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condition, and thus introducing water might falsify the result, there
is interposed a tube containing fragments of fused chloride of cal-
cium, by which the hydrogen gas is completely dried before it
comes into contact with the oxide of copper. When the apparatus
has been filled with pure dry hydrogen, heat is applied to the bulb
containing the black oxide of copper, and, as soon as its temperature
has been raised to dull redness, decomposition commences. The
oxide of copper becomes glowing red, and then, even if the heat of
the lamp be much reduced, the reaction still goes on ; the glowing
gradually pervades the whole mass, water is formed and condensed
on the colder parts of the tube in considerable quantity, and when
the reaction is completed, there remains in the bulb a porous mass
of pure metallic copper. It is necessary, however, to determine
exactly the quantity of water formed : for this purpose, a small tube
filled with fragments of fused chloride of calcium is attached to the
bulb tube by means of a caoutchouc connector, and the stream of
hydrogen gas is allowed to continue through the apparatus until
the residual copper has become quite cold, and all traces of water
have been carried into the chloride of calcium tube, where it is com-
pletely absorbed and retained. The oxide of copper tube having
been weighed before and after the experiment, the loss found is the
oxygen which has been carried off: the chloride of calcium tube
having been also weighed before and after, the gain gives the weight
of water formed, and hence the composition of water may easily
be calculated, thus :



254 CONSTITUTION OF WATER.

100 parts of black oxide of copper give
79-85 of metallic copper, and lose
20-15 of oxygen, which form
22-67 of water ;

consequently, 22-67 of water consist of

20.15 of oxygen,
2-52 of hydrogen ;

or in 100 parts,

88-9 of oxygen,
11-1 of hydrogen.

To determine the combining equivalents of these substances, or,
in theoretical language, their atomic weights, it is first necessary to
ascertain what grounds there are for deciding on the atomic consti-
tution of water.

It has been already mentioned that at one time equal volumes of
all gases were considered to contain the same number of atoms ;
and hence, as water is formed by the union of one volume of oxy-
gen and two of hydrogen, it was considered by some chemists to
consist of one atom of oxygen united to two of hydrogen ; there-
fore, if we take oxygen as the standard of atomic weights, and call its
equivalent number 100, the result would be that 88-9 : 11-1 :: 100 :
12-48= weight of two atoms of hydrogen ; and hence the atomic
weight of hydrogen should be 6-24. To the adoption of this number
there are very many objections : 1st, The ground upon which it was
first adopted has been proved to be false, as the same volumes of all
gases certainly do not contain the same number of chemical atoms ;
and, although hydrogen enters so much into combination, it is but
very rarely that it does so in the proportion of 6-24. Likewise wa-
ter, in all the modes in which it is capable of combining, does so in
a quantity containing 12 - 48 and not 6-24 ; and, finally, water in com-
bination allies itself to a variety of metallic oxides, all of which
there is the strongest reason for supposing to be composed of one
equivalent or atom of the metal united with one of oxygen.

Water is therefore assumed to be composed of an equivalent of
each of its constituents ; and according as the standard of oxygen or
of hydrogen is taken, its atomic weight is

One atom of oxygen 100-00 8

One atom of hydrogen . . . 12-24 1

112-24 "9

Water is colourless, transparent, destitute of taste and smell. If
agitated, it solidifies at a temperature of 32 F. (0 Centigrade), but
if preserved quiescent, it may be cooled much lower without freez-
ing ; if it be then touched or shaken, a portion is immediately con-
verted into ice, and the temperature of the whole is raised to 32.
In freezing, water expands very much, and exerts therein so great
a force as to burst the strongest vessels in which it is contained.
It is thus that the surfaces of the hardest rocks are gradually crum-
bled into soil fit for the support of vegetable life j the water perco-



SOLUTION OF GASES IN WATER. 255

latmg into minute crevices and fissures during the warmer months,
and, when frozen in winter, breaking down, by repeated and in-
creasing expansive efforts during successive years, the substance
of masses which would appear, from compactness and hardness, fit-
ted to withstand the severest effects of time and climate.

The specific gravity of steam, such as it would be at the standard
temperature and pressure, is found to be 620-1, atmospheric air be-
ing 1000-0. Two volumes of steam contain two of hydrogen and
one of oxygen ; hence there is a condensation of three volumes to
two produced by the union of the gases. The calculated result is
thus found :

Two volumes of hydrogen . 68-8x2= 137-6

One volume of oxygen =1102-6

give two volumes of vapour of water 124<0-2
hence one volume of vapour of water = 620-1

In forming steam at 212, and a pressure of 30 inches mercury,
water expands to 1696 times its volume according to the determina-
tion of Gay Lussac, which gives almost exactly the proportion of a
cubic inch of water forming a cubic foot of steam, which contains
1728 cubic inches.

The solution of gases in water appears to be governed by prin
ciples similar to those which regulate the solvent action of water
upon solid bodies. In some instances there certainly takes place
chemical union, as in the case of muriatic acid gas and ammonia ; and
in these cases the condensation of the gases by the water is accom-
panied by the evolution of considerable heat. But in other cases,
particularly where the quantity of gas is small, the result appears
to be merely a mechanical distribution of the molecules of the gas
throughout the mass of the liquid. The following is a table of the
quantity of these gases absorbed by water without combination
100 volumes of water at 60 Fahr. and 30 inches bar. absorb of

Sulphuretted hydrogen 253 volumes

Sulphurous acid 438

Chlorine 206

Carbonic acid 100

Nitrous oxide 76

Olefiant gas 12'5

Oxygen 3'7

Nitrogen ) , fi

Hydrogen J '

The mixture of nitrogen and oxygen, which constitutes the air
we breathe, is absorbed by water, and it is to the air thus dissolved
that water, in great part, owes its refreshing taste. If water be
boiled this air is expelled, and this should be done if it be wished
to saturate the water with any other gas, as the power of water to
absorb any other gas is remarkably diminished by the presence of
even a small quantity of air. If water already saturated with one
gas be exposed to the action of a second, it lets a portion of the
first escape, and absorbs a corresponding quantity of the second.
In this way, a very small quantity of a sparingly soluble gas may
expel a large quantity of one much more soluble. A familiar exam-
ple of this fact consists in taking a glass half full of Champagne, and



256 CHEMICAL RELATIONS OF WATER.

having formed the palm of the hand into a hollow cup, to strike the
top of the glass, closing the glass and flattening the hand at the same
time ; the air above the wine is thus forcibly compressed, and a por-
tion is then absorbed, under pressure, by the fluid, from which a
quantity of carbonic acid is expelled, greater than that of air absorb-
ed in the proportion of 1060 to 16, and thus the effervescent char-
acter of the wine restored.

Notwithstanding this character of neutrality, which renders wa-
ter so useful as a vehicle or solvent for more energetic bodies, wa-
ter is actively engaged in a great variety of chemical reactions, in
which its elements, separating from one another, appear in an iso-
lated state, or, by combining with other substances which may be
present, generate new compounds. Even, however, independent of
decomposition, water plays a most important part in chemical theory,
from the numerous classes of compounds of which it forms a con-
stituent. The generality of saline bodies, in crystallizing, retain a
quantity of water, often more than one half their weight j this is
termed water of crystallization. On the application of a moderate
heat this water separates, and frequently the salt dissolves in its
own water of crystallization on being heated, undergoing what is
termed watery fusion.

Salts containing water of crystallization often attract still more
if surrounded by damp air, and fall into a liquid state : such are
termed deliquescent salts ; others, on the contrary, give off their
water of crystallization even at ordinary temperatures, if the air be
moderately dry ; some, such as the carbonate and sulphate of soda,
losing all j others, as the phosphate of soda, only a portion of that
constituent : these are termed efflorescent salts ; when the efflores-
cence is complete, they lose their crystalline arrangement and fall
to powder.

Graham has shown, that in many classes of salts, particularly in
the sulphates, one portion of the water is much more intimately
united than the remainder 5 that, in fact, in addition to the mere
water of crystallization, which may be removed without injury, there
is water essential to the constitution of the salt, and replaced by
other bodies when the salt enters into combination. Thus, in the
common crystallized sulphate of copper there are five atoms of wa-
ter, of which four are removable by a temperature of 150, but the
fifth withstands a temperature of 300. The formula of this salt is,
therefore, not Cu.O. . So 3 + 5H.O.,but Cu.O. . S.0 3 H.O. +4H.O.,the
four atoms being water of crystallization; now if this salt be com-
bined with sulphate of potash, this fifth atom of water disappears,
and the double salt is (Cu.O. . S.0 8 ) (K.O. . S.O S ) +4H.0. 5 the K.O.
S.0 3 having entered into the place of the water which had been ex-
pelled ; water thus circumstanced is termed constitutional water,
as being necessary to the complete constitution of the substance.

Water, in combination with the stronger acids, is capable of act-
ing as a base, and, indeed, we know of the existence of many acids
only in the form of their compounds with water. Thus the nitric,
the chloric, the oxalic, the acetic acids, have never been* obtained
in a separate form; what we generally term those acids being, in
reality, compounds of these acids with water salts of water. Oil



WATER IN CHEMICAL COMBINATION. 25?

of vitriol is a compound of sulphuric acid and water, sulphate of
water, and it combines with more water, producing great heat, to
form a sulphate of water with excess of base, precisely as the sul-
phate of copper combines with more oxide of copper to produce a
corresponding basic salt. The salts of water possess the most com-
plete similarity with those of zinc and copper, and it is from their
comparative study that the evidence in favour of the view inculca-
ted already, of hydrogen being a volatile metal, has been in greatest
part derived.

Water combines also with bases : the majority of metallic oxides
combine with water, often with the evolution of considerable heat.
The slacking of lime, which is the act of combination of dry lime
with water, produces so much heat as to ignite gunpowder, and r
when in large quantity, to become red hot. Ships laden with lime
nave often been burned at sea, from water getting into the hold
among the lime, and so much heat being evolved as to set the ship
on fire. Barytes and strontia produce, in slacking, still more heat.
Potash retains water so strongly that it can only be obtained free
from it by the direct combustion of the metal, potassium, in dry
oxygen gas or air. In relation to these powerful bases, water ap-
pears, therefore, to act the part of a feeble acid.

The compounds of water have been generally termed by chemists
hydrates ; thus, hydrate of lime, hydrated oxide of copper, hydrated
sulphuric acid, hydrated sulphate of zinc. The very different func-
tions performed by water, in the various modes of combination it
affects, render it necessary to adopt a definite principle of nomen-
clature in this respect. In the subsequent pages I shall employ the
vford-hydi-ate only where the water is combined with a base, such
as a metallic oxide ; thus, hydrate of lime, hydrate of potash, hy-
drated oxide of lead. Where the water is united to an acid, I shall,
in all cases in which the true chemical nature of the compound
comes into play, term it a salt ; as sulphate of water, oxalate of
water, &c. ; but where no strict theoretical explanation is involved,
I shall continue to use the common name, .as oil of vitriol, strong
sulphuric acid, oxalic acid, aquafortis, &c. There is no name pecu-
liarly applicable to the form of compounds which contains constitu-
tional Avater, but it will serve as well to characterize the absence or
deprivation of this water by the word anhydrous, the ordinary
name of the substance being supposed to include the combined wa-
ter j thus, common sulphate of zinc, freed from water of crystalliza-
tion, is Zn.O. . S.O 3 . H.O. ; anhydrous sulphate of zinc is Zn.O. . S.0 3 .
When there exists water of crystallization in a salt, it is of course
included when the salt is spoken of as crystallized. In formula?,
for the purpose of distinguishing between water of crystallization
and water more closely united, the latter will always be marked by
the symbols of its constituents, the former by the two initial letters
of the Latin word aqua, aq. ; thus the crystallized oxalic acid is
C 2 O 3 . H.O. + 2Aq. The phosphate of soda is P 2 O 5 +2Na.O. . H.O.-f-



Water does not exist in nature in a perfectly pure condition. It
contains dissolved a small portion of atmospheric air and of carbonic
acid, and also certain quantities of solid impurities, of which com

K K



258 MINERAL WATERS. PEROXIDE OF HYDROGEN.

mon salt, sulphates and carbonates of lime, and chloride of magne-
sium, are the most important. In particular localities, the water is-
suing from the earth contains iron, and often sulphuretted hydrogen ;
also traces of iodine and bromine ; and occasionally the quantity of
these foreign matters present is so great as to confer upon the wa-
ter medicinal properties, and to make such springs, under the name
of mineral springs, spas, be resorted to for the purpose of preserving
or recovering health. These impurities arise from the water, in
percolating through the porous rocky strata, of which the mount-
ains and general crust of the earth are composed, dissolving in small
quantity almost all the substances it meets. Hence rain-water or
snow-water, collected at a distance from houses, is the purest water
which can be obtained in nature. It contains only some carbonic
acid and air dissolved. The sea being the general reservoir into
which all the rivers of the earth discharge their waters, contains
in a concentrated form all the materials which the river waters had
carried down. Although not of absolutely the same constitution all
over the globe, yet it varies so little that the deviation may be ex-
plained by local circumstances. It is in many countries the source
from which common salt and sulphate of magnesia are derived.
When sea-water freezes, the ice contains scarcely a trace of saline
matter, so that, when melted, it forms a sweet, drinkable wafer.
Hence, in voyages in the Northern Seas, supplies of fresh water are
obtained by chopping blocks of ice from the frozen surface of the
ocean. To obtain water pure for chemical purposes, it is necessary
to distil it. The saline and fixed impurities remain behind, the pure
water passes over. Its purity may be ascertained by means of the
reagents fitted to detect the most important impurities. Thus, if
free from common salt, it will give no precipitate with a solution of
nitrate of silver j if free from lime, it will not be affected by oxalic
acid ; and if it be not rendered turbid by nitrate of barytes, it can
not contain sulphuric acid.

From the inactivity of water, and the facility with which it may
be obtained pure, as well as the important part which it plays in the
economy of nature, it is taken as the standard with which the prop-
erties of other bodies, when numerically determined, are compared.
Thus the specific heats of solids and liquids are reduced to a scale,
water being taken as 1-000. The specific gravities of liquids and
soli'ds are also taken in numbers, that of water being the standard.
If, however, the specific gravities, and heats of gases and vapours
were reduced to the standard of water, the fractions by which they
should be expressed would be inconveniently small ; and hence, for
this class of bodies, a better suited standard substance is found in
atmospheric air.

Of Oxygenated Water. Peroxide of Hydrogen.

H.O. + O., orH.O,.

This singular substance was first discovered by Thenard ; its
preparation is somewhat circuitous and indirect, oxygen and hydro-
gen not combining with each other directly in any other proportions
but those which form water.
For its preparation, peroxide of Liarium must be first procured ; this is prepared



PEROXIDE OF HYDROGEN, PREPARATION, ETC. 259

by placing pure barytes (oxide of barium) in a porcelain tube, which is heated to
redness in a charcoal furnace, and then a stream of pure oxygen gas passed over it
as long as it is absorbed ; the barytes absorbs as much more oxygen as it already
contained, and from Ba.O. becomes Ba.0 2 .

This substance may be still more easily prepared by mixing pure barytes with
its weight of chlorate of potash, and heating it nearly to redness ; when the disen-
gagement of oxygen from the chlorate of potash has set in, the mass becomes
glowing red at one point, and this appearance spreads over the whole mass like tinder.
The barytes burns, as it were, in the atmosphere of oxygen, and .forms the deutox-
ide of barium. If, then, the residual mass be washed with water, the chloride of
potassium, which remains from the chlorate of potash, is dissolved, and the deutox-
ide of barium, combining with an equivalent of water to form a bulky, white, insol-
uble hydrate, remains behind, and may be collected on a filter.

The best mode of obtaining peroxide of hydrogen from this substance is that pro-
posed by Pelouze. To dilute hydrofluoric acid (fluoride of hydrogen), the peroxide
of barium is added until the acidity of the liquor is completely neutralized. The
reaction is very simple ; the fluorine combines with the barium, while all the oxy-
gen is transferred to the hydrogen, which the fluoric acid abandons ; thus,
H.F.-j-Ba.0 2 =Ba.F.-|-H.02.

The fluoride of barium is insoluble, and may be collected on a filter along with
the excess of peroxide of barium ; the liquor contains only pure oxygenated water.
Fluosilicic acid, which is cheaper, and more convenient than the fluoric acid, may
also be used in this decomposition. The fluosilicate of barium separates as an in-
soluble white powder, and the peroxide of hydrogen remains dissolved.

Thenard's plan consisted in dissolving the peroxide of barium in dilute muriatic
acid, and then precipitating the barytes by sulphuric acid. The muriatic acid
which became free was then neutralized by another portion of peroxide of barium,
and this again precipitated by sulphuric acid. When the liquor had become strong
enough, the free muriatic acid was removed by the cautious addition of sulphate of
silver, and the sulphuric acid then evolved was precipitated by the addition of pure
barytes, carefully avoiding an excess.

The weak solution of peroxide of hydrogen thus obtained must be placed, along
with a capsule of sulphuric acid, under the exhausted receiver of the air-pump.
The water being more volatile, evaporates first, and the liquor gradually becomes
more concentrated, until, finally, the peroxide of hydrogen remains pure behind. If
left too long in the exhausted vessel, it evaporates itself without alteration.

Peroxide of hydrogen is a thick, colourless liquid. Its specific gravity is 1-452.
It has a nauseous taste, and irritates the skin. It bleaches and destroys all vegeta-
ble colours. Its reactions are generally so violent that it must be diluted with
many times its volume of water before they can be accurately observed.

Its most curious property is, that by being put in contact with any one of a great
number of solid substances, it is decomposed with great rapidity, being resolved
into oxygen and water. Black oxide of manganese is one of the most active. If a
little of this substance, in powder, be introduced into strong peroxide of hydrogen, in
a graduated tube, over mercury, the latter is decomposed almost explosively, disen-
gaging 475 times its volume of oxygen, the oxide of manganese remaining perfectly
unaltered. Platinum, gold, silver, quicksilver, particularly if the metal be in the
form of leaf or sponge, produce the same effect ; and if the peroxide of hydrogen
be put into contact with an oxide of these metals, as oxide of silver, it is not mere-
ly decomposed itself, but the oxide is also decomposed, the oxygen and metal both
becoming free. In the dark, and with strong peroxide of hydrogen, a flash of light is
seen to accompany its decomposition, and the rabe becomes red hot. The decompo-
sition of the oxide of silver cannot, however, be referred to the great heat produced,
as, even if the peroxide of hydrogen be diluted with fifty times its volume of water,
oxide of silver produces complete decomposition, evolution of oxygen, and separa-
tion of metallic silver; yet the effervescence is not very energetic, and the liquor
does not become sensibly warm to the hand.

With other metals, the oxygen, in place of becoming free, enters into combinatipn,
forming an oxide of a higher degree ; thus, with the oxides of lead and bismuth
there are formed peroxides of those metals ; with arsenic there is formed arsenic
acid. The animal substances fibrine and albumen, which are so similar in most
respects, are distinguished from each other by their action on this body, fibrine de-
composing it with rapidity, while albumen is without effect. It is highly probable,
that in the decomposition of water by the voltaic pile, some of this compound may



260 PREPARATION OF NITROGEN GAS.

be formed, as the quantity of oxygen collected is frequently smaller than it should
be ; and a portion of the process of bleaching, by exposing the wetted cloth to the
action of light and air, may possibly be carried on by the formation and subsequent
decomposition of this substance.

Peroxide of hydrogen, when kept for any length of time, even in a dilute condi-
tion, gradually decomposes, oxygen being given off, and water remaining behind.
The presence of an acid in the liquor retards this action very much, while the pres-
ence of an alkali accelerates it. It was in great part from the remarkable charac-
ters of this body that Berzelius derived his evidence in favour of the existence of a
catalytic force influencing chemical action, which has been described already.

Of Nitrogen.

N.

It has been already noticed, that the substance by which the ox-
ygen is diluted in atmospheric air, so as to render it suitable to the
respiration of animals, is called nitrogen, from its being the basis
of nitric acid and nitre (the nitre former). It is also called by some
chemists azote, from its incapability of supporting life ; but, as a
great number of gases resemble it in that respect, the former name
is the more characteristic, and it alone will be hereafter used.
As nitrogen exists in great quantity in the air we breathe, it is
most easily obtained by acting upon a con-
fined portion of the air so as to abstract the
oxygen, when the residual gas is found to
be nitrogen almost completely pure. Thus,
if a small piece of phosphorus, laid in a cup
e/, floating on water, be set on fire, and a bell
glass, a, be inverted over it, the phosphorus,
in burning, unites with the oxygen of the
air, and forms white fumes of phosphoric
acid. At first, from the great expansion of
the air caused by the high temperature of the flame, some bubbles
escape from under the edge of the glass ; but soon, even before the
phosphorus has ceased to burn, the water begins to rise in the bell,
and, finally, the clouds of phosphoric acid gradually dissolving in the
water, the residual gas will be found to occupy about four fifths of
the original volume of the air, and to be colourless as the air had
been before. Any other burning body would answer the same pur-



Online LibraryRobert KaneElements of chemistry → online text (page 36 of 101)