Let us suppose the column to shorten, without change in the
quantity of matter, until the molecules are so squeezed together as to
resemble the closed file across Pall Mall. During these changes of
density, would the action of the molecules upon a beam of heat passing
among them at all resemble the action of the crowd upon the runner?
We must answer this question by direct experiment. To form our molecular
crowd we place, in the first instance, a gas or vapor in a tube 38
inches long, the ends of which are closed with circular windows,
air-tight, but formed of a substance which offers little or no
obstruction to the calorific waves. Calling the measured value of a heat
beam passing through this tube 100, we carefully determine the
proportionate part of this total absorbed by the molecules in the tube.
We then gather precisely the same number of molecules into a column 10.8
inches long, the one column being thus three and a half times the length
of the other. In this case also we determine the quantity of radiant
heat absorbed. By the depression of a barometric column, we can easily
and exactly measure out the proper quantities of the gaseous body. It is
obvious that one mercury inch of vapor, in the long tube, would
represent precisely the same amount of matter--or, in other words, the
same number of molecules--as 31/2 inches in the short one; while 2
inches of vapor in the long tube would be equivalent to 7 inches in the
short one.
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