SPECIFIC GRAVITY.
It is recorded of the great Dr. Wollaston, that when Sir Humphry Davy
placed in his hand, what was then considered to be _the_ scientific
wonder of the day--viz., a small bit of the metal potassium, he
exclaimed at once, "How heavy it is," and was greatly surprised, when
Sir Humphry threw the metal on water, to see it not only take fire, but
actually _float_ upon the surface; here, then, was a philosopher
possessing the deepest learning, unable, by the sense of touch and by
ordinary handling, to state correctly whether the new substance (and
that a metal), was heavy or light; hence it is apparent that the
property of specific gravity is one of importance, and being derived
from the Latin, means _species_, a particular sort or kind; and
_gravis_, heavy or weight--i.e., the particular weight of every
substance compared with a fixed standard of water.
[Illustration: Fig. 65. A. A large cylindrical vessel containing water,
in which the egg sinks till it reaches the bottom of the glass. B. A
similar glass vessel containing half brine and half water, in which the
egg floats in the centre--viz., just at the point where the brine and
water touch.]
We are so constantly in the habit of referring to a standard of
perfection in music and the arts of painting and sculpture, that the
youngest will comprehend the office of water when told that it is the
philosopher's unit or starting-point for the estimation of the relative
weights of solids and liquids. A good idea of the scope and meaning of
the term specific gravity, is acquired by a few simple experiments,
thus: if a cylindrical [Page 49] glass, say eighteen inches long, and
two and a half wide, is filled with water, and another of the same size
is also filled, one half with water and the other half with a saturated
solution of common salt, or what is commonly termed brine, a most
amusing comparison of the relative weights of equal bulks of water and
brine, can be made with the help of two eggs; when one of the eggs is
placed in the glass containing water, it immediately sinks to the
bottom, showing that it has a greater specific gravity than water; but
when the other egg is placed in the second glass containing the brine,
it sinks through the water till it reaches the strong solution of salt,
where it is suspended, and presents a most curious and pretty
appearance; seeming to float like a balloon in air, and apparently
suspended upon nothing, it provokes the inquiry, "whether magnetism has
anything to do with it?" The answer, of course, is in the negative, it
merely floats in the centre, in obedience to the common principle, that
all bodies float in others which are heavier than themselves; the brine
has, therefore, a greater weight than an equal bulk of water, and is
also heavier than the egg. A pleasing sequel to this experiment may be
shown by demonstrating how the brine is placed in the vessel without
mixing with the water above it; this is done by using a glass tube and
funnel, and after pouring away half the water contained in the vessel
(Fig. 65.), the egg can be floated from the bottom to the centre of the
glass, by pouring the brine down the funnel and tube. The saturated
solution of salt remains in the lower part of the vessel and displaces
the water, which floats upon its surface like oil on water, carrying the
egg with it.
[Illustration: Fig. 66. A vessel half full of water, and as the brine is
poured down the tube the egg gradually rises.]
The water of the Dead Sea is said to contain about twenty-six per cent.
of saline matter, which chiefly consists of common salt. It is perfectly
clear and bright, and in consequence of the great density, a person may
easily float on its surface, like the egg on the brine, so that if a
ship could be heavily laden whilst floating on the water of the Dead
Sea, it would most likely sink if transported to the Thames. This
illustration of specific gravity is also shown by a model ship, which
being first floated on the brine, will afterwards sink if conveyed to
another vessel containing water. One of the tin model ships sold as a
magnetic [Page 50] toy answers nicely for this experiment, but it must be
weighted or adjusted so that it just floats in the brine, A; then it
will sink, when placed, in another vessel containing only water.
[Illustration: Fig. 67.
A. Vessel containing brine, upon which the little model floats.
B. Vessel containing water, in which the ship sinks.]
Another amusing illustration of the same kind is displayed with goldfish,
which swim easily in water, floating on brine, but cannot dive to
the bottom of the vessel, owing to the density of the saturated solution
of salt. If the fish are taken out immediately after the experiment, and
placed in fresh water, they will not be hurt by contact with the strong
salt water.
These examples of the relative weights of equal bulks, enable the
youthful mind to grasp the more difficult problem of ascertaining the
specific gravity of any solid or liquid substance; and here the strict
meaning of terms should not be passed by. _Specific_ weight must not be
confounded with _Absolute_ weight; the latter means the entire amount of
ponderable matter in any body: thus, twenty-four cubic feet of sand
weigh about one ton, whilst specific weight means the _relation_ that
subsists between the _absolute weight_ and the _volume_ or _space_ which
that _weight_ occupies. Thus a cubic foot of water weighs sixty-two and
a half pounds, or 1000 ounces avoirdupois, but changed to gold, the
cubic foot weighs more than half a ton, and would be equal to about
19,300 ounces--hence the relation between the cubic foot of water and
that of [Page 51] gold is nearly as 1 to 19.3; the latter is therefore
called the specific gravity of gold.
Such a mode of taking the specific gravity of different
substances--viz., by the weight of equal bulks, whether cubic feet or
inches, could not be employed in consequence of the difficulty of
procuring exact cubic inches or feet of the various substances which by
their peculiar properties of brittleness or hardness would present
insuperable obstacles to any attempt to fashion or shape them into exact
volumes. It is therefore necessary to adopt the method first devised by
Archimedes, 600 B.C., when he discovered the admixture of another metal
with the gold of King Hiero's crown.
This amusing story, ending in the discovery of a philosophical truth,
may be thus described:--King Hiero gave out from the royal treasury a
certain quantity of gold, which he required to be fashioned into a
crown; when, however, the emblem of power was produced by the goldsmith,
it was not found deficient in weight, but had that appearance which
indicated to the monarch that a surreptitious addition of some other
metal must have been made.
It may be assumed that King Hiero consulted his friend and philosopher
Archimedes, and he might have said, "Tell me, Archimedes, without
pulling my crown to pieces, if it has been adulterated with any other
metal?" The philosopher asked for time to solve the problem, and going
to take his accustomed bath, discovered then specially what he had never
particularly remarked before--that, as he entered the vessel of water,
the liquid rose on each side of him--that he, in fact, displaced a
certain quantity of liquid. Thus, supposing the bath to have been full
of water, directly Archimedes stepped in, it would overflow. Let it be
assumed that the water displaced was collected, and weighed 90 pounds,
whilst the philosopher had weighed, say 200 pounds. Now, the train of
reasoning in his mind might be of this kind:--"My body displaces 90
pounds of water; if I had an exact cast of it in lead, the same _bulk_
and _weight_ of liquid would overflow; but the weight of my body was,
say 200 pounds, the cast in lead 1000 pounds; these two sums divided by
90 would give very different results, and they would be the specific
gravities, because the rule is thus stated:--'Divide the gross weight by
the loss of weight in water, the water displaced, and the quotient gives
the specific gravity.'" The rule is soon tested with the help of an
ordinary pair of scales, and the experiment made more interesting by
taking a model crown of some metal, which may be nicely gilt and
burnished by Messrs. Elkington, the celebrated electro-platers of
Birmingham. For convenience, the pan of one scale is suspended by
shorter chains than the other, and should have a hook inserted in the
middle; upon this is placed the crown, supported by very thin copper
wire. For the sake of argument, let it be supposed that the crown weighs
17½ ounces avoirdupois, which are duly placed in the other scale-pan,
and without touching these weights, the crown is now placed in a vessel
of water. It might be supposed that directly the crown enters the water,
it would gain weight, in consequence of being wetted, [Page 52] but the
contrary is the case, and by thrusting the crown into the water, it may
be seen to rise with great buoyancy so long as the 17½ ounces are
retained in the other scale-pan; and it will be found necessary to place
at least two ounces in the scale-pan to which the crown is attached
before the latter sinks in the water; and thus it is distinctly shown
that the crown weighs only about 15½ ounces in the water, and has
therefore _lost_ instead of _gaining_ weight whilst immersed in the
liquid. The rule may now be worked out:
Ounces.
Weight of crown in air 17½
Ditto in water 15½
---
Less in water 2
17½ / 2 = 8¾
The quotient 8¾ demonstrates that the crown is manufactured of
copper, because it would have been about 19¼ if made of pure gold.
[Illustration: Fig. 68. A. Ordinary pair of scales. B. Scale-pan,
containing 17½ ounces, being the weight of the crown in air. C. Pan,
with hook and crown attached, which is sunk in the water contained in
the vessel D; this pan contains the two ounces, which must be placed
there to make the crown sink and exactly balance B.]
[Page 53]
_Table of the Specific Gravities of the Metals in common use._
Platinum 20.98
Gold 19.26 to 19.3 and 19.64
Mercury 13.57
Lead 11.35
Silver 10.47 to 10.5
Bismuth 9.82
Copper 8.89
Iron 7.79
Tin 7.29
Zinc 6.5 to 7.4
The simple rule already explained may be applied to all metals of any
size or weight, and when the mass is of an irregular shape, having
various cavities on the surface, there may be some difficulty in taking
the specific gravity, in consequence of the adhesion of _air-bubbles_;
but this may be obviated either by brushing them away with a feather,
or, what is frequently much better, by dipping the metal or mineral
first into alcohol, and then into water, before placing it in the vessel
of water, by which the actual specific gravity is to be taken.
The mode of taking the specific gravity of liquids is very simple, and
is usually performed in the laboratory by means of a thin globular
bottle which holds exactly 1000 grains of pure distilled water at 60°
Fahrenheit. A little counterpoise of lead is made of the exact weight of
the dry globular bottle, and the liquid under examination is poured into
the bottle and up to the graduated mark in the neck; the bottle is then
placed in one scale-pan, the counterpoise and the 1000-grain weight in
the other; if the liquid (such as oil of vitriol) is heavier than water,
then more weight will be required--viz., 845 grains--and these figures
added to the 1000 would indicate at once that the specific gravity of
oil of vitriol was 1.845 as compared with water, which is 1.000. When
the liquid, such as alcohol, is lighter than water, the 1000-grain
weight will be found too much, and grain weights must be added to the
same scale-pan in which the bottle is standing, until the two are
exactly balanced. If ordinary alcohol is being examined, it will be
found necessary to place 180 grains with the bottle, and these figures
deducted from the 1000 grains in the other scale-pan, leave 820, which,
marked with a dot before the first figure (_sic_ .820), indicates the
specific gravity of alcohol to be less than that of water.
The difference in the gravities of various liquids is displayed in a
very pleasing manner by an experiment devised by Professor Griffiths, to
whom chemical lecturers are especially indebted for some of the most
ingenious and beautiful illustrations which have ever been devised. The
experiment consists in the arrangement of five distinct liquids of
various densities and colours, the one resting on the other, and
distinguished not only by the optical line of demarcation, but by little
balls of wax, which are adjusted by leaden shot inside, so as to sink
through [Page 54] the upper strata of liquids, and rest only upon the
one that it is intended to indicate.
The manipulation for this experiment is somewhat troublesome, and is
commenced by procuring some pure bright quicksilver, upon which an iron
bullet (black-leaded, or painted of any colour) is placed, or one of
those pretty glass balls which are sold in such quantities at the
Crystal Palace.
Secondly. Put as much white vitriol (sulphate of zinc) into a half pint
of boiling water as it will dissolve, and, when cold, pour off the clear
liquid, make up a ball of coloured wax (say red), and adjust it by
placing little shot inside, until it sinks in a solution of sulphate of
copper and floats on that of the white vitriol.
Thirdly. Make a solution of sulphate of copper in precisely the same
manner, and adjust another wax ball to sink in water, and float on this
solution.
Fourthly. Some clear distilled water must be provided.
Fifthly. A little cochineal is to be dissolved in some common spirits of
wine (alcohol), and a ball of cork painted white provided.
Finally. A long cylindrical glass, at least eighteen inches high, and
two and a half or three inches diameter, must be made to receive these
five liquids, which are arranged in their proper order of specific
gravity by means of a long tube and funnel.
The four balls--viz., the iron, the two wax, and the cork balls, are
allowed to slide down the long glass, which is inclined at an angle; and
then, by means of the tube and funnel, pour in the tincture of
cochineal, and all the balls will remain at the bottom of the glass. The
water is poured down next, and now the cork ball floats up on the water,
and marks the boundary line of the alcohol and water. Then the solution
of blue vitriol, when a wax ball floats upon it. Thirdly, the solution
of white vitriol, upon which the second wax ball takes its place; and
lastly, the quicksilver is poured down the tube, and upon this heavy
metallic fluid the iron or glass ball floats like a cork on water.
[Illustration: Fig. 69. Long cylindrical glass, 18 × 3 inches,
containing the five liquids.]
The tube may now be carefully removed, pausing at each liquid, so that
no mixture take place between them; and the result is the arrangement of
five liquids, giving the appearance of a cylindrical glass painted with
[Page 55] bands of crimson, blue, and silver; and the liquids will not
mingle with each other for many days.
A more permanent arrangement can be devised by using liquids which have
no affinity, or will not mix with each other--such as mercury, water,
and turpentine.
The specific weight or weights of an equal measure of air and other
gases is determined on the same principle as liquids, although a
different apparatus is required. A light capped glass globe, with
stop-cock, from 50 to 100 cubic inches capacity, is weighed full of air,
then exhausted by an air-pump, and weighed empty, the loss being taken
as the weight of its volume of air; these figures are carefully noted,
because _air_ instead of _water_ is the standard of comparison for all
gases. When the specific gravity of any other gas is to be taken, the
glass globe is again exhausted, and screwed on to a gas jar provided
with a proper stop-cock, in which the gas is contained; and when perfect
accuracy is required, the gas must be dried by passing it over some
asbestos moistened with oil of vitriol, and contained in a glass tube,
and the gas jar should stand in a mercurial trough. (Fig. 70.) The
stop-cocks are gradually turned, and the gas admitted to the exhausted
globe from the gas jar; when full, the cocks are turned off, the globe
unscrewed, and again weighed, and by the common rule of proportion, as
the weight of the air first found is to the weight of the gas, so is
unity (1.000, the density of air) to a number which expresses the
density of the gas required. If oxygen had been the gas tried, the
number would be 1.111, being the specific gravity of that gaseous
element. If chlorine, 2.470. Carbonic acid, 1.500. Hydrogen being much
less than air, the number would only be 69, or decimally 0.069.
[Illustration: Fig. 70. A. Glass globe to contain the gas. B. Gas jar
standing in the mercurial trough, D. C. Tube containing asbestos
moistened with oil of vitriol.]
A very good approximation to the correct specific gravity (particularly
where a number of trials have to be made with the same gas, such as
[Page 56] ordinary coal gas) is obtained by suspending a light paper
box, with holes at one end, on one arm of a balance, and a counterpoise
on the other. The box can be made carefully, and should have a capacity
equal to a half or quarter cubic foot; it is suspended with the holes
downward, and is filled by blowing in the coal gas until it issues from
the apertures, and can be recognised by the smell. The rule in this case
would be equally simple: as the known weight of the half or quarter
cubic foot of common air is to the weight of the coal gas, so is 1.000
to the number required. (Fig. 71.)
[Illustration: Fig. 71. A. The balance. B. The paper box, of a known
capacity. C. Gas-pipe blowing in coal-gas, the arrows showing entrance
of gas and exit of the air.]
As an illustration of the different specific weights of the gases, a
small balloon, containing a mixture of hydrogen and air, may be so
adjusted that it will just sink in a tall glass shade inverted and
supported on a pad made of a piece of oilcloth shaped round and bound
with list. On passing in quickly a large quantity of carbonic acid, the
little balloon will float on its surface; and if another balloon,
containing only hydrogen, is held in the top part of the open shade, and
a sheet of glass carefully slid over the open end, the density of the
gases (although they are perfectly invisible) is perfectly indicated;
and, as a climax to the experiment, a third balloon can be filled with
laughing gas, and may be placed in the glass shade, taking care that the
one full of pure hydrogen does not escape; the last balloon will sink to
the bottom of the [Page 57] jar, because laughing gas is almost as
heavy as carbonic acid, and the weight of the balloon will determine its
descent. (Fig. 72.)
[Illustration: Fig. 72. Inverted large glass shade, containing half
carbonic acid and half common air.]
[Illustration: Fig. 73. A. Inverted glass shade, containing the
material, B, for generating carbonic acid gas. C. The soap-bubble. D D.
The glass tube for blowing the bubbles. E. Small lantern, to throw a
bright beam of light from the oxy-hydrogen jet upon the thin
soap-bubble, which then displays the most beautiful iridescent colours.]
A soap-bubble will rest most perfectly on a surface of carbonic acid
gas, and the aerial and elastic cushion supports the bubble till it
bursts. The experiment is best performed by taking a glass shade twelve
inches broad and deep in proportion, and resting it on a pad; half a
pound of sesquicarbonate of soda is then placed in the vessel, and upon
this is poured a mixture of half a pint of oil of vitriol and half a
pint of water, the latter being previously mixed and allowed to cool
before use. An enormous quantity of carbonic acid gas is suddenly
generated, and rising to the edge, overflows at the top of the glass
shade. A well-formed soap-bubble, detached neatly from the end of a
glass-tube, oscillates gently on the surface of the heavy gas, and
presents a most curious and pleasing appearance. The soapy water is
prepared by cutting a few pieces of yellow soap, and placing them in a
two-ounce bottle containing distilled water. (Fig. 73.) The specific
gravity of the gases, may therefore be either greater, or less than
atmospheric air, [Page 58] which has been already mentioned as the
standard of comparison, and examined by this test the vapours of some of
the compounds of carbon and hydrogen are found to possess a remarkably
high gravity; in proof of which, the vapour of ether may be adduced as
an example, although it does not consist only of the two elements
mentioned, but contains a certain quantity of oxygen. In a cylindrical
tin vessel, two feet high and one foot in diameter, place an ordinary
hot-water plate, of course full of boiling water; upon this warm surface
pour about half an ounce of the best ether; and, after waiting a few
minutes until the whole is converted into vapour, take a syphon made of
half-inch pewter tube, and warm it by pouring through it a little hot
water, taking care to allow the water to drain away from it before use.
After placing the syphon in the tin vessel, a light may be applied to
the extremity of the long leg outside the tin vessel, to show that no
ether is passing over until the air is sucked out as with the
water-syphon; and after this has been done, several warm glass vessels
may be filled with this heavy vapour of ether, which burns on the
application of flame. Finally, the remainder of the vapour may be burnt
at the end of the syphon tube, demonstrating in the most satisfactory
manner that the vapour is flowing through the syphon just as spirit is
removed by the distillers from the casks into cellars of the
public-houses. (Fig. 74.)
[Illustration: Fig. 74. A. Tin vessel containing the hot-water plate, B,
upon which the ether is poured. C. The syphon. D. Glass to receive the
vapour. E. Combustion of the ether vapour in another vessel.]
[Page 59]
Before dismissing the important subject of specific gravity (or, as it
is termed by the French _savants_, "density"), it may be as well to
state that astronomers have been enabled, by taking the density of the
earth and by astronomical observations, to calculate the gravity of the
planets belonging to our solar system; and it is interesting to observe
that the density of the planet Venus is the only one approaching the
gravity of the earth:--
The Earth 1.000
The Sun .254
The Moon .742
Mercury 2.583
Venus 1.037
Mars .650
Jupiter .258
Saturn .104
Herschel .220