of three or four thousand parsecs to the galactic seg- 
ment is mostly a matter of convenience and approximation; the intention is merely to sug- 
gest that practically every known object except spiral nebulae and globular clusters is 
within a thousand parsecs or so of the central plane of a greatly extended, indefinitely bounded 
stellar stratum. 
6 Shapley, Harlow, Mt. Wilson Contr., No. 152, 1917 (1-28), fig. 1; No. 161, 1918 (1-35), 
section V. 
Ubid., No. 152, 1917 (1-28), p. 22, footnote 2. 
^Ibid., No. 115, 1915 (1-21), p. 11 and fig. 1. 
9 Pickering, E. C, Ann. Obs. Harvard Coll., Cambridge, Mass., 26, 1891 (260-286). 
"Adams, W. S., and van Maanen, A., Astr. J. Albany, N. Y., 27, 1913, p. 187. 
" Melotte, P. J., Mem. R. Astr. Soc, London, 60, 1915 (175-186). 
12 Kapteyn, J. C, Mt. Wilson Contr., Nos. 82 and 147, Astrophys. /., Chicago, III., 40, 
1914 (43-126), 47, 1918 (104-133, 146-178, 255-282). 
"Plummer, H. C, Mon. Not. R. Astr. Soc, London, 73, 1913 (174-191). 
" Charlier, C. V. L., Meddelanden Lunds Astr. Obs., Lund, Series 2, No. 14, 1916 (1-108). 
15 Bailey, S. I., Ann. Obs. Harvard Coll., Cambridge, Mass., 60, 1908, No. VIII. 
i^Newcomb, S., Pub. Carnegie Inst., Washington, D. C, No. 10, 1904 (1-32). 
" Shapley, Harlow, Mt. Wilson Contr., No. 157, 1918 (1-26), p. 23. 
18 Barnard, E. E., Astrophys. J., Chicago, III., 49, 1919 (1-23). 
A COMPARISON OF CERTAIN ELECTRICAL PROPERTIES OF 
ORDINARY AND URANIUM LEAD 
By p. W. Bridgman 
Jefferson Physical Laboratory, Harvard University 
Communicated, June 17, 1919 
A comparison of the physical properties of chemical isotopes is of 
significance because of the light it may throw on the corresponding 
mechanisms. Comparisons of the properties of ordinary and uranium 
lead have hitherto been made with respect to the atomic volume,^ ther- 
moelectric quality, 2 and emission spectra.^ No differences have been 
detected, except possibly a very slight shift in one of the spectrum lines. 
It is not to be expected that large differences exist with regard to other 
physical properties, but nevertheless a verification by direct experiment 
is not without interest. 
Through the kindness of Prof. T. W. Richards there was made avail- 
able for me 20 grams of lead of radio-active origin on which he has 
already published chemical data,^ and also a similar quantity of puri- 
