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67
are the motion of the air (the strength of the wind), haze, and
pressure. The time of year, loo, may have an influence, but this is
in any case much less than that of Ihe daily varialions. The CO,
percentage seems in general to be somewhat greater in winler than
in summer. My figures also give a slight increase from June to
September (see p. 52).
To the non-periodical fluctuations belong different average figures
for different years. For the CO, percenlage during Ihe years 1898
—1901 Brown and EscOoMBE obtained the average values of 2,ss,
2.88, 2.86, and 3.11 per ten thousand. Reınau (1920, p. 6) considers
that the high value for 1901 was due to the fact that it was a rainy
summer and that there was therefore less assimilalory aclivily in
the vegelation. ManiÉ-Davy (Comples rend., 1880, p. 32) also found
an inverse proportionalily between condilions favourable for assi-
milation and the average CO, percentage for the years 1876—1879.
Far greater varialions than those mentioned above are seen in
the case of the air near the ground, expecially where thick vegetalion
is present. It is a well-known fact that large quanlilies of carbon
dioxide are found in the air enclosed in cavilies of the ground.
Fovor, PETTENKOFER, and others have made numerous invesligations
into the CO, percentage of the ground-air under varying condilions
(see WoLLny, Forsch. d. Agrikulturphysik, Vol. 5). It varies between
about 0.3 and 3.5%. The influence of the temperature is of special
importance. The CO, percentage rises rapidly wilh the temperature
up to about 60°, and this is no doubt largely due to the fact that
the CO, production is a biological process, induced by bacteria and
fungi. The dampness of the ground also has a great influence.
The CO, percentage rises with walering. The water too probably
works here purely physically, by driving out the carbon dioxide
present in the ground through absorption.
In regard to the quantities of CO, carried over to the atmosphere
from the ground hardly any statements are to be found. À priori
it is of course probable that the carbon dioxide of the air paitly
comes from the ground, and that all the fac ors that increase the
CO, production of the ground also increase the CO, percentage in
the lowest regions of the air. Purely theorelical discussions on the
basis of the CO, values of the ground-air or the giving off of CO,
from isolated clods of soil (see on this point PETERSEN, Versuchs-
slationen 1871, p. 165) will be found in LETTS and BLAKE (p. 215) and
Reınau (1920, p. 158). Besides high temperature and dampness, wind
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