188 
CHEMISTRY: F. W. CLARKE 
exact composition and configuration which give to normal lead its uniformity 
and stability. Whether or not the process of change can continue until 
normal lead is formed, it is impossible for us to say. 
The atoms of the chemical elements, are, as I have already said, extremely 
complex, but their structure is not yet completely understood. To some 
part of each kind of atom its chemical properties and its spectrum are proba- 
bly due. It is conceivable that this part may be the earliest to form, with 
its surrounding rings or envelopes at first not quite adjusted to permanent 
stability. With the final adjustment the isotopes as such should disappear, 
and the normal element be completed. This is speculation, and its legitimacy 
remains to be established. A careful comparison of the spectra of the ele- 
ments from thallium up to uranium might furnish some evidence as to its 
validity. The spectrum of uranium, for example, may contain lines which 
really belong to some of its derivatives. 
Note. — Since this paper was written, one by Professor Barrell 14 has appeared, 
in which the use of the uranium-lead ratio for determining the age of minerals 
is defended. There are also two papers by Holmes and Lawson, 15 and another 
by Holmes, 16 in which the same position is taken. There is evidently room 
for further discussion of the subject, but as yet I see no good reason to change 
my own views. 
[Published by permission of the Director of the U. S. Geological Survey.] 
1 J. Amer. Chem. $oc, Easton, Pa., 37, 1915, (1027). 
2 Ibid., 36, 1914, (1329). 
3 Ibid., 38, 1916, (2613). 
4 Ibid., 39, 1917, (531). 
5 Zsch. Elektrochem., Halle, 20, 1914, (457). 
6 Monatsh. Chem., Vienna, 36, 1915, (355). 
7 Nature, London, 94, 1915, (615). 
8 Chem. Abst., 11, 1917, (3173). From Physik. Zs., 18, 1917, (114). 
• Washington, U. S. Geol. Survey, Bidls. 78 and 90. Also in Bull. 591, pp. 366-368. Hille- 
brand discusses the mode of occurrence of these minerals, much as I have done. 
10 See Joly, Phil. Mag., (6), 22, 1911, (354). and Becker, Bull. Geol. Soc. Amer., 19, 1908, 
(134). 
11 Amer. J. Sci., New Haven, (4), 23, 1907, (86). 
12 Bull. Geol. Soc. Amer., (4), 19, 1908, (134). See also Zambonini, Rome, Atti Acc. Lincei 
(5), 20, part 2, 1911, (131). 
13 Popidar Science Monthly, January, 1873. 
14 Bull. Geol. Soc. Amer., 28, 1918, (745). 
15 Phil. Mag., (6), 28, 1914, (823), 29, 1915, (682). 
16 Proc. Geologist's Assoc., London, 26, 1915, (289). 
