[1] _Phil. Mag._, Aug. 1868.
[2] _Nature_, March 31, 1870.
These, of course, are exceedingly rough estimates, for they are
derived from measurements some of which are still confessedly very
rough; but if at the present time, we can form even a rough plan for
arriving at results of this kind, we may hope that, as our means of
experimental inquiry become more accurate and more varied, our
conception of a molecule will become more definite, so that we may be
able at no distant period to estimate its weight with a greater degree
of precision.
A theory, which Sir W. Thomson has founded on Helmholtz's splendid
hydrodynamical theorems, seeks for the properties of molecules in the
ring vortices of a uniform, frictionless, incompressible fluid. Such
whirling rings may be seen when an experienced smoker sends out a
dexterous puff of smoke into the still air, but a more evanescent
phenomenon it is difficult to conceive. This evanescence is owing to
the viscosity of the air; but Helmholtz has shewn that in a perfect
fluid such a whirling ring, if once generated, would go on whirling
for ever, would always consist of the very same portion of the fluid
which was first set whirling, and could never be cut in two by any
natural cause.
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