of the real
value, hence no deductions could be drawn from the observed facts.
For this reason larger volumes of air were taken, and a current of air,
whose volume could be accurately measured by known methods, was passed
through condensers capable of retaining the carbonic acid. But in this
case the air must pass very slowly through it, so that the process may
last several hours; and since the air is continually in motion, owing to
vertical and horizontal currents, the experiment may be begun with the
air of one place, and concluded with air from a far distant spot. For
example, if an experiment lasting twenty-four hours was made in Paris
when the air moved but four meters per second (nine or ten miles per
hour), it might be begun with air from the Department of the Seine, and
end with air from the Department of the Rhone, or the Belgian frontier,
according to the direction of the wind.
So long as we had no analytical methods of sufficient delicacy to
estimate with certainty the hundredth, or at least the tenth of a
milligramme of carbonic acid, it was very difficult to determine the
quantity in the air at a given time and place. It is frequently possible
to analyze upon the plain air that has descended from the heights
above, and to examine by bright daylight the effect of night upon the
atmosphere.
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