importance, and to prove this, it is only necessary to employ a test
glass (see figure) provided with a lateral tube, A, that terminates in a
lower tubulure, B, above which there is a contraction, C. Upon pouring
water into the lateral tube until the level reaches D, and placing
a particle of camphor on its surface, the camphor will be seen to
continually move about, even when the liquid has reached the upper
edge of the vessel. To reduce the level to various heights, it is only
necessary to revolve the tube in the cork through which it is fitted to
the tubulure. In proceeding thus, agitation or _collision_ of the water
is avoided; and yet if the test glass is very clean, the camphor will
continue to move at every level of the water.

But, some one will doubtless say, how do you explain the stoppage in the
motions of the camphor on the surface of water contained in vessels that
are not perfectly clean? Before answering this question, let us say in
the first place that the cause of the motions under consideration is due
to nothing else but the evaporation of this concrete oil--to effluvia
that escape from all parts and that exert upon the body whence they
emanate a recoiling action exactly like that which manifests itself in
an aelopile mounted upon a brasier, or, better yet, in the explosion of
a sky-rocket. A portion of these camphory vapors, as well as a small
portion of the camphor itself, dissolves in the water and forms upon its
surface an oily layer which is at first very slight, but the thickness
of which may increase in time until it becomes (especially if the vessel
is narrow) a mechanical obstacle to the gyration of the small fragments
of camphor that it imprisons, and whose evaporation it prevents. Now,
as this layer of volatile oil may and does evaporate, in fact, after a
certain length of time, the camphor then resumes its gyratory motions;
but there is not the least reason in the world for saying on that
account that it "has _habituated_ itself to the cause which had at first