GRAVITATION.
The Forms of Matter--Shape of the Earth--Probability of the Mass
forming this Planet having existed in a Nebulous State--Zodiacal
Lights--Comets--Volatilization of Solid Matter by Artificial
means--The principle of Gravitation--Its Influence through
Space and within the smallest Limits--Gravitating powers of the
Planets--Density of the Earth--Certainty of Newton’s Law of the
Inverse Square--Discovery of Neptune--State of a Body relieved from
Gravitation--Experiment explaining Saturn’s Ring, &c.--General
inference.
Let us suppose the earth--consisting of three conditions of matter;
the solid, the fluid, and the aëriform--to be set free from that power
by which it is retained in its present form of a spheroid flattened
at the poles, but still subject to the influences of its diurnal and
annual rotations. Agreeably to the law which regulates the conditions
of all bodies moving at high velocities, the consequence of such a
state of things would be, that our planet would instantly spread itself
over an enormous area. The waters and even the solid masses of this
globe would, in all probability, present themselves amidst the other
phenomena of space in a highly attenuated state, revolving in an orbit
around the sun, as a band of nebulous matter, which might sometimes
be rendered sensible to sight by still reflecting solar light, or by
condensation in the form of flights of shooting stars.[13]
This may be illustrated by experiment. If upon a rapidly revolving
disc we place a ball of dust, it will be almost immediately spread
out, and its particles will arrange themselves in a series of regular
curves, varying with the velocity of the motion. In addition to the
disintegration which would arise from the tendency of the atoms to fly
from the centre, the motion, in space, of the planetary mass would
naturally occasion a trailing out, and the only degree of uniformity
which this orb could, under these imaginary conditions, possibly
present, would be derived from the combined effects of motions in
different directions.
Amid the remoter stars, some remarkable cloud-like appearances are
discovered. These nebulæ, presenting to the eye of the observer
only a gleaming light, as from some phosphorescent vapour, were
long regarded as indications of such a condition as that which we
have just been considering. Astronomers saw, in those mysterious
nebulæ, a confirmation of their views, which regarded all the orbs
of the firmament as having once been thin sheets of vapour, which
had gradually, from irregular bodies traversing space, been slowly
condensed about a centre, and brought within the limits of aggregating
agencies, until, after the lapse of ages, they become sphered stars,
moving in harmony amid the bright host of heaven.[14] Geologists seized
on those views with eagerness, as confirming theoretical conclusions
deduced from an examination of the structure of the earth itself, and
explained by them the gradual accretion of atoms into crystalline rocks
from a cooling mass.
The researches of modern astronomers, aided by the magnificent
instruments of Lord Rosse,[15] have, however, shown that many of the
most remarkable nebulæ are only clusters of stars; so remote from
us, that the light from them appears blended into one diffused sheet
or luminous film. There are, however, the Magellanic clouds, and
other singular patches of light, exhibiting changes which can only
be explained on the theory of their slow condensation. There is no
evidence to disprove the position that world-formation may still be
going on; that a slow and gradual aggregation of particles, under the
influence of laws with which we are acquainted, may be constantly in
progress, to end, eventually, in the formation of a sphere.
May we not regard the zodiacal light as the remains of a solar
luminiferous atmosphere, which once embraced the entire system of which
it is the centre?[16] Will not the strange changes which have been
_seen to take place_ in cometary bodies, even whilst they were passing
near the earth,--as the division of Biela’s comet and the ultimate
formation of a second nucleus from the detached portion,--strongly
tend to support the probability of the idea that attenuated matter
has, in the progress of time, been condensed into solid masses, and
that nebulous clouds must still exist in every state of tenuity in the
regions of infinite space,[17] which, in the mysterious processes of
world-formation, will, eventually, become stars, and reflect across
the blue immensity of heaven, in brightness, that light which is the
necessary agent of organisation and all manifestations of beauty?
The inferences drawn from a careful study of the condition of our own
globe are in favour of the assumption of the existence of nebulous
matter. By the processes of art and manufacture, by the operation of
those powers on which organisation and life depend, solid matter is
constantly poured off in such a state that it cannot be detected, _as
matter_, by any of the human senses. Yet a thousand results, daily and
hourly accumulating as truths around us, prove that the solid metals,
the gross earths, and the constituents of animal and vegetable life,
all pass away invisible to us, and become “thin air.” We know that,
floating around us, these volatilized bodies exist in some material
form, and numerous experiments in chemistry are calculated to convince
us, that the most attenuated air is capable, with a slight change
of circumstances, of being converted into the condition of solid
masses. Hydrogen gas, the lightest, the most ethereal of the chemical
elements, dissolves iron and zinc, arsenic, sulphur, and carbon; and
from the transparent combinations thus formed, we can with facility
separate those ponderous bodies. Such substances must exist in our
own atmosphere; why not in the regions of space? Whether this planet
ever floated a mass of nebulous matter, only known by its dim and
filmy light, or comet-like rushed through space with widely eccentric
orbit, are questions which can only receive the reply of speculative
minds. Whether the earth and the other members of the Solar System were
ever parts of a Central Sun,[18] and thrown from it by some mighty
convulsion, though now revolving with all the other masses around that
orb, chained in their circuits by some infinite power, is beyond the
utmost refinements of science to discover. This hypothesis is, however,
in its sublime conception, worthy of the master-mind that gave it birth.
All we know is, that our earth is an oblate sphere, which, by the
effects of its rotation around an axis, is somewhat enlarged at the
equator and flattened at the poles;--that it maintains its regular
course around the sun, in virtue of the operation of two forces, one
of which, acting constantly, would eventually draw it into the body
of the sun itself; but that it is opposed by the other, centrifugal
force, and the varying momentum of the revolving mass;--that the same
force acting from the centre of the earth itself, and from the centre
of every particle of its substance, resolves the whole into a globular
form.
The principle of Gravitation[19] is that force which resides in every
form of matter, by which particle is attracted by particle, and mass
by mass, the less towards the greater. What this may be, we scarcely
dare to speculate. In the vast area of its action, which opens before
the eye of the mind, we see a power spanning all space, and linking
together every one of those myriads of worlds which spangle the robe
of the Infinite, and we are compelled to pause. Is this principle of
gravitation a property of matter, or is it a power higher than the
more tangible forces, is the question which presses on the mind. If
we regard it as a subtile principle pervading all space, we compel
ourselves to look beyond it for another power yet more refined; and we
cannot halt until, ascending from the limitable to the illimitable, we
resolve gravitation and its governing influences to the centre of all
power--the will of the eternal Creator.
Science has developed the grand truth, that it is by the exercise
of this all-pervading influence that the earth is retained in its
orbit--that the pellucid globe of dew which glistens on the leaf is
bound together--that the débris which float upon the lake accumulate
into one mass--that the sea exhibits the phenomena of the tides--and
the aërial ocean its barometric changes. In all things this force is
active, and throughout nature it is ever present. Our knowledge of the
laws which it obeys, enables us to conclude that the sun and distant
planets are consolidated masses like this earth. We find that they have
gravitating power, and by comparing this influence with that exerted
by the earth, we are enabled to weigh the mass of one planet against
another. In the balance of the astronomer, it is as easy to poise the
remote star, as it is for the engineer to calculate the weight of the
iron tunnel of the Menai Straits, or any other mechanical structure.
Thus throughout the universe the balance of gravitating force is
unerringly sustained. If one of the most remote of those gems of light,
which flicker at midnight in the dark distance of the starry vault,
was, by any power, removed from its place, the disturbance of these
delicately balanced mysteries would be felt through all the created
systems of worlds.
From the peculiarity of the laws which this power called gravity obeys,
it has been inferred that it acts from centres of force; it is proved
that its power diminishes in the inverse ratio of the square of the
distance, and that the gravitating power of every material body is in
the direct proportion of its mass. In astronomical calculations we have
first to learn the mass of our earth. Experiment informs us that the
density of our hardest rock is not above 2·8; but from the enormous
pressure to which matter must be subjected, at great depths from the
surface, the weight of the superincumbent mass constantly increasing,
it is quite certain that the earth’s density must be far more than
this. Maskelyne determined the attraction of large masses by a plummet
and line on the mountain Schehallion.[20] Cavendish, with exceedingly
delicate apparatus, observed the attraction of masses of known
weight and size upon each other. Applying the powers of arithmetical
calculation, and the data obtained from the small experiments to the
larger phenomena, Maskelyne determined the earth’s mean density to
be 4·71, whilst Cavendish made it 5·48, but the more recent refined
investigations of Baily have determined it to be 5·67.[21]
From data thus obtained by severe inductive experiments and
mathematical analyses, the astronomer, by observing the deviations of
a distant star, is enabled to determine the influence of those stellar
bodies near which it passes, and, hence, to calculate the relative
magnitudes of each. The accuracy of the law is in this way put to the
severest test, and the precision of astronomical prediction is the
strongest proof of its universality and truth.
Rolling onward its lonely way, in the far immensity of our system,
the planet Uranus was discovered by the elder Herschel,--so great
its distance that its diminished light could scarcely be detected
by the most powerful telescopes; but since its discovery its path
has been carefully watched, and some irregularities noticed. Most
of these disturbances were referable to known causes; but a little
alteration in its rate of motion observed when the planet was in one
portion of its vast orbit was unexplained. Convinced of the certainty
of Newton’s law, and having determined that the attraction of known
masses was insufficient to produce the disturbance observed, these
deviations were referred to the gravitating influence of a mass
beyond the known limits of our Solar System. By the investigations
of Adams in England,[22] and Le Verrier in France,[23] the place of
the hypothetical mass was determined, and its size computed. As a
grand confirmation of the great law, and to the glory of those two
far-searching minds, who do honour to their respective countries and
their age, the hypothesis became a fact, in the discovery of the planet
Neptune in the place determined by rigorous calculation. Astronomy
affords other examples of the sublime truth of the law of gravitation,
than which science can afford no more elevated poetry.
So completely is all nature locked in the bonds of this infinite power,
that it is no poetic exaggeration to declare, that the blow which rends
any earthly mass is conveyed by successive impulses to every one of the
myriads of orbs, which are even too remote for the reach of telescopic
vision.
An illustrative experiment must close our consideration of relative
operations of rotation and gravitation. We well know that a body in
a fluid state would, if suspended above the earth, it being at the
same time free to take any form, naturally assume that of a flattened
spheroid, from the action of the mass of the earth upon it: whereas the
force of cohesive attraction acting equally from all sides of a centre,
would, if uninfluenced, necessarily produce a perfect sphere. The best
method of showing that this would be the case, is as follows:--
Alcohol and water are to be mixed together until the fluid is of the
same specific gravity as olive oil. If, when this is effected, we
drop globules of the oil into the mixed fluid, it will be seen that
they take an orbicular form;--and, of course, in this experiment the
power of the earth’s gravitating influence is neutralized. The same
drops of oil under any other conditions would be flattened. Simple as
this illustration is, it tells much of the wondrous secret of those
beautifully balanced forces of cohesion and of gravitation; and from
the prosaic fact we rise to a great philosophical truth. Our experiment
may lead us yet farther in exemplification of known phenomena. If we
pass a steel wire through one of those floating spheres of oil, and
make it revolve rapidly and steadily, thus imitating the motion of a
planet on its axis, the oil spreads out, and we have the spheroidal
form of our earth. Increase the rapidity of this rotation, and when a
certain rate is obtained the oil widens into a disc, a ring separates
itself from a central globe, and at a distance from it still revolves
around it.[24] Here we have a miniature representation of the ring
of Saturn. This is a suggestive experiment, the repetition of which,
by reflective minds, cannot fail to lead to important deductions. The
phenomena of cohesion, of motion, and gravitation, are all involved;
and we produce results resembling, in a striking manner, the conditions
which prevail in the planetary spaces, under the influence of the same
powers. If we take a glass globe, and having filled it with a fluid of
the proper density, drop into it large and small globules of oil, we
may produce an instructive representation of the stellar vault, with
its beautiful spheres of light revolving in their respective orbits;
and though crossing each other’s paths, still moving in obedience to
attracting and repelling forces--onward in perfect harmony.
From the centre of our earth to the utmost extremity of the
universe--from the infinitely small to the immensely vast--gravitation
exerts its force. It is met on all sides by physical powers acting in
antagonism to it, but, like a ruling spirit, it restrains them in their
wildest moods.
The smallest dust which floats upon the wind
Bears this strong impress of the Eternal Mind.
In mystery round it, subtile forces roll;
And gravitation binds and guides the whole.
In every sand, before the tempest hurl’d,
Lie locked the powers which regulate a world,
And from each atom human thought may rise
With might to pierce the mysteries of the skies,--
To try each force which rules the mighty plan,
Of moving planets, or of breathing man;
And from the secret wonders of each sod,
Evoke the truths, and learn the power of God.
FOOTNOTES:
[13] Three hypotheses may be used to account for this most curious
phenomenon.
1st. The body shines by its own light, and then explodes like a
sky-rocket, breaking into minute fragments too small to be any longer
visible to the naked eye.
2nd. Such a body, having shone by its own light, suddenly ceases to
be luminous. “The falling stars and other fiery meteors which are
frequently seen at a considerable height in the atmosphere, and which
have received different names according to the variety of their figure
and size, arise from the fermentation of the effluvia of acid and
alkaline bodies which float in the atmosphere. When the more subtile
parts of the effluvia are burned away, the viscous and earthy parts
become too heavy for the air to support, and by their gravity fall to
the earth.”--Keith’s _Use of the Globes_. According to Sir Humphry
Davy, in the Philosophical Transactions for 1847, “the luminous
appearances of shooting stars and meteors cannot be owing to any
inflammation of elastic fluids, but must depend upon the ignition of
solid bodies.”
3. The body shines by the reflected light of the sun, and ceases to be
visible by its passing into the earth’s shadow, or, in other words,
is eclipsed. Upon the two former suppositions the fact of the star’s
disappearance conveys to us no knowledge of its position, or of its
distance from the earth; and all that can be said is, that if it be
a satellite of the earth, the great rapidity of its motion involves
the necessity of its being at no great distance from the earth’s
surface--much nearer than the moon; while the resistance it would
encounter in traversing the air would be so great that it is probably
without the limits of our atmosphere. _Sir J. W. Lubbock leans to the
third hypothesis._--Sir J. W. Lubbock, _On Shooting Stars_: Phil. Mag.
No. 213, p. 81.
Sir J. Lubbock also published a supplementary paper on the same
subject, in No. 214, p. 170.
Mr. J. P. Joule entertains an hypothesis with respect to Shooting
Stars similar to that advocated by Chladni to account for meteoric
stones, and he reckons the ignition of these miniature planetary bodies
by their violent collision with our atmosphere, to be a remarkable
illustration of the doctrine of the equivalency of heat to mechanical
power, or _vis viva_.
If we suppose a meteoric stone of the size of a six-inch cube to enter
our atmosphere at the rate of eighteen miles per second of time, the
atmosphere being 1/100 of its density at the earth’s surface, the
resistance offered to the motion of the stone will in this case be at
least 51,600 lbs.; and if the stone traverse twenty miles with this
amount of resistance, sufficient heat will thereby be developed to give
1° Fahrenheit to 6,967,980 lbs. of water. Of course by far the largest
portion of this heat will be given to the displaced air, every particle
of which will sustain the shock, whilst only the surface of the stone
will be in violent collision with the atmosphere. Hence the stone may
be considered as placed in a blast of intensely heated air, the heat
being communicated from the surface to the centre by conduction. Only
a small portion of the heat evolved will therefore be received by the
stone; but if we estimate it at only 1/100 it will still be equal
to 1° Fahrenheit per 69,679 lbs. of water, a quantity quite equal
to the melting and dissipation of any materials of which it may be
composed.--Mr. J. P. Joule, _On Shooting Stars_: Phil. Mag. No. 216, p.
348.
[14] “Laplace conjectures that in the original condition of the solar
system, the sun revolved upon his axis, surrounded by an atmosphere
which, in virtue of an excessive heat, extended far beyond the orbits
of all the planets, the planets as yet having no existence. The heat
gradually diminished, and as the solar atmosphere contracted by
cooling, the rapidity of its rotation increased by the laws of rotatory
motion; and an exterior zone of vapour was detached from the rest,
the central attraction being no longer able to overcome the increased
centrifugal force. This zone of vapour might in some cases retain
its form, as we see it in Saturn’s ring; but more usually the ring
of vapour would break into several masses, and these would generally
coalesce into one mass, which would revolve about the sun,”--Whewell’s
_Bridgewater Treatise_.
The following passage is translated by the same author from Laplace:--
“The anterior state (a state of cloudy brightness) was itself preceded
by other states, in which the nebulous matter was more and more
diffuse, the nucleus being less and less luminous. We arrive in this
manner at a nebulosity so diffuse, that its existence could scarce be
suspected. Such is in fact the first state of the nebula which Herschel
carefully observed by means of his telescope.”
Sir William Herschel has the following observations on these remarkable
masses:--
“The nature of planetary nebulæ, which has hitherto been involved in
much darkness, may now be explained with some degree of satisfaction,
since the uniform and very considerable brightness of their apparent
disc accords remarkably well with a much condensed, luminous fluid;
whereas, to suppose them to consist of clustering stars will not so
completely account for the milkiness or soft tint of their light, to
produce which it would be required that the condensation of the stars
should be carried to an almost inconceivable degree of accumulation.
“How far the light that is perpetually emitted from millions of suns
may be concerned in this shining fluid, it might be presumptuous to
attempt to determine; but notwithstanding the inconceivable subtilty
of the particles of light, when the number of the emitting bodies
is almost infinitely great, and the time of the continual emission
indefinitely long, the quantity of emitted particles may well become
adequate to the constitution of a shining fluid or luminous matter,
provided a cause can be found that may retain them from _flying off,
or reunite them_.”--_Observations on Nebulous Stars_: Philosophical
Transactions, vol. lxxxi. A.D. 1791.
In addition, the following Memoirs on the same subject, by Sir
William Herschel, have been published in the Philosophical
Transactions:--_Catalogue of 1000 Nebulæ and Clusters of Stars_, vol.
lxxvi.; _Catalogue of another 1000, with remarks on the Heavens_, vol.
lxxix.; _Catalogue of 500 more, with remarks as above_, vol. xcii.;
_Of such as have a cometary appearance_, vol. ci.; _Of planetary
nebulæ_, ibid.; _Of stellar nebulæ_, ibid.; _On the sidereal part of
the heavens, and its connection with the nebulous_, vol. civ.; _On the
relative distances of clusters of nebulous stars_, vol. cviii.
[15] Lord Rosse’s beautiful telescopes have been formed upon principles
which appear to embrace the best possible conditions for obtaining
a reflecting surface which should reflect the greatest quantity of
light, and retain that property little diminished for a length of
time. The alloy used for this purpose consists of tin and copper in
atomic proportions, namely, one atom of tin to four atoms of copper,
or by weight 58·9 to 126·4.--_On the Construction of large Reflecting
Telescopes_: by Lord Rosse. Report of the Fourteenth Meeting of the
British Association, 1844, p. 79.
[16] The best description of the Zodiacal Light occurs in a letter
furnished by Sir John Herschel to the _Times_ newspaper in March,
1843:--“The zodiacal light, as its name imports, invariably appears
in the zodiac, or, to speak more precisely, in the plane of the sun’s
equator, which is 7° inclined to the zodiac, and which plane, seen from
the sun, intersects the ecliptic in longitude 78° and 258°, or so much
in advance of the equinoctial points: in consequence it is seen to the
best advantage at, or a little after, the equinoxes; after sunset,
at the spring, and before sunrise, at the autumnal equinox; not only
because the direction of its apparent axis lies at those times more
nearly perpendicular to the horizon, but also because at those epochs
we are approaching the situation when it is seen most completely in
section.
“At the vernal equinox the appearance of the zodiacal light is that
of a pretty broad pyramidal, or rather lenticular, body of light,
which begins to be visible as soon as the twilight decays. It is very
bright at its lower or broader part near the horizon, and, if there
be broken clouds about, often appears like the glow of a distant
conflagration, or of the rising moon, only less red, giving rise, in
short, to amorphous masses of light such as have been noticed by one of
your correspondents as possibly appertaining to the comet. At higher
altitudes, its light fades gradually, and is seldom traceable much
beyond the Pleiades, which it usually, however, attains and involves,
and (what is most to my present purpose) its axis at the vernal equinox
is always inclined (to the northward of the equator) at an angle of
between 60° and 70° to the horizon, and it is most luminous at its
base, resting on the horizon, where also it is broadest, occupying,
in fact, an angular breadth of somewhere about 10° or 12° in ordinary
clear weather.”
[17] “The assumption that the extent of the starry firmament is
literally infinite has been made by one of the greatest of astronomers,
the late Dr. Olbers, the basis of a conclusion that the celestial
spaces are, in some slight degree, deficient in _transparency_; so that
all beyond a certain distance is, and must remain for ever, unseen; the
geometrical progression of the extinction of light far outrunning the
effect of any conceivable increase in the power of our telescopes. Were
it not so, it is argued, every part of the celestial concave ought to
shine with the brightness of the solar disc, since no visual ray could
be so directed as not, in some point or other of its infinite length,
to encounter such a disc.”--_Edinburgh Review_, p. 185, for January,
1848; _Etudes d’Astronomie Stellaire_.
[18] In the _Astronomische Nachrichten_ of July, 1846, appeared a
Memoir by M. Mädler, _Die Centralsonne_. The conclusions arrived at by
Mädler may be understood from the following quotation from a French
translation, made by M. A. Gautier, in the _Archives des Sciences
Physiques et Naturelles_, for October, 1846:--“Quoiqu’il résulte
de ce qui précède que la région du ciel que j’ai adoptée satisfait
à toutes les conditions posées plus haut, il n’en est pas moins
convenable de la soumettre à toutes les épreuves possibles. Plusieurs
essais de combinaisons différentes m’ont convaincu qu’on ne pourrait
trouver aucun autre point dans le ciel qui pût tenir lieu, même d’une
manière approchée, que celui que j’ai adopté. On pourrait maintenant
m’addresser l’objection que, si la région du ciel où se trouve le
centre de gravité de notre système d’étoiles fixes, est déterminée
par ce qui précède entre certaines limites, il n’en résulte pas la
nécessité de choisir Alcyone pour ce centre, attendu qu’il pourrait
bien tomber sur quelqu’autre étoile située dans le groupe ou dans son
voisinage. Mais outre que c’est tout près de là que se trouve le groupe
le plus brillant et le plus riche en étoiles de tout le ciel, et qu’il
ne s’agit point ici d’un point arbitraire situé dans le voisinage
peu apparent et qui n’ait rien qui le distingue, il ne se trouve nul
part, même dans la région voisine, une aussi exacte concordance des
mouvements propres qu’ici, et ces mouvements correspondent mieux que
tous les autres aux conditions établies plus haut. Or si l’on doit
considérer ce groupe central, entre les étoiles également éloignées, on
peut présumer que la plus brillante de beaucoup présente la plus grande
masse. Outre cela Alcyone, considérée optiquement, est au milieu du
groupe des Pleïades; et son mouvement propre, déterminé par Bessel, est
plus exactement en accord avec la moyenne de ceux des autres Pleïades;
ainsi que des étoiles de cette région jusqu’à 10° de distance. _Je puis
donc établir comme conséquence de tout ce qui précède, que le groupe
des Pleïades est le groupe central de l’ensemble du système des étoiles
fixes, jusqu’aux limites extérieures déterminées par la Voie Lactée;
et que Alcyone est l’étoile de ce groupe qui paraît être, le plus
vraisemblablement, le vrai Soleil central._”
[19] See the article _On Gravitation_, Penny Cyclopædia, from the pen
of the Astronomer-Royal.
[20] Delambre dates the commencement of modern astronomical observation
in its most perfect form from Maskelyne, who was the first who gave
what is now called a standard catalogue (A.D. 1790) of stars; that
is, a number of stars observed with such frequency and accuracy, that
their places serve as standard points of the heavens. His suggestion
of the _Nautical Almanack_, and his superintendence of it to the end
of his life, from its first publication in 1767, are mentioned in the
_Almanack_ (vol. i. p. 364); his _Schehallion Experiment on Attraction_
in vol. iii. p. 69; and the character of his _Greenwich Observations in
Greenwich Observatory_ in vol. ii. p. 442.
[21] _Experiments to determine the Density of the Earth._ By Henry
Cavendish, Esq., F.R.S. and F.A.S.--Philosophical Transactions, 1798.
[22] Adams: _An Explanation of the observed irregularities in the
motion of Uranus, on the hypothesis of disturbance caused by a more
distant Planet_.--Appendix to Nautical Almanack for 1851.
[23] Le Verrier: _Premier Mémoire sur la théorie d’Uranus_, Comptes
Rendus, vol. xxi.; _Sur la planête qui produit les anomalies observées
dans le mouvement d’Uranus_.--_Ib._ vol. xxiii.
[24] The experiment alluded to is one of a series by M. Plateau, who
thus describes his arrangement of the fluid:--“We begin by making a
mixture of alcohol and distilled water, containing a certain excess
of alcohol, so that when submitted to the trial of the test tube
it lets the small sphere of oil fall to the bottom rather rapidly.
When this point is obtained, the whole is thrown upon filters, care
being taken to cover the funnels containing these last with plates
of glass; this precaution is taken in order to prevent, as much as
possible, the evaporation of the alcohol. The alcoholic liquor passes
the first through the filters, ordinarily carrying with it a certain
number of very minute spherules of oil When the greater part has thus
passed, the spherules become more numerous; what still remains in the
first filters, namely, the oil and a residue of alcoholic liquor, is
then thrown into a single filter placed on a new flask. This last
filtration takes place much more slowly than the first, on account of
the viscosity of the oil; it is considerably accelerated by renewing
the filter once or twice during the operation. If the funnel has been
covered with sufficient care, the oil will collect into a single mass
at the bottom of the flask under a layer of alcoholic liquor.”--_On the
Phenomena presented by a free Liquid Mass withdrawn from the action of
Gravity._ By Professor Plateau, of the University of Ghent. Translated
from the _Memoirs of the Royal Academy of Brussels_, vol. xvi.; in the
_Scientific Memoirs_, vol. iv. part 13.