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IX
VAPOUR-PRESSURE OF MERCURY
applied on account of this pressure to readings of barometers
and manometers. And it is the smallness of this pressure,
and not any special property of mercury itself, that explains
why the influence of mercury-vapour is negligible in discharge-
phenomena, although it is always present in Geissler tubes.
formula used appears to be satisfactorily established, and to be sufficiently tested
as far as the single hypothesis contained in it is concerned; so that it merits
at least as much confidence as an observation of such small quantities, which
must be difficult and deceptive. In addition to this, I may add that up to 50° I
could discover no perceptible pressure; whereas 0.10 mm., as given by Regnault,
or even 0.04 mm., as given by Hagen, could not have escaped observation.