282 
CHEMISTRY: HARKINS AND WILSON 
the composition of the odd and even groups as found for the elements of 
smaller atomic weight hold so far into the system as the radioactive 
elements. The general form of the system indicates at least that the 
heavy atoms contain more heHum than independent hydrogen units, 
and this seems in accord with the fact that uranium loses alpha particles 
in eight steps without any apparent loss of a hydrogen nucleus. 
The stabiHty with which the hydrogen nuclei which are not con- 
tained in heUum groups, but which generally occur in threes (H3 in the 
table) , are built into the complex atom is not in general indicated with any 
degree of accuracy, but in the case of Kthium it seems to be great, for 
lithium shows the extremely large packing effect equal to 1.57%, which 
might seem doubtful if it were not for the extreme care taken by Rich- 
ards and Willard^ in the determination of the atomic weight of Kthium. 
The atomic weight of hydrogen used in these papers (1.0078) is probably 
not in error^o by so much as one part in 5000. 
The hydrogen-heUum system here presented is entirely in accord 
with, but independent of, the astronomical theory that the order in 
which the elements appear in the stars is first nebulium, hydrogen and 
helium, then such of the Kghter elements as calcium, magnesium, 
oxygen, and nitrogen, and finally iron, and the other heavy metals, 
although in the present system it has not been found necessary to in- 
clude nebuHum. Some of the nebulae give bright line spectra of nebu- 
lium, hydrogen and helium; such Orion stars as those of the Trapezium 
give lines for hydrogen and heHum; while those that are more developed 
show magnesium, sihcon, oxygen and nitrogen, and some of the other 
low atomic weight elements in addition. Bluish white stars such as 
Sirius give narrow and faint lines for iron, sodium, and magnesium, 
and the solar stars give a much weaker hydrogen spectrum, and many 
more and stronger Hues for iron and the heavy metals.^^ The astro- 
nomical theory that the heavier elements are thus formed from those of 
smaller atomic weight is of extreme interest, but the evidence for it 
is somewhat uncertain, since it is possible that it is the difference in 
the density of the different elements which is the effective factor in caus- 
ing the spectra to appear in the order in which they are found to occur. 
The relative brightness of the different Knes varies greatly, such lines as 
the calcium H and K lines being extremely strong, and this also inter- 
feres with the determination of the order of the appearance of the ele- 
ments in the stars. On the other hand the evidence presented in these 
papers seem to give some support to the theory of the evolution of the 
heavier atoms from those which are lighter, but the hydrogen-helium 
