through the helices, and observing that the magnets swung perfectly
free, it was first arranged that the bismuth cylinders within the
helices had their central or neutral points opposite to the poles of
the magnets. All being at rest the number on the scale marked by the
cross wire of the telescope was 572. The cylinders were then moved,
one up the other down, so that two of their ends were brought to bear
simultaneously upon the magnetic poles: the magnet moved promptly, and
after some oscillations [Footnote: To lessen these a copper damper was
made use of.] came to rest at the number 612; thus moving from a
smaller to a larger number. The other two ends of the bars were next
brought to bear upon the magnet: a prompt deflection was the
consequence, and the final position of equilibrium was 526; the
movement being from a larger to a smaller number. We thus observe a
manifest polar action of the bismuth cylinders upon the magnet; one
pair of ends deflecting it in one direction, and the other pair
deflecting it in the opposite direction.
Substituting for the cylinders of bismuth thin cylinders of iron, of
magnetic slate, of sulphate of iron, carbonate of iron, protochloride
of iron, red ferrocyanide of potassium, and other magnetic bodies, it
was found that when the position of the magnetic cylinders was the
same as that of the cylinders of bismuth, the deflection produced by
the former was always opposed in direction to that produced by the
latter; and hence the disposition of the force in the diamagnetic body
must have been precisely antithetical to its disposition in the
magnetic ones.
But it will be urged, and indeed has been urged against this
inference, that the deflection produced by the bismuth cylinders may
be due to induced currents excited in the metal by its motion within
the helices. In reply to this objection, it may be stated, in the
first place, that the deflection is permanent, and cannot therefore be
due to induced currents, which are only of momentary duration. It has
also been urged that such experiments ought to be made with other
metals, and with better conductors than bismuth; for if due to
currents of induction, the better the conductor the more exalted will
be the effect. This requirement was complied with.
Cylinders of antimony were substituted for those of bismuth. This
metal is a better conductor of electricity, but less strongly
diamagnetic than bismuth. If therefore the action referred to be due
to induced currents we ought to have it greater in the case of
antimony than with bismuth; but if it springs from a true diamagnetic
polarity, the action of the bismuth ought to exceed that of the
antimony. Experiment proves this to be the case. Hence the
deflection produced by these metals is due to their diamagnetic, and
not to their conductive capacity. Copper cylinders were next
examined: here we have a metal which conducts electricity fifty times
better than bismuth, but its diamagnetic power is nearly null; if the
effects be due to induced currents we ought to have them here in an
enormously exaggerated degree, but no sensible deflection was produced
by the two cylinders of copper.
It has also been proposed by the opponents of diamagnetic polarity to
coat fragments of bismuth with some insulating substance, so as to
render the formation of induced currents impossible, and to test the
question with cylinders of these fragments. This requirement was also
fulfilled. It is only necessary to reduce the bismuth to powder and
expose it for a short time to the air to cause the particles to become
so far oxidised as to render them perfectly insulating. The
insulating power of the powder was exhibited experimentally;
nevertheless, this powder, enclosed in glass tubes, exhibited an
action scarcely less powerful than that of the massive bismuth
cylinders.
But the most rigid proof, a proof admitted to be conclusive by those
who have denied the antithesis of magnetism and diamagnetism, remains
to be stated. Prisms of the same heavy glass as that with which the
diamagnetic force was discovered, were substituted for the metallic
cylinders, and their action upon the magnet was proved to be precisely
the same in kind as that of the cylinders of bismuth. The enquiry was
also extended to other insulators: to phosphorus, sulphur, nitre,
calcareous spar, statuary marble, with the same invariable result:
each of these substances was proved to be polar, the disposition of
the force being the same as that of bismuth and the reverse of that of
iron. When a bar of iron is set erect, its lower end is known to be a
north pole, and its upper end a south pole, in virtue of the earth's
induction. A marble statue, on the contrary, has its feet a south
pole, and its head a north pole, and there is no doubt that the same
remark applies to its living archetype; each man walking over the
earth's surface is a true diamagnet, with its poles the reverse of
those of a mass of magnetic matter of the same shape and position.
An experiment of practical value, as affording a ready estimate of the
different conductive powers of two metals for electricity, was
exhibited in the lecture, for the purpose of proving experimentally
some of the statements made in reference to this subject. A cube of
bismuth was suspended by a twisted string between the two poles of an
electro-magnet. The cube was attached by a short copper wire to a
little square pyramid, the base of which was horizontal, and its sides
formed of four small triangular pieces of looking-glass. A beam of
light was suffered to fall upon this reflector, and as the reflector
followed the motion of the cube the images cast from its sides
followed each other in succession, each describing a circle about
thirty feet in diameter. As the velocity of rotation augmented, these
images blended into a continuous ring of light. At a particular
instant the electro-magnet was excited, currents were evolved in the
rotating cube, and the strength of these currents, which increases
with the conductivity of the cube for electricity, was practically
estimated by the time required to bring the cube and its associated
mirrors to a state of rest. With bismuth this time amounted to a score
of seconds or more: a cube of copper, on the contrary, was struck
almost instantly motionless when the circuit was established.
********************
XIV. PHYSICAL BASIS OF SOLAR CHEMISTRY.
[Footnote: From a discourse delivered at the Royal Institution of
Great Britain, June 7, 1861.]
OMITTING all preface, attention was first drawn to an experimental
arrangement intended to prove that gaseous bodies radiate heat in
different degrees. Near a double screen of polished tin was placed an
ordinary ring gas-burner, and on this was placed a hot copper ball,
from which a column of heated air ascended. Behind the screen, but so
situated that no ray from the ball could reach the instrument, was an
excellent Thermo-electric pile, connected by wires with a very
delicate galvanometer. The pile was known to be an instrument whereby
heat is applied to the generation of electric currents; the strength
of the current being an accurate measure of the quantity of the heat.
As long as both faces of the pile are at the same temperature, no
current is produced; but the slightest difference in the temperature
of the two faces at once declares itself by the production of a
current, which, when carried through the galvanometer, indicates by
the deflection of the needle both its strength and its direction.
The two faces of the pile were in the first instance brought to the
same temperature; the equilibrium being shown by the needle of the
galvanometer standing at zero. The rays emitted by the current of hot
air already referred to were permitted to fall upon one of the faces
of the pile; and an extremely slight movement of the needle showed
that the radiation from the hot air, though sensible, was extremely
feeble. Connected with the ring-burner was a holder containing oxygen
gas; and by turning a cock, a stream of this gas was permitted to
issue from the burner, strike the copper ball, and ascend in a heated
column in front of the pile. The result was, that oxygen showed
itself, as a radiator of heat, to be quite as feeble as atmospheric
air.
A second holder containing olefiant gas was then connected with the
ring-burner. Oxygen and air had already flowed over the ball and
cooled it in some degree. Hence the olefiant gas laboured under a
disadvantage. But on permitting the gas to rise from the ball, it
casts an amount of heat against the adjacent face of the pile
sufficient to impel the needle of the galvanometer almost to 90°. This
experiment proved the vast difference between two equally invisible
gases with regard to their power of emitting radiant heat.
The converse experiment was now performed. The thermo-electric pile
was removed and placed between two cubes filled with water kept in a
state of constant ebullition; and it was so arranged that the
quantities of heat falling from the cubes on the opposite faces of the
pile were exactly equal, thus neutralising each other. The needle of
the galvanometer being at zero, a sheet of oxygen gas was caused to
issue from a slit between one of the cubes and the adjacent face of
the pile. If this sheet of gas possessed any sensible power of
intercepting the thermal rays from the cube, one face of the pile
being deprived of the heat thus intercepted, a difference of
temperature between its two faces would instantly set in, and the
result would be declared by the galvanometer. The quantity absorbed
by the oxygen under those circumstances was too feeble to affect the
galvanometer; the gas, in fact, proved perfectly transparent to the
rays of heat. It had but a feeble power of radiation: it had an
equally feeble power of absorption.
The pile remaining in its position, a sheet of olefiant gas was caused
to issue from the same slit as that through which the oxygen had
passed. No one present could see the gas; it was quite invisible, the
light went through it as freely as through oxygen or air; but its
effect upon the thermal rays emanating from the cube was what might be
expected from a sheet of metal. A quantity so large was cut off, that
the needle of the galvanometer, promptly quitting the zero line, moved
with energy to its stops. Thus the olefiant gas, so light and clear
and pervious to luminous rays, was proved to be a most potent
destroyer of the rays emanating from an obscure source. The
reciprocity of action established in the case of oxygen comes out
here; the good radiator is found by this experiment to be the good
absorber.
This result, now exhibited before a public audience for the first
time, was typical of what had been obtained with gases generally.
Going through the entire list of gases and vapours in this way, we
find radiation and absorption to be as rigidly associated as positive
and negative in electricity, or as north and south polarity in
magnetism. So that if we make the number which expresses the
absorptive power the numerator of a fraction, and that which expresses
its radiative power the denominator, the result would be, that on
account of the numerator and denominator varying in the same,
proportion, the value of that fraction would always remain the same,
whatever might be the gas or vapour experimented with.
But why should this reciprocity exist? What is the meaning of
absorption? what is the meaning of radiation? When you cast a stone
into still water, rings of waves surround the place where it falls;
motion is radiated on all sides from the centre of disturbance. When
a hammer strikes a bell, the latter vibrates; and sound, which is
nothing more than an undulatory motion of the air, is radiated in all
directions. Modern philosophy reduces light and heat to the same
mechanical category. A luminous body is one with its atoms in a state
of vibration; a hot body is one with its atoms also vibrating, but at
a rate which is incompetent to excite the sense of vision; and, as a
sounding body has the air around it, through which it propagates its
vibrations, so also the luminous or heated body has a medium, called
aether, which accepts its motions and carries them forward with
inconceivable velocity. Radiation, then, as regards both light and
heat, is the transference of motion from the vibrating body to the
aether in which it swings: and, as in the case of sound, the motion
imparted to the air is soon transferred to surrounding objects,
against which the aerial undulations strike, the sound being, in
technical language, absorbed; so also with regard to light and heat,
absorption consists in the transference of motion from the agitated
aether to the molecules of the absorbing body.
The simple atoms are found to be bad radiators; the compound atoms
good ones: and the higher the degree of complexity in the atomic
grouping, the more potent, as a general rule, is the radiation and
absorption. Let us get definite ideas here, however gross, and purify
them afterwards by the process of abstraction. Imagine our simple
atoms swinging like single spheres in the aether; they cannot create
the swell which a group of them united to form a system can produce.
An oar runs freely edgeways through the water, and imparts far less of
its motion to the water than when its broad flat side is brought to
bear upon it. In our present language the oar, broad side vertical,
is a good radiator; broad side horizontal, it is a bad radiator.
Conversely the waves of water, impinging upon the flat face of the
oar-blade, will impart a greater amount of motion to it than when
impinging upon the edge. In the position in which the oar radiates
well, it also absorbs well. Simple atoms glide through the aether
without much resistance; compound ones encounter resistance, and hence
yield up more speedily their motion to the aether. Mix oxygen and
nitrogen mechanically, they absorb and radiate a certain amount of
heat. Cause these gases to combine chemically and form nitrous oxide,
both the absorption and radiation are thereby augmented hundreds of
times!
In this way we look with the telescope of the intellect into atomic
systems, and obtain a conception of processes which the eye of sense
can never reach. But gases and vapours possess a power of choice as
to the rays which they absorb. They single out certain groups of rays
for destruction, and allow other groups to pass unharmed. This is
best illustrated by a famous experiment of Sir David Brewster's,
modified to suit present requirements. Into a glass cylinder, with
its ends stopped by discs of plate-glass, a small quantity of nitrous
acid gas is introduced; the presence of the gas being indicated by its
rich brown colour. The beam from an electric lamp being sent through
two prisms of bisulphide of carbon, a spectrum seven feet long and
eighteen inches wide is cast upon the screen. Introducing the
cylinder containing the nitrous acid into the path of the beam as it
issues from the lamp, the splendid and continuous spectrum becomes
instantly furrowed by numerous dark bands, the rays answering to which
are intercepted by the nitric gas, while the light which falls upon
the intervening spaces is permitted to pass with comparative impunity.
Here also the principle of reciprocity, as regards radiation and
absorption, holds good; and could we, without otherwise altering its
physical character, render that nitrous gas luminous, we should find
that the very rays which it absorbs are precisely those which it would
emit. When atmospheric air and other gases are brought to a state of
intense incandescence by the passage of an electric spark, the spectra
which we obtain from them consist of a series of bright bands. But
such spectra are produced with the greatest brilliancy when, instead
of ordinary gases, we make use of metals heated so highly as to
volatilise them. This is easily done by the voltaic current. A
capsule of carbon filled with mercury, which formed the positive
electrode of the electric lamp, has a carbon point brought down upon
it. On separating the one from the other, a brilliant arc containing
the mercury in a volatilised condition passes between them. The
spectrum of this arc is not continuous like that of the solid carbon
points, but consists of a series of vivid bands, each corresponding in
colour to that particular portion of the spectrum to which its rays
belong. Copper gives its system of bands; zinc gives its system; and
brass, which is an alloy of copper and zinc, gives a spectrum made up
of the bands belonging to both metals.
Not only, however, when metals are united like zinc and copper to form
an alloy, is it possible to obtain the bands which belong to them. No
matter how we may disguise the metal--allowing it to unite with oxygen
to form an oxide, and this again with an acid to form a salt; if the
heat applied be sufficiently intense, the bands belonging to the metal
reveal themselves with perfect definition. Into holes drilled in a
cylinder of retort carbon, pure culinary salt is introduced. When the
carbon is made the positive electrode of the lamp, the resultant
spectrum shows the brilliant yellow lines of the metal sodium.
Similar experiments made with the chlorides of strontium, calcium,
lithium, [Footnote: The vividness of the colours of the lithium
spectrum is extraordinary; the spectrum, moreover, contained a blue
band of indescribable splendour. It was thought by many, during the
discourse, that I had mistaken strontium for lithium, as this blue
band had never before been seen. I have obtained it many times since;
and my friend Dr. Miller, having kindly analysed the substance made
use of, pronounces it pure chloride of lithium.--J. T.] and other
metals, give the bands due to the respective metals. When different
salts are mixed together, and rammed into holes in the carbon; a
spectrum is obtained which contains the bands of them all.
The position of these bright bands never varies, and each metal has
its own system. Hence the competent observer can infer from the bands
of the spectrum the metals which produce it. It is a language
addressed to the eye instead of the ear; and the certainty would not
be augmented if each metal possessed the power of audibly calling out,
'I am here!' Nor is this language affected by distance. If we find
that the sun or the stars give us the bands of our terrestrial metals,
it is a declaration on the part of these orbs that such metals enter
into their composition. Does the sun give us any such intimation?
Does the solar spectrum exhibit bright lines which we might compare
with those produced by our terrestrial metals, and prove either their
identity or difference? No. The solar spectrum, when closely
examined, gives us a multitude of fine dark lines instead of bright
ones. They were first noticed by Dr. Wollaston, but were multiplied
and investigated with profound skill by Fraunhofer, and named after
him Fraunhofer's lines. They had been long a standing puzzle to
philosophers. The bright lines yielded by metallic vapours had been
also known to us for years; but the connection between both classes of
phenomena was wholly unknown, until Kirchhoff, with admirable
acuteness, revealed the secret, and placed it at the same time in our
power to chemically analyse the sun.
We have now some difficult work before us. Hitherto we have been
delighted by objects which addressed themselves as much to our
aesthetic taste as to our scientific faculty; we have ridden
pleasantly to the base of the final cone of Etna, and must now
dismount and march through ashes and lava, if we would enjoy the
prospect from the summit. Our problem is to connect the dark lines of
Fraunhofer with the bright ones of the metals. The white beam of the
lamp is refracted in passing through our two prisms, but its different
components are refracted in different degrees, and thus its colours
are drawn apart.
Now the colour depends solely upon the rate of oscillation of the
atoms of the luminous body; red light being produced by one rate, blue
light by a much quicker rate, and the colours between red and blue by
the intermediate rates. The solid incandescent coal-points give us a
continuous spectrum; or in other words they emit rays of all possible
periods between the two extremes of the spectrum. Colour, as many of
you know, is to light what _pitch_ is to sound. When a violin-player
presses his finger on a string he makes it shorter and tighter, and
thus, causing it to vibrate more speedily, heightens the pitch.
Imagine such a player to move his fingers slowly along the string,
shortening it gradually as he draws his bow, the note would rise in
pitch by a regular gradation; there would be no gap intervening
between note and note. Here we have the analogue to the continuous
spectrum, whose colours insensibly blend together without gap or
interruption, from the red of the lowest pitch to the violet of the
highest. But suppose the player, instead of gradually shortening his
string, to press his finger on a certain point, and to sound the
corresponding note; then to pass on to another point more or less
distant, and sound its note; then to another, and so on, thus sounding
particular notes separated from each other by gaps which correspond to
the intervals of the string passed over; we should then have the exact
analogue of a spectrum composed of separate bright bands with
intervals of darkness between them. But this, though a perfectly true
and intelligible analogy, is not sufficient for our purpose; we must
look with the mind's eye at the oscillating atoms of the volatilised
metal.
Figure these atoms as connected together by springs of a certain
tension, which, if the atoms are squeezed together, push them again
asunder, and if the atoms are drawn apart, pull them again together,
causing them, before coming to rest, to quiver for a certain time at a
certain definite rate determined by the strength of the spring. Now
the volatilised metal which gives us one bright band is to be figured
as having its atoms united by springs all of the same tension, its
vibrations are all of one kind. The metal which gives us two bands
may be figured as having some of its atoms united by springs of one
tension, and others by springs of a different tension. Its vibrations
are of two distinct kinds; so also when we have three or more bands we
are to figure as many distinct sets of springs, each capable of
vibrating in its own particular time and at a different rate from the
others. If we seize this idea definitely, we shall have no difficulty
in dropping the metaphor of springs, and substituting for it mentally
the forces by which the atoms act upon each other. Having thus far
cleared our way, let us make another effort to advance.