Francis Lieber.

Library of universal knowledge. A reprint of the last (1880) Edinburgh and London edition of Chambers' encyclopaedia, with copious additions by American editors (Volume 13) online

. (page 161 of 203)
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997.129 avoirdupois ounces, or 62.82 avoirdupois pounds. It is convenient to remember
tiiat a cubic foot of water weighs about 1000 ounces avoirdupois, as the error result-
ing from employing this estimate does not amount to much more than ?i J () ih of the
Whole, For aOriform bodies, the standard is atmospheric air. a cubic inch ol which, at a
temperature of 32 Fahr., weighs -32098, and at 60" Fahr., -30935 grains troy. The specific
gravity of solid bodies is best measured by the hydrostatic balance a figure of which is
given under Archimedes, Principle of (q.v.) which gives the weight of a volume of
water equal in bulk to the solid, by which it is only necessary to divide the weight of
the solid in air to obtain the specific gravity; that of liquids may be obtained by the are-
ometer (q.v.), or by comparing the weight lost by a solid body in the liquid and in water,
and dividing the former by the latter or by means of the specific-gravity battle, which
holds exactly 1000 grains of distilled water in its standard condition. The bottle is
emptied of water, filled with the liquid, and then weighed; the result gives the weight
of a volume of the fluid equal in bulk to 1000 grains of the standard, and hence this
weight divided by 1000 gives the specific gravity. The specific gravity of an aeriform
fluid is determined by weighing a glass globe filled first with the fluid and then with
atmospheric air. Annexed is a table of the specific gravity of a few of the more com-
mon substances:

SOLIDS (METALS).



Sp. Gr.
I ridiom Jmmmered) 23.

1'latinum xM.15

Gold 10.35

Mercury 14.

Lead... 11.35

Silver 10.74

Bismuth 9.82

Cobalt 7.8!

Copper 8.78



Sp. Gr.

Iron 7.78

Tin 7.v'!

Zinc 7.19

Antimony 0.70

Arsenic " '< '

Aluminium 2.07

Calcium 1.58

Sodium 97

Potassium 86



OTHER SOLIDS.

Sp. Gr. I Sp. Gr.

Loadstone 4.0.3 \ Honey \ 45

Ruby 4.28 i Lignum-vitee

Topaz 4.0:; evil 1 10

Diamond 3.52 ; Amber

Limestone 2.70 Spanish mahogany..,.

Chalk ;-.45 J^u-lish oak

Glass. Flint jj.yo , Butter .114



Flint


2 53




89


Clav


2.16


A.sh


84


Ivory


1 92


Elm


07


Bone


1 66






Sand


. 1.50


Cork...


. M4



Sp. Gr.

Sulphuric acid i.si

Nit ric acid 1.5

Aqua regia 1.33

Blood 1.04

Oil of cinnamon 1.04

Oil of cloves 1.03

Milk 1 03

Tar 1.01



Sp. Gr.

Champagne jvine l.

Burgundy w me f)9

Whisky, average.. '.)

Oil of turpentine !87

Brandy 84

Alcohol, pure 80

Ether, sulphuric [72



Hydriodic acid 4.34 Oxygen 'ill

Chlorine t>. 14 Olefiant gas

Sulphurous acid ;>.>:> Nitrogen"

Cyanogen l.SO I'nissic acid

Carbonic acid 1.52 Ammonia

Muriatic acid 1.^8 Hydrogen

SPECIFIC LEGACY. Sec LEGACY.

SPECIFIC PERFORMANCE, in English law, is the compt.^ory execution or carrying
out of a contract in its details, the court of chancery generallv having alone jurisdiction
to enforce specific performance. In Scotland, the corresponding phrase is implement
As a general rule the courts do not attempt to enforce specific' performance, hnt as ;t
substitute give the party injured by the breach of contract satisfaction iu the shape of
damages.

SPEC TACLES, for the purpose of aiding the sight when impaired by nsre or other
(see SIGHT, DEKBCTfi OF), were invented during the 13th century. The 'merit is attributed



Specter
Spectrum.

by some to Alexander di Spina, a Florentine monk, and by others to Ro^er Bacon. At
first they were exceedingly clumsy, both in the lenses themselves and also in their frames;
aild but little improvement took place in them until the beginning of this century, when
light metal frames were introduced, instead of the cumbrous horn or tortoise-shell mount-
ings, which are still occasionally seen, and have obtained the name of goggles. So skilli'ul
are the workmen of Wolvcrhampton, where they are chiefly made, in the manufacture of
steel frames, that some of exquisite workmanship are now turned out, which, with their
lenses complete, are under a quarter of an ounce in weight. They have consequently
displaced gold, silver, and all other materials, when comfort and effectiveness are pre
1'errcd to useless show combined with inconvenience. The lenses themselves are nearly
always made of the best optical glass, and by the best makers are ground with extreme
care. Many profess to be made of " pebbles," or rock-crystal ; but lenses really made of
that material are exceedingly rare, and have no real advantage over good glass.

SPECTEE BAT. Pliylloatoma, a genus of bats having two membranous crests on the
nose, the one leaf-like, the other in the form of a horseshoe. This gives to their face
that peculiar appearance from which they derive their popular as well as their scientific
name. The species are numerous, and all natives of the West Indies and South America.

SPECTROPHONE and PHOTOPHONE or RADIOPHONE. The action of these
instruments depends upon the phenomena of radiant energy which within a few years
have been observed by different physicists, but whose practical results, if the time has
arrived when the term may be used, have been principally accomplished by prof. Alex-
ander Graham Bell, the inventor of the speaking telephone in common use, in conjunc-
tion with Mr. Sumner Tainter. The invention of the photophone arose from the employ-
ment by \Villoughby Smith of selenium as a resistance medium in testing submarine
cables. It was found that the resistance of selenium to the galvanic current varied con-
siderably, and the discovery was made that this was caused by the action of light, which
lessened the resistance. When selenium is exposed to the action of the solar spectrum,
the maximum effect is produced, according to Sale, just outside of the red end of the
spectrum, in a point nearly coincident with the maximum of the heat rays (see Radiation
in article HEAT, ante), but according to Adams it is produced in the greenish-yellow or
most luminous part of the spectrum, and he moreover found that selenium was sensitive
to the cold light of the moon. E. W. Siemens discovered that heat and light produced
opposite effects upon some extremely sensitive varieties of selenium. In some of his
experiments the resistance on exposure to light was only one-fifteenth of what it was in
the dark. It occurred to prof. Bell to substitute the telephone for the galvanometer
hitherto used in these experiments on account of its great sensitiveness to electrical influ-
ences; but in doing so it was necessary to vary the action of light so that the inter-
missions from light to darkness should be sudden, in order to produce a succession of
changes in the conductivity of the selenium corresponding in frequency to the musical
vibrations within the limits of hearing; and upon further consideration it appeared to
him that all the audible effects obtained from varieties of electricity could also be pro-
duced by variations of light acting on selenium. In an article in Science of Sept. 11,
1880, he says: " I saw that the effect could be produced at the extreme distance at which
selenium would respond to the action of a luminous body, but that this distance could
be indefinitely increased by the use of a parallel beam of light, so that we could telephone
from one place to another without the necessity of a conducting wire between the trans-
mitter and receiver. It was evidently necessary, in order to reduce this idea to practice,
to devise an apparatus to be operated on by the voice of the speaker, by which variations
could be produced in a parallel beam of light, corresponding to the variations in the air
produced by the voice." But a difficulty was found in the fact that the resistance of
selenium was too great to respond sufficiently to the action of light; this, however, was
overcome by reducing this resistance from some half million ohms to 800 in the dark,
and to 155 in the light. The fundamental features of the selenium photophone are best
given in prof. Bell's own words: " We have devised about fifty forms of apparatus for
varying a beam of light in the manner required. The best and simplest form consists
of a plain mirror of flexible material such as silvered mica or microscopic glass. Against
the back of this mirror the speaker's voice is directed. The light reflected from this
mirror is thus thrown into vibration corresponding to those of the diaphragm itself. In
arranging the apparatus for the purpose of reproducing sound at a distance any power-
ful source of light may be used, but we have experimented chiefly with sunlight. For
this purpose a large beam is concentrated by means of a lens upon "the diaphragm mirror,
and after reflection is again rendered parallel by means of another lens. The beam is
received at a distant station upon a parabolic reflector, in the focus of which 13 placed a
sensitive selenium cell, connected in a local circuit with a battery and telephone." The
loudest effects obtained from light were produced by rapidly interrupting the beam by
a perforated rotating disk, revolving over the face of another perforated disk, with holes
corresponding. Audible musical tones were produced from the light of a candle.

The experiments connected with the construction of this apparatus led toothers with
other substances than selenium, and also without the use of telephone or battery. A
thin sheet of hard rubber was held close to the ear while abeam of intermittent light was
thrown upon it by a lens, the result being the production of a musical note, and this



Specter,
Spectrum,

effect was intensified by arranging the hard rubber as a diaphragm and listening through
a hearing-tube. The remarkable though natural conclusion was reached " that so uitds
can be produced by the action of a variable light from $>ibst<incai of all kinds ic/ien. in (Jte
form of thin diaphragms." Subsequently prof . Bell arrived at the conclusion that wno-
nnixnemi under the influence of intermittent lit/Id it it j>n>]>crfy of all matter. Various experi-
ments were made with different fibrous and porous materials, such as cotton-wool, worsted }
silks, sponge, lamp-black, etc. These articles were inclosed in a conical cavity, con-
tained in a piece of brass, and closed by a flat plate of glass through which an intermit-
tent beam of light was thrown upon them. A hearing-tube communicated with the cavity.
Mr. Tain tor found that the darkest shades produced the best effects. Black worsted
especially gave an extremely loud sound. Cotton-wool darkened with lamp-black gave
so loud a sound as to suggest the use of lamp-black alone. Of this substance a tea-
spoonful, placed in a test-tube and exposed to an intermittent beam of sunlight produced
the loudest sound of all, and a piece of smoked glass, with the smoked surface receiving
the intermittent beam, gave a fine effect. Upon smoking the interior of the conical
cavity of the receiver above-mentioned, and then exposing it to the intermittent beam,
"the effect was perfectly startling. The sound was so loud as to be actually painful to
the ear placed closely against the end of the l*earing-tube." The various experiments
above alluded to will probably be of great importance in telephony as indicating that
lamp-black may be substituted for selenium in an electrical receiver. 51. Mercadier
passed an intermittent beam from an electric lamp through a prism and found a differ-
ence in the audible effects in different parts of the spectrum. These experiments were
repeated by prof. Bell, with somewhat different results. Under conditions not neces-
sary to describe here, "sounds were obtained in every part of the visible spectrum
excepting the extreme half of the violet, as well as in the ultra-red. A continuous
increase in the loudness of the sound W 7 as observed upon moving the receiver gradually
from the violet into the ultra-red. The point of maximum Bound lay very far out in the
ultra-red. Beyond this point the sound began to decrease, and then stopped so suddenly
that a very slight motion of the receiver made all the difference between almost maximum
sound and complete silence." Removing the smoked wire gauze from the receiver and
substituting red worsted different results were obtained, the" maximum effect bcinsr pro-
duced in the green at that part where the red worsted appeared to be black. On either
side of this point the sound gradually died away. On substituting green silk for the red
worsted the maximum effects were "found in the red. A test tube containing the vapor
of sulphuric ether was then substituted for the receiver, but no effects were observed till
a certain point far out in the ultra-red was reached, Avhen a musical tone was suddenly
produced, which disappeared as suddenly further on. With the vapor of iodine the
maximum effect was in the green. These, and experiments with other substances, led
to the conclusion that " the nature of the rays that produce sonorous effects in different
substances depends upon the nature of the substances that are exposed to the beam, and
that the sounds are in every case due to those raj's of the spectrum that are absorbed by
the body." These phenomena led prof. Bell to the construction of a new instrument for
use in spectrum analysis, which was described and exhibited to the philosophical society
of Washington last April. " The eye-piece of a spectroscope is removed and sensitive
substances are placed in the focal point of the instrument behind an opaque diaphragm
containing a slit. These substances are put in communication with the ear by means of
a hearing-tube, and thus the instrument is converted into a veritable ' spectrophone.'
Suppose we smoke the interior of our spectrophonic receiver and till the cavil}' with
peroxide of nitrogen gas. We have then a combination that gives us good sounds in all
parts of the spectrum, visible and invisible, except the ultra-violet. ?sow pass a rapidly
interrupted beam of light through some substance whose absorption spectrum is to be
investigated, and bands of sound and silence are observed upon exploring the spectrum,
the silent positions corresponding to the absorption bands. Of course the ear cannot for
one moment compete with the eye in the examination of the visible part of the spectrum;
but in the invisible part beyond the red, where the eye is useless, the ear is invaluable.
In working in this region of the spectrum, lamp-black alone may be used in the spectro-
phonic receiver. Indeed, the sounds produced by this substance in the ultra-red are so
well marked as to constitute our instrument a most reliable and convenient substitute for
the thermo-pile." See Science for May 28, 1881. Prof. Bell recognizes the fact that
the spectrophone must always be no more than an adjunct to the spectroscope, but
believes that it will have a wide and independent field of usefulness in the investigation
of absorption spectra in the ultra-red.

SPECTEOSCOPE, the instrument by the aid of which spectral phenomena (see SPEC
TRUM) may be most conveniently studied. It consists essentially of, first, an illuminated
slit, from which parallel rays of light proceed; secondly, a prism or train of prisms, to
separate the differently refrangible rays; and thirdly, a telescope, to view a magnified
image of the spectrum produced. See Svx.

SPEC TEUM (Gr.) is a term applied in optics to the colored image of the sun or other
luminous body, produced by refraction through a prism' (q. v.), by disfraction (q.v.)
through a fine grating, etc. In what follows we shall confine ourselves to the spectrum
produced by a prism, as that which is commonly used. Besides, so far as we have at



Spectrum.



698



present occasion to examine it, it presents very nearly the same appearance as spectra
produced by other processes.

The solar spectrum was first carefully examined by Newton, who deduced from his
observations the composite nature of white light, and the different refrangibilities of its
various colored constituents. A ray of sunlight enters a darkened room through a small
hole, in a shutter. It proceeds in a straight line to the opposite wall, and forms as a
circular white spot, an image of the sun." If the edge of a glass prism be interposed in
the path of this ray, the white spot disappears, aud the spectrum is produced. In this
ijrm of experiment, its shape is that of a rectangle with semicircular ends.

The breadth of the spectrum is equal to the diameter of the spot; and it is brilliantly
colored, the one end being red, and the other end violet. Between tlie.se we, have grada-
tions of color, and the whole appeared to Newton to be divisible iuto seven differently
colored spaces, which he called red, oiauge, yellow, green, blue, indigo, violet. -This
was in accordance with the speculations of old days, when analogies were constantly
looked for, and seems to have been suggested to Newton by the common musical scale.
It is impossible, however, to settle precisely the exact boundary between any two of
these fancied species of color; and, besides, such a description of the spectrum (though
complete enough for mere popular language) is totally inadequate to express our present
knowledge of the subject. In order to study the spectrum a little more closely, suppose
we have pieces of colored glass which allow only one definite color to pass. With u red
glass placed at the hole in the shutter, the prism being removed, the effect would be to
ren.ler the spot red, without changing its position. Introduce the prism, and the effect
is to change the position of the spot without altering its size or color.

Similarly, with a violet glass we have a violet spot, and so on; the change of
position, due to refraction, befng least for red and greatest for violet. It thus appears
that the spectrum formed in this way, is made up of a series of circular spots, of the
various colors of which white light consists all of the same size, and having their centers
ranged along a line, so that each overlaps those next it. The only parts of the spectrum
which UK pure, i.e., where no two or more colors are mixed, are the ends; so that, by
this process, it is impossible to separate definitely the rays of different refrungibility. so
as to see, for instance, whether any are wanting. How, then, are we to ascertain whether
sunlight, contains rays of every refrangibility from red to violet? The obvious method
is to milks the spot not circular, but long and very narrow, a process mentioned by
Newton himself. To make this spot thus narrow, a method commonly employed, is to
set the prism about half-way between the shutter and the screen and to place before it a
l.::i-t. such that, if the prism were removed, thore would be an image of the hole in
t!)3 sh'itter nearly equal to it in size. The hole must, therefore, be a narrow slit, paral-
lel to the edge of the prism. When this arrangement is adjusted we have a pure spec-
trum, and we find it to be (at first sight) continuous. Thus, it appears that sunlight con-
tain, ray. s of every refrangibility, from the highest to the lowest; and that Newton's
sevenfold division of it, though sometimes convenient for popular reference, has no
scientific basis. Besides, what we can see is not the whole spectrum but a mere fraction
, of it; for beyond the red end, there are invisible rays recognized at once by their halt-
ing powers; and beyond the violet, there are invisible rays more powerful than the
visible in producing chemical changes, as on a photographic plate, and which can bo
changed into visible rays by fluorescent substances. See PHOSPHOKESCKXCF. The
breadth of the visible spectrum evidently depends on the length of the slit, its length on
the difference of refrangibility of red and violet.

If we cut a narrow slit in the screen on which the spectrum falls, in a direction per-
pendicular to its length, the light which passes through has a definite refrangibility, and
can no longer be drawn out by a prism into a spectrum. This experiment also is due to
Newton.

If the slit in the shutter be very narrow, and the prism be adjusted to the most favor-
able position (so that the incident and refracted rays make equal angles with the sur-
faces on which they impinge, and from which they escape, respectively), we see that
after all the solar spectrum is not continuous. It is found jto be crossed at intervals by
dark bands, showing the absence of rays of certain definite refrangibilities. The phe-
nomenon is found to be the same whatever be the substance of the prism; so that these
rays are really wanting in sunlight.

This important discovery was made by Wollaston, but the bands were first carefully
observed and measured by Fraunhofer, from whom they are commonly called Fraun-
hofcr's lines. We owe to Fraunhofer the invaluable suggestion of employing a telescope
to examine the spectrum. The refracted rays are received directly on the object-glass of
the telescope, which forms an image of the spectrum to be examined with the aid of the
eye-piece, the screen being dispensed with. Wollastonhad seen only five lines; Fraun-
hofer at once discovered four hundred; Brewster, with more perfect apparatus, counted
two thousand ; and now, with a train of prisms, and powerful telescopes, their number
seems beyond computation. They show every variety of breadth and distinctness, and
are grouped in the most irregular manner. For reference, Fraunhofer selected some of
the more prominent, to which he attached the earlier letters of the alphabet. By their
help he was enabled to measure refractory indices (see REFRACTION) with a precision



699



Spectrum.



completely unlocked for. If the light of a candle, a bright gas-flame, a white hot wire,
or a lime-ball in the oxhydrogen flame, he examined in the same way, no anch lutes are
seen. But some of them, and others not apparently belonging to sunlight, were found
by Fraunliofer in the spectra of various fixed stars while the light of the moon and
planets seemed to give spectra similar to that of sunlight.

The first to throw any light on this subject was Brewster. He showed that when
light passed through nitrous acid gas its spectrum was interrupted by countless lines;
and that they increased in number and breadth by the application of heat 10 the gas, so
that at a high temperature a thin layer of this gas is opaque to direct sunlight. lieiiee it
was n:i:ural to conclude that the dark bands in the solar spectrum are caused by absorp-
tion in some medium lying between us and the sun. It is to be observed, however, that
this is on the supposition that light as it comes immediately from tlie sun would give,
like that of the lime ball, a continuous spectrum. But Brewster went further. He
showed that some of Fraunhofer's lines depend on the altitude of the sun, that is* on the
greater or less space of air, fog, and vapor through which his rays must pass before
reaching the earth. Some of tliem, then, are caused by absorption in tlie earth's atmos-
phere.

But we must now look to another class of phenomena. A spirit lamp flame gives a
very feeble spectrum; and, if a little common salt be put on the wick, although the
flame becomes instantly very much brighter, no alteration is produced on the spectrum,
save the appearance of a bright yellow line, crossing it at the place where the dark line
called by Fraunliofer D appears in sunlight. On examining this line carefully Fraun-
liofer found that it, like D, is double and he verified that these two rays were exactly
(so far as refractive index goes) two of those wanting iu sunlight, and in the light of
some of the stars.

About the same time Talbot and Herschel (q.v.) showed that the colors given by
Litliia, Strontia, etc., in a spirit flame were, like that produced by common salt, due to
the production of light of several perfectly definite refraugibilities; so that the spectrum
of the lamp-flame was crossed in each case by a series of bright lines always the same
when the same body was placed in the flame; and they suggested (in 1825) the applica-
tion of this method to the qualitative analysis of minerals, etc., when the presence of
extremely minute quantities of different bodies has to be ascertained. This was, in
reality, tiie foundation of SPECTKUM ANALYSIS; and the method was. we may >ay,
almost complete so far as practice is concerned. The theory, however, was left incom-
plete, so far as regards the cause ol'durk lines in the solar spectrum. Foucault (in 1849)
seems to have been^the first ^o approach the true explanation. An experiment of
his, from which, however, he drew no inferences, contains the complete theory. When
salt is placed in the voltaic arc (Electric Light, q.v.) the spectrum gives the double bright



Online LibraryFrancis LieberLibrary of universal knowledge. A reprint of the last (1880) Edinburgh and London edition of Chambers' encyclopaedia, with copious additions by American editors (Volume 13) → online text (page 161 of 203)