[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.]
As for ourself, we confess that, after repeating the curious experiments
of Mr. Dutrochet day after day, and scrupulously following his
directions, we have, in the presence of our results, that were exactly
identical with his, almost been tempted to believe ourself to be the
victim of some occult power, or at least of some optical illusion,
the true cause of which remained a mystery to us. Finally, after
many fruitless attempts to find a key to the enigma that engaged our
attention, the light finally dawned upon us, and then shone straight in
our eyes.
In comparing the last results of our experiments with those that we had
obtained previously, we saw, for example, that the camphor moved in the
test glasses at a level that was notably higher than that at which its
gyration took place the day before, or the day before that. And yet we
had always used the same vessels, the same water, and particles detached
from the same lump of camphor.
To what, then, could be due the difference observed between the two
levels at which we had, in the first and last place, seen the
camphor execute its movements? In the absence of any answer that was
satisfactory, we finally suspected that the difference that we had
noticed was ascribable to the fact that, after the numerous washings
that the apparatus had been submitted to in having water poured into
them to repeat the experiments, they had gradually been freed from
impurities of whatever nature they might have been, and which, unbeknown
to us, might have soiled their sides.
Starting with this idea, which was as yet a hyphothetical one, we began
to wash our hands, glasses, etc., at first with very dilute sulphuric
acid, and then with ammonia. Afterward we rinsed them with quantities of
water and dried them carefully with white linen rags that had been used
for no other purpose; and finally we plunged them again into very clean
water. We thus cut the Gordian knot, and were on the right track.
In fact, on again repeating Mr. Dutrochet's experiments, with that
minute care as to cleanliness that we had observed to be absolutely
necessary, we saw crumble away, one after another, all the pieces of
the scaffolding that this master had with so much trouble built up. The
camphor moved in all our vessels, of glass or metal, and of every form,
at all heights. The immersed bodies, such as glass tubes, table knives,
pieces of money, etc., had lost their pretended "sedative effect" on a
pretended "activity of the water," and on the vessels that contained
it. The so-called phenomenon of habit "transported from physiology into
physics," no longer existed.
The likening of the apparatus employed to obtain motions of camphor
upon water, with the entirely physiological apparatus by means of which
nature effects a circulation of the liquid contained in the internodes
of _Chara vulgaris_, had proved a grave error that was to be erased from
the science into which it had been introduced by its author with entire
good faith. The true cause of _life_ had not then been unveiled, and the
new agent designated as _diluo-electricity_ vanished before the very
simple and authentic fact that camphor moves rapidly upon the surface
of very pure mercury, in which no one would assuredly suppose that that
volatile substance could dissolve.
Mr. Dutrochet attaches great importance to the manner in which the water
is poured (with or without agitation) into the vessel with which
the experiment is performed. The matter is in fact of little or no
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 ælopile 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
influenced it, and that, too, in modifying itself in such a way as to
render null the influence of a cause that has not ceased to be present"
(Dutrochet, _l.c._., p. 50).
We have been enabled to convince ourself of the existence of this oily
layer of camphor when it was of a certain thickness by introducing under
the water on which it, had formed, a few drops of sulphuric ether whose
sudden evaporation produced sufficient cold to instantaneously congeal
the layer in question and thus render it perfectly visible to the eye.
The slight layer of greasy matter that habitually lines the sides of
vessels from whence no effort has been made to remove it, produces
effects exactly like those of the oil of camphor, that is to say, that
in measure as it becomes thicker it likewise arrests the motions of the
concrete volatile essence.
This is precisely what happens in a test-glass in which we see the
camphor in motion become immovable if the level of the water be raised a
few centimeters, and, more especially, if it be raised to the upper edge
of the apparatus. In its slow ascent the liquid _licks_ up, so to speak,
the oily layer that lines the inner surface of the vessel, and this
material spreads over the surface of the water and forms thereupon a
layer which, in spreading over the bit of camphor itself, prevents its
evaporation, and, consequently, its motions. The existence of the layer
under consideration cannot be doubted, since it is made to disappear by
causing the water to-overflow from the edges of the vessel, and, more
easily still, by spreading a piece of filtering paper over the liquid in
which the camphor is in a state of rest. As soon as the paper is
removed (without the water being touched by the fingers, it should be
understood), the camphor resumes its motions and afterward continues
them at all levels.
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
that are endowed with very great agility. Nothing is more curious than
to see the particle of camphor successively ascend and descend the
strongly pronounced curves presented by the mercury near the sides of
the vessel that contains it. On raising the temperature of the metal
slightly, the motions of the camphor on its surface are accelerated, and
the same effects occur with water that has been slightly heated.
The experiments that we have just called attention to show what
importance slight impurities may have upon certain results. "They
prove," says our learned colleague Mr. Daquin, "that there exists upon
polished substances an imperceptible coating of those fatty matters
which serve to-day to explain Moser's images." We find therein also a
manifest proof and a rational explanation of those grave errors into
which the presence of these fatty matters, that have hitherto been
scarcely suspected, led so clever and so distinguished a scientist as
the illustrious discoverer of endosmosis.--_N. Joly, in La Nature_.
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