The fingers themselves, provided they are very clean, have no power to
stop the gyration. The following experiment, which is easy to repeat, is
an unquestionable proof of this.

Wash carefully the middle finger with aqua ammonia, and afterward with
plenty of water, and then dip it into a drinking glass in which a
fragment of camphor is rapidly moving, and the gyration will not be
stopped. But it will be made to stop instantly if the finger in
its natural state (that is, covered with the fatty substances that
ordinarily soil the fingers, especially in summer) be dipped into this
same glass.

_Movements of Camphor upon Mercury_.--In order to study the motions of
camphor, mercury possesses, as compared with water, a great advantage,
and that is that we can easily assure ourselves of the degree of
cleanliness of this metal by means of the condensed breath. The
vapory-deposits thereon in a uniform manner if the mercury is perfectly
clean, but forms variously shaded and more persistent spots if it is
soiled by foreign bodies But it is extremely difficult to clean mercury
completely. To do so Mr. Boisgiraud and I take distilled mercury and
leave it for a long time in contact with concentrated sulphuric acid,
taking care to often shake the mixture. Then, after removing the greater
part of the acid, we throw the metal into a vessel containing quick lime
in powder, and finally pass it through a filter containing a few holes
in its lower part.

Purified by this process, mercury not only permits of the motions of
camphor on its surface, but renders visible the traces of the vapors
that escape from it, and which resemble small tadpoles with a long tail