"
But it may be urged that the change of density, in these experiments,
has not been carried far enough to justify the enunciation of a law of
molecular physics. The condensation into less than one-third of the
space does not, it may be said, quite represent the close file of men
across Pall Mall. Let us therefore push matters to extremes, and
continue the condensation till the vapor has been squeezed into a
liquid. To the pure change of density we shall then have added the
change in the state of aggregation. The experiments here are more easily
described than executed; nevertheless, by sufficient training,
scrupulous accuracy, and minute attention to details, success may be
insured. Knowing the respective specific gravities, it is easy, by
calculation, to determine the condensation requisite to reduce a column
of vapor of definite density and length to a layer of liquid of definite
thickness. Let the vapor, for example, be that of sulphuric ether, and
let it be introduced into our 38 inch tube till a pressure of 7.2 inches
of mercury is obtained. Or let it be hydride of amyl, of the same
length, and at a pressure of 6.6 inches. Supposing the column to
shorten, the vapor would become proportionally denser, and would, in
each case, end in the production of a layer of liquid exactly one
millimeter in thickness.[1] Conversely, a layer of liquid ether or of
hydride of amyl, of this thickness, were its molecules freed from the
thrall of cohesion, would form a column of vapor 38 inches long, at a
pressure of 7.
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