[Footnote: Continued from SUPPLEMENT No. 391, page 6240.]


To have these movements occur in a constant and invariable manner upon
the surface of water, and especially upon mercury, it is necessary to
take precautions in regard to cleanliness, this being something that
we have purposely neglected to mention to our readers. For we wished,
through this voluntary omission, to stimulate their sagacity by bringing
them face to face with difficulties that they will perhaps have
succeeded in overcoming, with causes of error that they will have
perceived, and the principal one of which is the want of absolute
cleanliness in the water, vessels, and instruments that they may have
used for the experiments.

Thus, very probably, they will have more than once seen the camphor
remain immovable when placed in vessels in which they had hoped to
be able to see it undergo its gyratory and other motions. Their
astonishment will have been no less than our own was when we noticed
the sudden cessation of the camphor's motions under the influence of
vitreous or metallic objects, such as glass rods or tubes, pieces of
gold, silver, or copper coin, table knives, etc., dipped into the liquid
in which such motions were taking place before the immersion of the
objects under consideration.

The instantaneously _sedative_ power of the human fingers, or of a hair,
will have, perhaps, reminded them of some sort of sorcery, or of some
diabolic art worthy of the great Albert.

[Illustration: APPARATUS FOR THE STUDY OF THE MOTIONS OF CAMPHOR.]