Letts & Buake—The Carbonic Anhydride of the Atmosphere. 205 
anhydride, and hence reduce the amount in the air. According 
to the same observer, even when there is no frost, but the tem- 
perature falls considerably, a quantity of warm, and therefore 
light ground air containing carbonic anhydride, escapes—more, 
indeed, than when the temperature rises, and in this way he 
explains the excess of the gas found at night time in the air. 
Influence of the Seasons. 
Considering the well known and powerful effect of plants in 
decomposing carbonic anhydride under the influence of sunlight, 
it might be expected that a marked decrease in the amount of that 
gas in the atmosphere would occur during that period of the year 
when the days are long and plant-life is most active, that is to say, 
in spring and summer, or, for our latitudes say, from April to 
September ; while, on the other hand, an increase might be antici- 
pated when plant-activity ceases, and the days are short, com- 
mencing with the fall of the leaf in October and lasting until about 
the end of March. Butit must be remembered that during spring 
and summer—and especially the latter—the conditions are most 
favourable for the production and evolution of carbonic anhydride 
from the soil; while in autumn, and especially in winter, with the 
fall in temperature, putrefactive and fermentative changes are 
arrested, and the production and evolution of carbonic anhy- 
dride from the soil cease to a more or less corresponding extent. 
The effect, then, of the seasons on the amount of atmospheric car- 
bonic anhydride should be to induce two sets of actions which 
are opposed to each other, and tend to maintain a state of equili- 
brium: the absorbent action of plants in summer being compen- 
sated more or less by the additional evolution of carbonic anhydride 
from the soil; while in autumn and winter the want of activity on 
the part of plants is also compensated by the decreased evolution 
of the gas from the soil. But to what extent these opposing 
actions will affect each other it is impossible to foresee. 
De Saussure was of opinion that there was a very decided 
merease in the amount of atmospheric carbonic anhydride in sum- 
mer, and a decrease in winter. Hasselbarth and Fittbogen also 
found the lowest proportion of the gas in December, while Spring 
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