THE COMPOSITION OF ATMOSPHERIC AIR 5 
The contaminations of CO: caused by large towns 
are also fairly localised. At Kew, about 6 miles west 
of the centre of London, on the average only 27 ppm 
more CO; are found in easterly than in westerly winds. 
Argon (A). The constancy of the composition of air 
with respect to its minor constituents has been investi- 
gated in the cases of A and He. The former was in- 
vestigated by Moissan [e. 16], who, after chemical 
removal of Oo, No, and CO by calcium metal, measured 
the remaining rare gases and obtained for the A content 
the figures shown in Table IV. The standard deviation 
of these analyses (excepting the value of 0.949 over 
the Atlantic Ocean which must be considered errone- 
ous) is 0.002 per cent, or 0.2 per cent of the A con- 
tent, and within these limits there are no significant 
variations. 
Taste IV. Survey or Argon Contents or AIR 
(After Moissan |[c. 16]) 
Location A (per cent) 
OWERRR. 20's Reet BES te ette Oe eee Seat ee 0.935 
St. Petersburg (Leningrad)............... 0.933 
JNUIDGTDG, 55855 6 SR oma ae ME eae eee 0.935 
Tonian Sea (87°N, 15°E).................. 0.936 
Witeninapreir ey. ar ctotn chic dternaniee sa aateaiveies 0.938 
IBETININ 5 1g oo baie Oe Se EO IE OEE ieee aeeen 0.932 
WODIG®. . 0:46 pig Ole ne eRe ee ere 0.936 
IMU [SING Gortcea gia lea chee cree ce cae Seca 0.935 
PONG). ocd eb ange es Sone eee ee eee 0.934 
TLOMDGIOD, soe cA pene ee oe Oe eRe Sone CR eee 0.983 
Atlantic Ocean (37°N, 24°W)............. 0.932 
Atlantic Ocean (48°N, 22°W)........... (0.949) 
Average (omitting the last value). ...| 0.9348 + 0.0006 
Helium (He). Similar results were also obtained with 
He in spite of the fact that vast quantities of He 
constantly escape from the earth’s crust, particularly 
from oil fields in the United States. It has been esti- 
mated that between eight and thirty million cubic 
metres of He are generated annually by radioactive 
processes. The amounts of He added to the atmosphere 
in this way are balanced by losses of He into the void 
of the universe, because He, owing to its lightness, is 
not permanently retained by the gravitational field of 
the earth. 
A world survey covering all continents and oceans 
from the Arctic to the Antarctic [12] showed no sig- 
nificant deviations even comparatively near oil fields 
in the United States. The value obtained was 5.239 
= 0.002 ppm with a standard deviation of +0.008 
ppm [11]. It is apparent that the turbulence of the 
troposphere quickly eliminates any nonuniformity re- 
sulting from localised He discharge. 
ATMOSPHERIC GASES OF CONSTANT 
PERCENTAGE 
While surveys over large areas have been carried 
out only for the four gases Oz, COs, A, and He, the 
constancy in the total percentage of these gases makes 
it plausible that other gases too must have a constant 
total percentage, unless they are subject to vapour- 
pressure equilibria (as is H.O) or to radiation equilibria 
(as is O3), or are simply the result of some industrial 
activity (as are SO: and I). 
Nitrogen (V2). For the analysis of N»2 no direct pre- 
cision method has been discovered. However, as the 
sum of nitrogen and rare gases (called “atmospheric 
nitrogen” by Krogh) has a constant value of 79.0215 
per cent, and as this sum is constant to at least 0.002 
per cent, N2 too must be constant to the same degree. 
Neon (Ne). The most reliable data for the Ne content 
of air are (1) a single analysis by Watson [c. 11] who 
found 18.2 ppm, (2) three analyses by Glueckauf [11] 
who found a mean of 18.21 + 0.04 ppm, and (8) recent 
analyses by Chackett, Paneth, and Wilson [6] who 
found 18.1 + 0.08 ppm. 
Krypton (Kr) and Xenon (Xe). Of the figures in the 
literature [c. 16], those by Moureu and Lepape and 
by Damkohler appear to be the most reliable.. As 
these values are accurate only to +10 per cent of their 
value, the Kr and Xe contents have recently been re- 
determined with a much higher accuracy by the author 
of this article. The figures in parts per million by 
volume are: 
Moureu + Lepape (1926) Ar: 1.0 +0.1, Xe:0.09 + 0.01 
Damkohler (1935) Kr: 1.08 + 0.1, Xe: 0.08 + 0.01 
Glueckauf + Kitt (unpub- 
lished) Kr: 1.14 + 0.01, Xe: 0.087 + 0.001 
Nitrous Oxide (V.O). The presence of nitrous oxide 
in atmospheric air was discovered by Adel [15, Chap. 
10] by means of an absorption band at 7.8 » in the 
solar spectrum. Since then further atmospheric ab- 
sorption bands have been discovered at 3.9 u, 4.5 p, 
and 8.6 uw. The recent chemical analysis by Slobod and 
Krogh [20] of N20 in air at ground level gave a value 
of 0.56 + 0.1 ppm, which is in agreement with the 
spectroscopic data. 
Methane (CH,). Methane was found by spectro- 
scopic identification of its absorption band in sunlight 
modified by the passage through the earth’s atmosphere 
over Columbus, Ohio (Migeotte), over Flagstaff, Ari- 
zona (Adel), and over Pontiac, Michigan (McMath 
Observatory) [15, Chap. 10]. From the latter data the 
CH, content of air has been estimated to be about 1.2 
ppm (by weight), that is, about 2.2 X 10-® by volume. 
It is possible that this figure is somewhat high, as 
during the process of distillation of air it is found that 
the Kr and Xe fraction contaims only an amount of 
CH, roughly equal to that of these gases (1.2 x 1075 
by volume). 
We are faced with the question of the origin of this 
CH, which is constantly destroyed by the ozone in 
atmospheric air. As a constant source of this CH, we 
may consider either decay of biological products, or 
gas escaping from oil wells, or both. The question of 
the relative extent of these two processes can be decided 
by determining the content of radiocarbon (4C) in the 
CH, of air. Methane from biological sources contains 
0.95 * 10-2 @ of “C per g of C, while mineral CH, 
is inactive. 
At the suggestion of the author, Prof. F. W. Libby 
at the University of Chicago analysed the “C content of 
