ABUNDANCE OF CHEMICAL ELEMENTS—SUESS “309 
Iron meteorites consist of metal and small amounts of sulfide only. 
Other meteorites are composed entirely of silicate. 
A large fraction of meteorites, primarily the so-called chondrites, 
contain all three phases in remarkably constant proportions. It is 
generally believed that the chondrites contain all the condensable, 
nonvolatile components of solar matter in approximately primeval 
proportion. In any case it seems unlikely that chemically similar 
elements were separated from each other when the meteorites formed 
from a gas cloud because of the incompleteness of the separation of 
the three main phases. 
Harold C. Urey compared the mean densities of meteorites with 
those of the terrestrial planets and the moon and concluded that aver- 
age chondrites and the moon probably contain the same ratio of metal 
to silicate, whereas the earth and the terrestrial planets contain rela- 
tively more metal. During the formation of the terrestrial planets 
a fraction of the silicate originally present escaped condensation. 
The much lower densities of the outer planets, Jupiter, Saturn, 
Uranus, Neptune, and Pluto, show that these planets have retained 
most of the volatile substances, a large part of the hydrogen and 
helium, the oxygen as water, nitrogen as ammonia, and carbon as 
methane. 
Clearly, chemical analysis of meteorites cannot tell us anything 
about the solar abundances of the rare gases and of elements that 
form such highly volatile compounds. The concentration of many 
other elements in meteorites, however, shows a surprising agreement 
with the results of astronomical data. The relative amounts of ele- 
ments such as sodium, aluminum, silicon, potassium, or calcium are 
found to be the same in the sun and in meteorites within the limits 
of errors of the analytical methods. Some values for heavier ele- 
ments, for example strontium and barium, are also in perfect 
agreement. 
True differences undoubtedly exist in the case of the lightest ele- 
ments, particularly for lithium, beryllium, and boron. The concen- 
trations of these elements on the surface of the sun are much smaller 
than those expected from the data on meteorites. The abundance 
of lithium relative to silicon on the sun is less than one-hundredth 
of that in meteorites. Beryllium and boron may be almost completely 
absent. 
The deficiency of these elements on the sun is now understood as a 
consequence of thermonuclear reactions in the sun’s interior. These 
reactions use up these elements as well as the heavy hydrogen isotope, 
deuterium. At the high temperatures of the sun’s interior these ele- 
ments react with protons and form either helium or heavier nuclear 
species. The nature of these reactions is now known in detail. It 
is also known that the isotopes of carbon and nitrogen participate 
