All
considerations, those we have indicated as well as others which might
be invoked (for example, the recent researches of M. Spring on the
limit of visibility of fluorescence), give this result:--that there
are, in this space, some twenty thousand millions of molecules. Each
of these must receive in the space of a millimetre about ten thousand
shocks, and be ten thousand times thrust out of its course. The free
path of a molecule is then very small, but it can be singularly
augmented by diminishing the number of them. Tait and Dewar have
calculated that, in a good modern vacuum, the length of the free path
of the remaining molecules not taken away by the air-pump easily
reaches a few centimetres.
By developing this theory, we come to consider that, for a given
temperature, every molecule (and even every individual particle, atom,
or ion) which takes part in the movement has, on the average, the same
kinetic energy in every body, and that this energy is proportional to
the absolute temperature; so that it is represented by this
temperature multiplied by a constant quantity which is a universal
constant.
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