to occasion a deposit of "fulminating gold," the properties of which are
described immediately following.
_Properties of Chloride of Gold._--As sold in commerce it usually contains
excess of hydrochloric acid, and is then of a bright yellow color; but
when neutral and somewhat concentrated it is dark red (_Leo ruber_ of the
alchemists). It gives no precipitate with carbonate of soda, unless heat
be applied; the free hydrochloric acid present forms, with the alkali,
chloride of sodium, which unites with the chloride of gold, and produces a
double salt, chloride of gold and sodium, soluble in water.
Chloride of gold is decomposed with precipitation of metallic gold by
charcoal, sulphurous acid, and many of the vegetable acids; also by
protosulphate and protonitrate of iron. It tinges the cuticle of an
indelible purple tint. It is soluble in alcohol and in ether.
Gold, Fulminating.
This is a yellowish-brown substance, precipitated on adding ammonia to a
strong solution of chloride of gold.
It may be dried carefully at 212°, but _explodes violently_ on being
heated suddenly about to 290°. Friction also causes it to explode when
dry; but the moist powder may be rubbed or handled without danger. It is
decomposed by sulphuretted hydrogen.
Fulminating gold is probably an aurate of ammonia, containing 2 atoms of
ammonia to 1 atom of peroxide of gold.
Gold, Hyposulphite of.
Symbol, AuO S{2}O{2}. Atomic Weight, 253.
Hyposulphite of gold is produced by the reaction of chloride of gold upon
hyposulphite of soda.
The salt sold in commerce as sel d'or is a double hyposulphite of gold and
soda, containing one atom of the former salt to three of the latter, with
four atoms of water of crystallization. It is formed by adding one part of
chloride of gold, in solution, to three parts of hyposulphite of soda, and
precipitating the resulting salt by alcohol; the chloride of gold must be
added to the hyposulphite of soda, and not the soda salt to the gold.
_Properties._--Hyposulphite of gold is unstable and cannot exist in an
isolated state, quickly passing into sulphur, sulphuric acid, and metallic
gold. When combined with excess of hyposulphite of soda in the form of sel
d'or, it is more permanent.
Sel d'or occurs crystallized in fine needles, which are very soluble in
water. The commercial article is often impure, containing little else than
hyposulphite of soda, with a trace of gold. It may be analyzed by adding
a few drops of strong nitric acid (free from chlorine) diluting with
water, and afterwards collecting and igniting the yellow powder, which is
metallic gold.
Grape Sugar.
Symbol, C{24}H{28}O{28}. Atomic weight, 366.
This modification of sugar, often termed _granular sugar_, or _glucose_,
exists abundantly in the juice of grapes, and in many other varieties of
fruit. It forms the saccharine concretion found in honey, raisins, dried
figs, etc. It may be produced artificially by the action of fermenting
principles, and of dilute mineral acids, upon starch.
_Properties._--Grape sugar crystallizes slowly and with difficulty from
a concentrated aqueous solution, in small hemispherical nodules, which
are hard, and feel gritty between the teeth. It is much less sweet to
the taste than cane sugar, and not so soluble in Water (1 part dissolves
in 1-1/2 of cold water). Grape sugar tends to absorb oxygen, and hence
it possesses the property of decomposing the salts of the noble metals,
and reducing them by degrees to the metallic state, even without the aid
of lights The action however in the case of _nitrate of silver_ is slow,
unless the temperature be somewhat elevated. _Cane_ sugar does not possess
these properties to an equal extent, and hence it is readily distinguished
from the other variety.
Honey.
This substance contains two distinct kinds of sugar, grape sugar, and an
uncrystallizable substance analogous to, or identical with, the treacle
found associated with common sugar in the cane juice. The agreeable
taste of honey probably depends upon the latter, but its reducing power
on metallic oxides is due to the former. Pure grape sugar can readily
be obtained from inspissated honey, by treating it with alcohol, which
dissolves out the syrup, but leaves the crystalline portion.
Hydrochloric; Acid.
Symbol, HCl. Atomic weight, 37.
Hydrochloric acid is a volatile gas, Which may be liberated from the
salts termed chlorides by the action of sulphuric acid. The acid, by its
superior affinities, removes the base; thus,--
NaCl + HO SO{3} = NaO SO{3} + HCl.
_Properties._--Abundantly soluble in water, forming the liquid
hydrochloric or muriatic acid of commerce. The most concentrated solution
of hydrochloric acid has a sp. gr. 1·2, and contains about 40 per cent, of
gas; that commonly sold is somewhat weaker, sp; gr. 1·14 = 28 per cent.
real acid.
Pure hydrochloric acid is colorless, and fumes in the air. The yellow
color of the commercial acid depends upon the presence of traces of
perchloride of iron or organic matter; commercial muriatic acid also often
contains a portion of free chlorine and of sulphuric acid.
Hydriodic Acid.
Symbol, HI. Atomic weight, 127.
This is a gaseous compound of hydrogen and iodine, corresponding in
composition to the hydrochloric acid. It cannot, however, from its
instability, be obtained in the same manner, since, on distilling
an iodide with sulphuric acid, the hydriodic acid first formed is
subsequently decomposed into iodine and hydrogen. An aqueous solution of
hydriodic acid is easily prepared by adding iodine to water containing
sulphuretted hydrogen gas; a decomposition takes place, and sulphur is set
free; thus: HS + I = HI + S.
_Properties._--Hydriodic acid is very soluble in water, yielding a
strongly acid liquid. The solution, colorless at first, soon becomes brown
from decomposition, and liberation of free iodine. It may be restored to
its original condition by adding solution of sulphuretted hydrogen.
Hydrosulphuric Acid.
Symbol, HS. Atomic weighty 17.
This substance, also known as sulphuretted hydrogen, is a gaseous compound
of sulphur and hydrogen, analogous in composition to hydrochloric and
hydriodic acids. It is usually prepared by the action of dilute sulphuric
acid upon sulphuret of iron, the decomposition being similar to that
involved in the preparation of the hydrogen acids generally:--
FeS + HO SO{3} = FeO SO{3} + HS.
_Properties._--Cold water absorbs three times its bulk of hydrosulphuric
acid, and acquires the peculiar putrid odor and poisonous qualities of the
gas. The solution is faintly acid to test-paper, and becomes opalescent on
keeping, from gradual separation of sulphur. It is decomposed by nitric
acid, and also by chlorine and iodine. It precipitates silver from its
solutions, in the form of black sulphuret of silver; also copper, mercury,
lead, etc.; but iron and other metals of that class are not affected, if
the liquid contains free acid. Hydrosulphuric acid is constantly employed
in the chemical laboratory for these and other purposes.
Hydrosulphate of Ammonia.
Symbol, NH{4}S HS. Atomic weight, 51.
The liquid known by this name, and formed by passing sulphuretted hydrogen
gas into ammonia, is a double sulphuret of hydrogen and ammonium. In the
preparation, the passage of the gas is to be continued until the solution
gives no precipitate with sulphate of magnesia and smells strongly of
hydrosulphuric acid.
Properties,--Colorless at first, but afterwards changes to yellow, from
liberation and subsequent solution of sulphur. Becomes milky on the
addition of any acid. Precipitates, in the form of sulphuret, all the
metals which are affected by sulphuretted hydrogen; and, in addition,
those of the class to which iron, zinc, and manganese, belong.
Hydrosulphate of ammonia is employed in photography to darken the negative
image, and also in the preparation of iodide of ammonium; the separation
of silver from hyposulphite solutions, etc.
Hyposulphite of Soda.
Symbol, NaO S{2}H{2} + 5 HO. Atomic weight, 125.
The hyposulphite of soda commonly employed by photographers is a neutral
combination of hyposulphurous acid and the alkali soda. It is selected as
being more economical in preparation than any other hyposulphite adapted
for fixing.
Hyposulphite of soda occurs in the form of large translucent groups of
crystals, which include five atoms of water. These crystals are soluble
in water almost to any extent, the solution being attended with the
production of cold; they have a nauseous and bitter taste.
Hyposulphite of Gold. (_See_ Gold, Hyposulphite of.)
Hyposulphite of Silver. (_See_ Silver, Hyposulphite of.)
Iceland Moss.
_Cetraria Islandica._--A species of lichen found in Iceland and the
mountainous parts of Europe; when boiled in water, it first swells up, and
then yields a substance which gelatinizes on cooling.
It contains lichen starch; a bitter principle soluble in alcohol, termed
"cetrarine;" and common starch; traces of gallic acid and bitartrate of
potash are also present.
Iodine.
Symbol, I. Atomic weight, 126.
Iodine is chiefly prepared at Glasgow, from _kelp_, which is the fused
ash obtained by burning seaweeds. The waters of the ocean contain minute
quantities of the iodides of sodium and magnesium, which are separated and
stored up by the growing tissues of the marine plant.
In the preparation, the mother-liquor of kelp is evaporated to dryness
and distilled with sulphuric acid; the hydriodic acid first liberated is
decomposed by the high temperature, and fumes of iodine condense in the
form of opaque crystals.
_Properties._--Iodine has a bluish-black color and metallic lustre; it
stains the skin yellow, and has a pungent smell, like diluted chlorine.
It is extremely volatile when moist, boils at 350°, and produces dense
violet-colored fumes, which condense in brilliant plates. Specific gravity
4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000
parts for perfect solution: even this minute quantity however tinges the
liquid of a brown color. Alcohol and ether dissolve it more abundantly,
forming dark-brown solutions. Iodine also dissolves freely in solutions of
the alkaline iodides, such as the iodide of potassium, of sodium, and of
ammonium.
_Chemical Properties._--Iodine belongs to the chlorine group of elements,
characterized by forming acids with hydrogen, and combining extensively
with the metals (see chlorine). They are however comparatively indifferent
to oxygen, and also to each other. The iodides of the alkalies and
alkaline earths are soluble in water; also those of iron, zinc, cadmium,
etc. The iodides of lead, silver, and mercury are nearly or quite
insoluble.
Iodine possesses the property of forming a compound of a deep blue color
with starch. In using this as a test, it is necessary first to liberate
the iodine (if in combination), by means of chlorine, or nitric acid
saturated with peroxide of nitrogen. The presence of alcohol or ether
interferes to a certain extent with the result.
Iodide of Ammonium.
Symbol, NH{4}I. Atomic weight, 144.
This salt may be prepared by adding carbonate of ammonia to iodide of
iron, but more easily by the following process:--A strong solution of
hydrosulphate of ammonia is first made, by passing sulphuretted hydrogen
gas into liquor ammoniæ To this liquid iodine is added until the whole
of the sulphuret of ammonium has been converted into iodide. When this
point is reached, the solution at once colors brown from solution of free
iodine. On the first addition of the iodine, an escape of sulphuretted
hydrogen gas and a dense deposit of sulphur take place. After the
decomposition of the hydrosulphate of ammonia is complete, a portion of
hydriodic acid--formed by the mutual reaction of sulphuretted hydrogen
and iodine--attacks any carbonate of ammonia which may be present, and
causes an effervescence. The effervescence being over, the liquid is still
acid to test-paper, from excess of hydriodic acid; it is to be cautiously
neutralized with ammonia, and evaporated by the heat of a water-bath to
the crystallizing point.
The crystals should be thoroughly dried over a dish of sulphuric acid,
and then sealed in small tubes containing each about half a drachm of the
salt; by this means it will be preserved colorless.
Iodide of ammonium is very soluble in alcohol, but it is not advisable
to keep it in solution, from the rapidity with which it decomposes and
becomes brown.
The most common impurity of commercial iodide of ammonium is sulphate of
ammonia; it is detected by its sparing insolubility in alcohol.
Iodide of Cadmium.
Symbol, CdI. Atomic weight, 182.
This salt is formed by heating filings of metallic cadmium with iodine,
or by mixing the two together with addition of water. It is useful in
iodizing collodion intended for keeping, since it does not become brown
from liberation of free iodine with the same rapidity as the alkaline
iodides.
Iodide of cadmium is very soluble both in alcohol and water; the solution
yielding on evaporation large six-sided tables of a pearly lustre,
which are permanent in the air. The crystalline form of this salt is a
sufficient criterion of its purity.
Iodide of Iron.
Symbol, FeI. Atomic weight, 154.
Iodide of iron, in a fit state for photographic use, is easily obtained by
dissolving a drachm of iodine in an ounce of _proof spirit_--that is, a
mixture of equal bulks of spirits of wine and water--and adding an excess
of iron filings. After a few hours, a green solution is obtained without
the aid of heat. The presence of metallic iron in excess prevents the
liberation of iodine and deposit of peroxide of iron which would otherwise
speedily occur. It is very soluble in water and alcohol, but the solution
rapidly absorbs oxygen and deposits peroxide of iron; hence the importance
of preserving it in contact with metallic iron, with which the separated
iodine may recombine. By very careful evaporation, hydrated crystals of
protoiodide may be obtained, but the composition of the solid salt usually
sold under that name cannot be depended on.
The _periodide_ of iron, corresponding to the perchloride, has not been
examined, and it is doubtful if any such compound exists.
Iodide of Potassium.
Symbol, KI. Atomic weight, 166.
This salt is usually formed by dissolving iodine in solution of potash
until it begins to acquire a brown color; a mixture of iodide of potassium
and _iodate of potash_ (KO IO{5}) is thus formed; but by evaporation
and heating to redness, the latter salt parts with its oxygen, and is
converted into iodide of potassium.
_Properties._--It forms cubic and prismatic crystals, which should be
hard, and _very slightly or not at all deliquescent_. Soluble in less than
an equal weight of water at 60°; it is also soluble in alcohol, but not
in ether. The proportion of iodide of potassium contained in a saturated
alcoholic solution, varies with the strength of the spirit,--with
common spirits of wine, sp. gr. ·836, it would be about 8 grains to the
drachm; with alcohol rectified from carbonate of potash, sp. gr. ·823,
4 or 5 grains: with absolute alcohol, 1 to 2 grains. The solution of
iodide of potassium is instantly colored brown by free chlorine; also
very rapidly by peroxide of nitrogen; ordinary acids, however, act less
quickly, hydriodic acid being first formed, and subsequently decomposing
spontaneously.
Iodide of potassium, as sold in the shops, is often contaminated with
various impurities. The first and most remarkable is _carbonate of
potash_. When a sample of iodide of potassium contains much carbonate
of potash, it forms small and imperfect crystals, which are strongly
alkaline to test-paper, and become moist on exposure to the air, from the
deliquescent nature of the alkaline carbonate. _Sulphate of potash_ is
also a common impurity; it may be detected by chloride of barium.
_Chloride of potassium_ is another impurity; it is detected as
follows:--Precipitate the salt by an equal weight of nitrate of silver,
and treat the yellow mass with solution of ammonia; if any chloride of
silver is present, it dissolves in the ammonia, and after nitration is
re-precipitated in white curds by the addition of an excess of pure nitric
acid. If the nitric acid employed is not pure, but contains traces of free
chlorine, the iodide of silver must be well washed with distilled water
before treating it with ammonia, or the excess of free nitrate of silver
dissolving in the ammonia would, on neutralizing, produce chloride of
silver, and so cause an error.
_Iodide of potash_ is a fourth impurity often found in iodide of
potassium: to detect it, add a drop of dilute sulphuric acid, or a crystal
of citric acid, to the solution of the iodide; when, if much iodate be
present, the liquid will become yellow from liberation of free iodine.
The rationale of this reaction is as follows:--The sulphuric acid unites
with the base of the salt, and liberates hydriodic acid (HI), _a colorless
compound_; but if iodic acid (IO{5}) be also present, it decomposes the
hydriodic acid first formed, oxidizing the hydrogen into water (HO),
and setting free the iodine. The immediate production of a yellow color
on adding a weak acid to aqueous solution of iodide of potassium is,
therefore, a proof of the presence of an iodate. As iodate of potash is
thought to render collodion insensitive (?), this point should be attended
to.
Iodide of potassium may be rendered very pure by recrystallizing from
spirit, or by dissolving in strong alcohol of sp. gr. ·823, in which
sulphate, carbonate, and iodate of potash are insoluble. The proportion
of iodide of potassium contained in saturated alcoholic solutions varies
with the strength of the spirit.
Solution of chloride of barium is commonly used to detect impurities in
iodide of potassium; it forms a white precipitate if carbonate, iodate,
or sulphate be present. In the two former cases the precipitate dissolves
on the addition of _pure_ dilute nitric acid, but in the latter it is
insoluble. The commercial iodide, however, is rarely so pure as to remain
quite clear on the addition of chloride of barium, a _mere opalescence_,
therefore, may be disregarded.
Iodide of Silver. (_See_ Silver, Iodide of.)
Iron, Protosulphate of.
Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.
This salt, often termed _copperas_ or _green vitriol_, is a most abundant
substance, and used for a variety of purposes in the arts. Commercial
sulphate of iron, however, being prepared on a large scale, requires
recrystallization to render it sufficiently pure for photographic purposes.
Pure sulphate of iron occurs in the form of large, transparent prismatic
crystals, of a delicate green color: by exposure to the air they gradually
absorb oxygen and become rusty on the surface. Solution of sulphate of
iron, colorless at first, afterwards changes to a red tint, and deposits
a brown powder; this powder is a _basic_ persulphate of iron, that is, a
persulphate containing an excess of the oxide or _base_. By the addition
of sulphuric or acetic acid to the solution, the formation of a _deposit_
is prevented, the brown powder being soluble in acid liquids.
The crystals of sulphate of iron include a large quantity of water of
crystallization, a part of which they lose by exposure to dry air. By a
higher temperature, the salt may be rendered perfectly _anhydrous_, in
which state it forms a white powder.
Aqueous solution of sulphate of iron absorbs the _binoxide of nitrogen_,
acquiring a deep olive-brown color: as this gaseous binoxide is itself a
reducing agent, the liquid so formed has been proposed as a more energetic
developer than the sulphate of iron alone.
Iron, Protonitrate of.
Symbol, FeO NO{5} + 7 HO. Atomic weight, 153.
This salt, by careful evaporation _in vacuo_ over sulphuric acid, forms
transparent crystals, of a light green color, and containing 7 atoms
of water, like the protosulphate. It is exceedingly unstable, and soon
becomes red from decomposition, unless preserved from contact with air.
The following process is commonly followed for preparing protonitrate of
iron:--
Take of nitrate of baryta 300 grains; powder and dissolve by the aid of
heat in three ounces of water; then throw in, by degrees, with constant
stirring, crystallized sulphate of iron, _powdered_, 320 grains. Continue
to stir for about five or ten minutes. Allow to cool, and filter from the
white deposit, which is the insoluble sulphate of baryta.
In place of nitrate of baryta, the nitrate of lead may be used (sulphate
of lead being an insoluble salt), but the quantity required will be
different. The atomic weights of nitrate of baryta and nitrate of lead are
as 131 to 166; consequently 300 grains of the former are equivalent to 380
grains of the latter.
Iron, Perchloride of.
Symbol, Fe{2}Cl{3}. Atomic weight, 164.
There are two chlorides of iron, corresponding in composition to the
protoxide and the sesquioxide respectively. The protochloride is very
soluble in water, forming a green solution, which precipitates a dirty
white protoxide on the addition of an alkali. The perchloride, on the
other hand, is dark brown, and gives a foxy-red precipitate with alkalies.
_Properties._--Perchloride of iron may be obtained in the solid form by
heating iron wire in excess of chlorine; it condenses in the shape of
brilliant and iridescent brown crystals, which are volatile, and dissolve
in water, the solution being acid to test-paper. It is also soluble in
alcohol, forming the _tinctura ferri sesquichloridi_ of the Pharmacopoeia.
Commercial perchloride of iron ordinarily contains an excess of
hydrochloric acid.
Litmus.
Litmus is a vegetable substance, prepared from various _lichens_, which
are principally collected on rocks adjoining the sea. The coloring matter
is extracted by a peculiar process, and afterwards made up into a paste
with chalk, plaster of Paris, &c.
Litmus occurs in commerce in the form of small cubes, of a fine violet
color. In using it for the preparation of test-papers, it is digested
in hot water, and sheets of porous paper are soaked in the blue liquid
so formed. The red papers are prepared at first in the same manner, but
afterwards placed in water which has been rendered faintly acid with
sulphuric or hydrochloric acid.
Mercury, Bichloride of.
Symbol, HgCl{2}. Atomic weight, 274.
This salt, also called corrosive sublimate, and sometimes _chloride of
mercury_ (the atomic weight of mercury being halved), may be formed by
heating mercury in excess of chlorine, or, more economically, by subliming
a mixture of persulphate of mercury and chloride of sodium.
_Properties._--a very corrosive and poisonous salt, usually sold in
semi-transparent, crystalline masses, or in the state of powder. Soluble
in 16 parts of cold, and in 3 of hot water; more abundantly so in alcohol,
and also in ether. The solubility in water may be increased almost to any
extent by the addition of free hydrochloric acid.
The protochloride of mercury is an insoluble white powder, commonly known
under the name of _calomel_.
Milk.
The milk of herbivorous animals contains three principal
constituents--fatty matter, caseine, and sugar; in addition to these,
small quantities of the chloride of potassium, and of phosphates of lime
and magnesia, are present.
The fatty matter is contained in small cells, and forms the greater part
of the cream which rises to the surface of the milk on standing. Hence
_skimmed_ milk is to be preferred for photographic use.
The second constituent, _caseine_, is an organic principle somewhat
analogous to albumen in composition and properties. Its aqueous solution
however does not, like albumen, _coagulate_ on boiling, unless _an acid_
be present, which probably removes a small portion of alkali with which
the caseine was previously combined. The substance termed "rennet," which
is the dried stomach of the calf, possesses the property of coagulating
caseine, but the exact mode of its action is unknown. Sherry wine is also
employed to curdle milk; but brandy and other spirituous liquids, when
free from acid and astringent matter, have no effect.
In all these cases a proportion of the caseine usually remains in a
soluble form in the _whey_; but when the milk is coagulated by the
addition of acids, the quantity so left is very small, and hence the
use of the rennet is to be preferred, since the presence of caseine
facilitates the reduction of the sensitive silver salts.
Caseine combines with oxide of silver in the same manner as albumen,
forming a white coagulum, which becomes _brick-red_ on exposure to light.
Sugar of milk, the third principal constituent, differs from both
cane and grape sugar; it may be obtained by evaporating _whey_ until
crystallization begins to take place. It is hard and gritty, and only
slightly sweet; slowly soluble, without forming a syrup, in about two and
a half parts of boiling, and six of cold water. It does not ferment and
form alcohol on the addition of yeast, like grape sugar, but by the action
of _decomposing animal matter_ is converted into lactic acid.
When skimmed milk is exposed to the air for some hours it gradually
becomes _sour_, from lactic acid formed in this way; and if then heated to
ebullition, the caseine coagulates very perfectly.
Nitric Acid.
Symbol, NO{5}. Atomic weight, 54.
Nitric acid, or _aqua-fortis_, is prepared by adding sulphuric acid
to nitrate of potash, and distilling the mixture in a retort. Sulphate
of potash and free nitric acid are formed, the latter of which, being
volatile, distils over in combination with one atom of water previously
united with sulphuric acid.
_Properties._--Anhydrous nitric acid is a solid substance, white and
crystalline, but it cannot be prepared except by an expensive and
complicated process.
The concentrated liquid nitric acid contains 1 atom of water, and has a
sp. gr. of about 1·5: if perfectly pure it is colorless, but usually it
has a slight yellow tint, from partial decomposition into peroxide of
nitrogen: it fumes strongly in the air.
The strength of commercial nitric acid is subject to much variation. An
acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly
met with. If the specific gravity is much lower than this (less than
1·36), it will scarcely be adapted for the preparation of peroxyline.
The yellow _nitrous acid_, so called, is a strong nitric acid partially
saturated with the brown vapors of peroxide of nitrogen; it has a high
specific gravity, but this is somewhat deceptive, being caused in part
by the presence of the peroxide. On mixing with sulphuric acid the color
disappears, a compound being formed which has been termed a _sulphate of
nitrous acid_.
_Chemical properties._--Nitric acid is a powerful oxidizing agent; it
dissolves all the common metals, with the exception of gold and platinum.
Animal substances, such as the cuticle, nails, etc., are tinged of a
permanent yellow color, and deeply corroded by a prolonged application.
Nitric acid forms a numerous class of salts, all of which _are soluble
in water_. Hence its presence cannot be determined by any precipitating
re-agent, in the same manner as that of hydrochloric and sulphuric acid.
_Impurities of Commercial Nitric Acid._--These are principally _chlorine_
and _sulphuric acid_; also peroxide of nitrogen, which tinges the acid
yellow, as already described. Chlorine is detected by diluting the acid
with an equal bulk of distilled water, and adding a few drops of nitrate
of silver,--a _milkiness_, which is chloride of silver in suspension,
indicates the presence of chlorine. In testing for sulphuric acid, dilute
the nitric acid as before, and drop in _a single drop_ of solution of
chloride of barium; if sulphuric acid be present, an insoluble precipitate
of sulphate of baryta will be formed.
Nitrous Acid. (_See_ Silver, Nitrate of.)
Nitrate of Potash.
Symbol, KO NO{5}. Atomic weight, 102.
This salt, also termed _nitre_ or _saltpetre_, is an abundant natural
product, found effloresced upon the soil in certain parts of the East
Indies. It is also produced artificially in what are called nitre-beds.
Nitrate of potash is _an anhydrous salt_,--it contains simply nitric acid
and potash, without any water of crystallization; still, in many cases,
a little water is retained mechanically between the interstices of the
crystals, and therefore it is better to dry before use. This may be done
by laying it in a state of fine powder upon blotting-paper, close to a
fire, or upon a heated metallic plate.
Nitrate of Baryta.
Symbol, BaO NO{5}. Atomic weight, 131.
Nitrate of baryta forms octahedral crystals, which are anhydrous. It is
considerably less soluble than the chloride of barium, requiring 12 parts
of cold and 4 of boiling water for solution. It may be substituted for the
nitrate of lead in the preparation of protonitrate of iron.
Nitrate of Lead.
Symbol, PbO NO{5}. Atomic weight, 166.
Nitrate of lead is obtained by dissolving the metal, or the oxide of lead,
in _excess_ of nitric acid, diluted with 2 parts of water. It crystallizes
on evaporation in white anhydrous tetrahedra and octahedra, which are
hard, and decrepitate on being heated; they are soluble in 8 parts of
water at 60°.
Nitrate of lead forms with sulphuric acid, or soluble sulphates, a white
precipitate, which is the insoluble sulphate of lead. The _Iodide_ of lead
is also very sparingly soluble in water.
Nitrate of Silver. (_See_ Silver, Nitrate of.)
Nitro-Hydrochloric Acid.
Symbol, NO{4} + Cl.
This liquid is the aqua-regia of the old alchemists. It is produced by
mixing nitric and hydrochloric acids: the oxygen contained in the former
combines with the hydrogen of the latter, forming water and liberating
chlorine, thus:--
NO{5} + HCl = NO{4} + HO + Cl.
The presence of free chlorine confers on the mixture the power of
dissolving gold and platinum, which neither of the two acids possesses
separately. In preparing aqua-regia it is usual to mix one part, by
measure, of nitric acid with four of hydrochloric acid, and to dilute
with an equal bulk of water. The application of a gentle heat assists the
solution of the metal; but if the temperature rises to the boiling point,
a violent effervescence and escape of chlorine takes place.
Oxygen.
Symbol, O. Atomic weight, 8.
Oxygen gas may be obtained by heating nitrate of potash to redness, but in
this case it is contaminated with a portion of nitrogen. The salt termed
chlorate of potash (the composition of which is closely analogous to that
of the nitrate, chlorine being substituted for nitrogen) yields abundance
of pure oxygen gas on the application of heat, leaving behind chloride of
potassium.
_Chemical Properties._--Oxygen combines eagerly with many of the chemical
elements, forming oxides. This chemical affinity however is not well
seen when the elementary body is exposed to the action of _oxygen in the
gaseous form_. It is the _nascent_ oxygen which acts most powerfully as
an oxidizer. By nascent oxygen is meant oxygen on the point of separation
from other elementary atoms with which it was previously associated; it
may then be considered to be in the liquid form, and hence it comes more
perfectly into contact with the particles of the body to be oxidized.
Illustrations of the superior chemical energy of nascent oxygen are
numerous, but none perhaps are more striking than the mild and gradual
oxidizing influence exerted by atmospheric air, as compared with the
violent action of nitric acid and bodies of that class which contain
oxygen loosely combined.
Oxymel.
This syrup of honey and vinegar is prepared as follows:--Take of
Honey 1 pound.
Acid, acetic, fortiss. (Beaufoy's acid) 11 drachms.
Water 13 drachms.
Stand the pot containing the honey in boiling water until a scum rises
to the surface, which is to be removed two or three times. Then add the
acetic acid and water, and skim once more if required. Allow to cool, and
it will be fit for use.
Potash.
Symbol, KO + HO. Atomic weight, 57.
Potash is obtained by separating the carbonic acid from carbonate of
potash by means of caustic lime. Lime is a more feeble base than potash,
but the carbonate of lime, being _insoluble_ in water, is at once formed
on adding milk of lime to a solution of carbonate of potash.
_Properties._--Usually met with in the form of solid lumps, or in
cylindrical sticks, which are formed by melting the potash and running
it into a mould. It always contain some atoms of water, which cannot be
driven off by the application of heat.
Potash is soluble almost to any extent in water, much heat being evolved.
The solution is powerfully alkaline and acts rapidly upon the skin; it
dissolves fatty and resinous bodies, converting them into soaps; Solution
of potash absorbs carbonic acid quickly from the air, and should therefore
be preserved in stoppered bottles; the glass stoppers must be wiped
occasionally, in order to prevent them from becoming immovably fixed by
the solvent action of the potash upon the silica of the glass.
The liquor potassæ of the London Pharmacopoeia has a sp. gr. of 1·063,
and contains about 5 per cent; of real potash. It is usually contaminated
with _carbonate_ of potash, which causes it to effervesce on the addition
of acids; also, to a less extent, with sulphate of potash, chloride of
potassium, silica, etc.
Potash, Carbonate of.
Symbol, KO CO{2}. Atomic weight, 70.
The impure carbonate of potash, termed _pearlash_, is obtained from the
ashes of wood and vegetable matter, in the same manner as carbonate of
soda is prepared from the ashes of seaweeds. Salts of potash and of soda
appear essential to vegetation, and are absorbed and approximated by
the living tissues of the plant. They exist in the vegetable structure
combined with organic acids in the form of salts, like the oxalate,
tartrate, etc., which when burned are converted into carbonates.
_Properties._--The pearlash of commerce contains large and variable
quantities of chloride of potassium, sulphate of potash, etc. A purer
carbonate is sold, which is free from sulphates, and with only a trace of
chlorides. Carbonate of potash is a strongly alkaline salt, deliquescent,
and soluble in twice its weight of cold water; insoluble in alcohol, and
employed to deprive it of water.
Pyrogallic Acid.
Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight. 84.
The term _pyro_ prefixed to gallic acid implies that the new substance is
obtained by the _action of heat_ upon that body. At a temperature of about
410° Fahr., gallic acid is decomposed, and a white sublimate forms, which
condenses in lamellar Crystals; this is pyrogallic acid.
Pyrogallic acid is very soluble in cold water, and in alcohol and ether;
the solution decomposes and becomes brown by exposure to the air. It gives
an indigo blue color with protosulphate of iron, which changes to dark
green if any persulphate be present.
Although termed an _acid_, this substance is strictly _neutral_; it does
not redden litmus-paper, and forms no salts. The addition of potash
or soda decomposes pyrogallic acid, at the same time increasing the
attraction for oxygen; hence this mixture may conveniently be employed for
absorbing the oxygen contained in atmospheric air. The compounds of silver
and gold are reduced by pyrogallic acid even more rapidly than by gallic
acid, the reducing agent absorbing the oxygen, and becoming converted into
carbonic acid and a brown matter insoluble in water.
Commercial pyrogallic acid is often contaminated with empyreumatic oil,
and also with a black insoluble substance known as _metagallic acid_,
which is formed when the heat is raised above the proper temperature in
the process of manufacture.
Sel D'or. (_See_ Gold, Hyposulphite of.)
Silver.
Symbol, Ag. Atomic Weight, 108.
This metal, the _luna_ or _diana_ of the alchemists, is found native in
Peru and Mexico; it occurs also in the form of sulphuret of silver.
When pure it has a sp. gr. of 10·5, and is very malleable and ductile;
melts at a bright red heat. Silver does not oxidize in the air, but
when exposed to an impure atmosphere containing traces of sulphuretted
hydrogen, it is slowly tarnished from formation of sulphuret of silver. It
dissolves in sulphuric acid, but the best solvent is nitric acid.
The standard coin of the realm is an alloy of silver and copper,
containing about one-eleventh of the latter metal. It may be converted
into nitrate of silver, sufficiently pure for photographic purposes,
by dissolving it in nitric acid and evaporating the solution to the
crystallizing point: or, if the quantity be small, the solution may be
boiled down to complete dryness, and the residue _fused_ strongly; which
decomposes the nitrate of copper, but leaves the greater portion of the
silver salt unaffected. (N. B. Nitrate of silver which has undergone
fusion contains nitrite of silver, and will require the addition of acetic
acid if used for preparing the collodion sensitive film.)
Silver, Ammonio-Nitrate of.
Crystallized nitrate of silver absorbs ammoniacal gas rapidly, with
production of heat sufficient to fuse the resulting compound, which
is white, and consists of 100 parts of the nitrate + 29·5 of ammonia.
The compound however which photographers employ under the name of
ammonio-nitrate of silver, may be viewed more simply as a solution of the
oxide of silver in ammonia, without reference to the nitrate of ammonia
necessarily produced in the reaction.
Very strong ammonia, in acting upon oxide of silver, converts it into
a black powder, termed _fulminating silver_, which possesses the most
dangerous explosive properties. Its composition is uncertain. In
preparing ammonio-nitrate of silver by the common process, the oxide
first precipitated occasionally leaves a little black powder behind, on
re-solution; this does not appear, however, according to the observations
of the author, to be fulminating silver.
In sensitizing salted paper by the ammonio-nitrate of silver, _free
ammonia_ is necessarily formed. Thus:--
Chloride of ammonium + oxide of silver in ammonia
= chloride of silver + ammonia + water.
Silver, Oxide of.
Symbol, AgO. Atomic weight, 116.
If a little potash or ammonia be added to solution of nitrate of silver,
a brown substance is formed, which, on standing, collects at the bottom
of the vessel. This is oxide of silver, displaced from its previous
state of combination with nitric acid by the stronger oxide, potash.
Oxide of silver is soluble _to a very minute extent_ in pure water, the
solution possessing an alkaline reaction to litmus; it is easily dissolved
by nitric or acetic acid, forming a neutral nitrate or acetate; also
soluble in ammonia (ammonio-nitrate of silver), and in nitrate of ammonia
hyposulphite of soda, and cyanide of potassium. Long exposure to light
converts it into a black substance, which is probably a suboxide.
_Properties of the Suboxide of Silver._--Suboxide of silver bears the same
relation to the ordinary brown protoxide of silver that subchloride bears
to protochloride of silver.
It is a black powder, which assumes the metallic lustre on rubbing, and
when treated with dilute acids is resolved into protoxide of silver which
dissolves, and metallic silver.
Silver, Chloride of.
Symbol, AgCl. Atomic weight, 144.
_Preparation of Chloride of Silver by double decomposition._--In order
to illustrate this, take a solution in water of chloride of sodium or
"common salt," and mix it with a solution containing nitrate of silver;
immediately a dense, curdy, white precipitate falls, which is the
substance in question.
In this reaction the elements change places; the chlorine leaves the
sodium with which it was previously combined, and crosses over to the
silver; the oxygen and nitric acid are released from the silver, and unite
with the sodium: thus
Chloride of sodium + nitrate of silver
= Chloride of silver + nitrate of soda.
This interchange of elements is termed by chemists _double decomposition_.
The essential requirements in two salts intended for the preparation of
chloride of silver, are simply that the first should contain chlorine,
the second silver, and that both should be soluble in water; hence the
chloride of potassium or ammonium may be substituted for the chloride of
sodium, and the sulphate or acetate for the nitrate of silver.
In preparing chloride of silver by double decomposition, the white clotty
masses which first form must be washed repeatedly with water, in order to
free them from soluble nitrate of soda, the other product of the change.
When this is done, the salt is in a pure state, and may be dried, etc., in
the usual way.
_Properties of Chloride of Silver._--Chloride of silver differs in
appearance from the nitrate of silver. It is not met with in crystals,
but forms a soft white powder resembling common chalk or whiting. It is
tasteless and insoluble in water; unaffected by boiling with the strongest
nitric acid, but sparingly dissolved by concentrated hydrochloric acid.
Ammonia dissolves chloride of silver freely, as do solutions of
hyposulphite of soda and cyanide of potassium. Concentrated solutions
of alkaline chlorides, iodides, and bromides are likewise solvents of
chloride of silver, but to a limited extent.
Dry chloride of silver heated to redness fuses, and concretes on cooling
into a tough and semi-transparent substance, which has been termed _horn
silver_ or _luna cornea_.
Placed in contact with metallic zinc or iron acidified with dilute
sulphuric acid, chloride of silver is reduced to the metallic state, the
chlorine passing to the other metal under the decomposing influence of the
galvanic current which is established.
_Preparation and properties of the Subchloride of Silver._--If a plate
of polished silver be dipped in solution of perchloride of iron, or of
bichloride of mercury, a _black stain_ is produced, the iron or mercury
salt losing a portion of chlorine, which passes to the silver and
converts it superficially into subchloride of silver. This compound
differs from the white chloride of silver in containing less chlorine
and more of the metallic element; the composition of the latter being
represented by the formula AgCl, that of the former may perhaps be written
as Ag{2}Cl. (?)
Subchloride of silver is interesting to the photographer as corresponding
in properties and composition with the ordinary chloride of silver
blackened by light. It is a pulverulent substance of a bluish-black color,
which is decomposed by ammonia, hyposulphite of soda, and cyanide of
potassium, into chloride of silver which dissolves, and insoluble metallic
silver.
Silver, Bromide of.
Symbol, AgBr. Atomic weight, 186.
This substance so closely resembles the corresponding salts containing,
chlorine and iodine, that a short notice of it will suffice.
Bromide of silver is prepared by exposing a silvered plate to the vapor
of bromine, or by adding solution of bromide of potassium to nitrate
of silver. It is an insoluble substance, slightly yellow in color, and
distinguished from iodide of silver by dissolving in strong ammonia and in
chloride of ammonium. It is freely soluble in hyposulphite of soda and in
cyanide of potassium.
Silver, Citrate of. (_See_ Citric Acid.)
Silver, Iodide of.
Symbol, AgI. Atomic weight, 234.
_Preparation and Properties of Iodide of Silver._--Iodide of silver may be
formed in an analogous manner to the chloride, viz. by the direct action
of the vapor of iodine upon metallic silver, or by double decomposition
between solutions of iodide of potassium and nitrate of silver.
When prepared by the latter mode it forms an impalpable powder, the color
of which varies slightly with the manner of precipitation. If the iodide
of potassium be in excess, the iodide of silver falls to the bottom of the
vessel nearly white; but with an excess of nitrate of silver it is of a
straw-yellow tint. This point may be noticed, because the yellow salt is
the one adapted for photographic use, the other being insensible to the
influence of light.
Iodide of silver is tasteless and inodorous; insoluble in water and in
dilute nitric acid. It is scarcely dissolved by ammonia, which serves
to distinguish it from the chloride of silver, freely soluble in that
liquid. Hyposulphite of soda and cyanide of potassium both dissolve iodide
of silver; it is also soluble in solutions of the alkaline bromides and
iodides.
Silver, Fluoride of.
Symbol, AgF. Atomic weight, 127.
This compound differs from those just described in being soluble in
water. The dry salt fuses on being heated, and is reduced by a higher
temperature, or by exposure to light.
Silver, Sulphuret of.
Symbol, AgS. Atomic weight, 124.
This compound is formed by the action of sulphur upon metallic silver, or
of sulphuretted hydrogen, or hydrosulphate of ammonia, upon the silver
salts; the decomposition of hyposulphite of silver also furnishes the
black sulphuret.
Sulphuret of silver is insoluble in water, and nearly so in those
substances which dissolve the chloride, bromide, and iodide, such as
ammonia, hyposulphites, cyanides, etc.; but it dissolves in nitric acid,
being converted into soluble sulphate and nitrate of silver.
Silver, Nitrate of.
Symbol, AgO NO{5}. Atomic weight, 170.
Nitrate of silver is prepared by dissolving metallic silver in nitric
acid. Nitric acid is a powerfully acid and corrosive substance, containing
two elementary bodies united in definite proportions. These are nitrogen
and oxygen; the latter being present in greatest quantity.
Nitric acid is a powerful solvent for the metallic bodies generally. To
illustrate its action in that particular, as contrasted with other acids,
place pieces of silver foil in two test-tubes, the one containing dilute
sulphuric, the other dilute nitric acid; on the application of heat a
violent action soon commences in the latter, but the former is unaffected.
In order to understand the cause of the difference, it must be borne in
mind that when a metallic substance dissolves in an acid, the nature of
the solution is unlike that of an _aqueous_ solution of salt or sugar.
If you take salt water, and boil it down until the whole of the water
has evaporated, you obtain the salt again, with properties the same as
at first; but if a similar experiment be made with a solution of silver
in nitric acid, the result is different: in that case you do not get
metallic silver on evaporation, but silver _combined with oxygen_ and
_nitric acid_, both of which are tightly retained, being, in fact, in a
state of chemical combination with the metal.
If we closely examine the effects produced by treating silver with nitric
acid, we find them to be of the following nature:--first, a certain
amount of oxygen is imparted to the metal, so as to form an _oxide_, and
afterwards this oxide dissolves in another portion of the nitric acid,
producing _nitrate_ of the oxide, or, as it is shortly termed, nitrate of
silver.
It is therefore the _instability_ of nitric acid, its proneness to part
with oxygen, which renders it superior to sulphuric acid in the experiment
of dissolving silver. Nitric acid stands high in the list of "oxidizing
agents," and it is important that the photographer should bear this fact
in mind.
_Properties of Nitrate of Silver._--In the preparation of nitrate of
silver, when the metal has dissolved, the solution is boiled down in order
to drive off the excess of nitric acid, and set aside to crystallize. The
salt, however, as so obtained is still acid to test-paper, and requires
either recrystallization, or a careful heating to about 300° Fahrenheit,
to render it perfectly neutral.
Pure nitrate of silver occurs in the form of white crystalline plates,
which are very heavy and dissolve readily in an equal weight of cold
water. The solubility is much lessened by the presence of free nitric
acid, and in the _concentrated_ nitric acid the crystals are almost
insoluble. Boiling alcohol takes up about one-fourth part of its weight
of the crystallized nitrate, but deposits nearly the whole on cooling.
Nitrate of silver has an intensely bitter and nauseous taste; acting as a
caustic, and corroding the skin by a prolonged application. Its aqueous
solution is perfectly neutral to test-paper.
Heated in a crucible the salt melts, and when poured into a mould and
solidified, forms the _lunar caustic_ of commerce. At a still higher
temperature it is decomposed, and bubbles of oxygen gas are evolved.
The melted mass, cooled and dissolved in water, leaves behind a black
powder, and yields a solution which is faintly alkaline to test-paper. The
alkalinity depends upon the presence of _nitrite_ of silver associated
with excess of oxide, in the form probably of a basic or _sub_-nitrite of
silver.[B]
[Footnote B: Nitrite of silver differs from the nitrate in containing less
oxygen, and is formed from it by the abstraction of two atoms of that
element.]
Solution of nitrate of silver is decomposed by iron, zinc, copper,
mercury, etc., the nitric acid and oxygen passing to the other metal, and
metallic silver being precipitated.
Silver, Nitrite of.
Symbol, AgO NO{3}. Atomic weight, 154.
Nitrite of silver is a compound of nitrous acid, or NO{3}, with oxide of
silver. It is formed by heating nitrate of silver, so as to drive off a
portion of its oxygen, or more conveniently, by mixing nitrate of silver
and nitrate of potash in equal parts, fusing strongly, and dissolving in a
small quantity of boiling water; on cooling, the nitrite crystallizes out,
and may be purified by pressing in blotting paper. Mr. Hadow describes
an economical method of preparing nitrite of silver in quantity, viz. by
heating 1 part of starch in 8 of nitric acid of 1·25 specific gravity,
and conducting the evolved gases into a solution of pure carbonate of
soda until effervescence has ceased. The nitrite of soda thus formed is
afterwards added to nitrate of silver in the usual way.
_Properties._--Nitrite of silver is soluble in 120 parts of cold water;
easily soluble in boiling water, and crystallizes, on cooling, in long
slender needles. It has a certain degree of affinity for oxygen, and tends
to pass into the condition of nitrate of silver; but it is probable that
its photographic properties depend more upon a decomposition of the salt
and liberation of nitrous acid.
_Properties of Nitrous Acid._--This substance possesses very feeble
acid properties, its salts being decomposed even by acetic acid. It is
an unstable body, and splits up, in contact with water, into binoxide
of nitrogen and nitric acid. The peroxide of nitrogen, NO{4}, is also
decomposed by water and yields the same products.
Silver, Acetate of.
Symbol, AgO (C{4}H{3}O{3}). Atomic weight, 167.
This is a difficultly soluble salt, deposited in lamellar crystals when
an acetate is added to a strong solution of nitrate of silver. If _acetic
acid_ be used in place of an acetate, the acetate of silver does not fall
so readily, since the nitric acid which would then be liberated impedes
the decomposition.
Silver, Hyposulphite of.
Symbol, AgO S{2}O{3} . Atomic weight, 164.
In order to understand, more fully how _decomposition_ of hyposulphite
of silver may affect the process of fixing, the peculiar properties
of this salt should be studied. With this view nitrate of silver and
hyposulphite of soda may be mixed in equivalent proportions, viz. about
twenty-one grains of the former salt to sixteen grains of the latter,
first dissolving each in separate vessels in half an ounce of distilled
water. These solutions are to be added to each other and well agitated;
immediately a dense deposit forms, which is hyposulphite of silver.
At this point a curious series of changes commences. The precipitate, at
first white and curdy, soon alters in color: it becomes canary-yellow,
then of a rich orange-yellow, afterwards liver-color, and finally black.
The _rationale_ of these changes is explained to a certain extent by
studying the composition of the hyposulphite of silver.
The formula for this substance is as follows:--
AgO S{2}O{2},
But AgO S{2}O{2} plainly equals AgS, or sulphuret of silver, and SO{3}, or
sulphuric acid. The acid reaction assumed by the supernatant liquid is due
therefore to sulphuric acid, and the black substance formed is sulphuret
of silver. The yellow and orange-yellow compounds are earlier stages of
the decomposition, but their exact nature is uncertain.
The instability of hyposulphite of silver is principally seen when, it is
in an isolated state: the presence of an excess of hyposulphite of soda
renders it more permanent, by forming a double salt.
In fixing photographic prints this brown deposit of sulphuret of silver
is very liable to form in the bath and upon the picture; particularly
so when the _temperature_ is high. To obviate it observe the following
directions:--It is especially in the reaction between _nitrate of silver_
and hyposulphite of soda that the blackening is seen; the chloride and
other _insoluble_ salts of silver being dissolved, even to saturation,
without any decomposition of the hyposulphite first formed. Hence, if the
print be washed in water to remove the soluble nitrate, a very much weaker
fixing bath than usual may be employed. This plan, however, involving a
little additional trouble, is, on that account, often objected to, and,
when such is the case, a _concentrated_ solution of hyposulphite of soda
must be used, in order to dissolve off the white hyposulphite of silver
before it begins to decompose. When the proofs are taken at once from
the printing frame and immersed in a _dilute_ bath of hyposulphite (one
part of the salt to six or eight of water), _a shade of brown_ may often
be observed to pass over the surface of the print, and a large deposit
of sulphuret of silver soon forms as the result of this decomposition.
On the other hand, with a strong hyposulphite bath there is little or no
discoloration and the black deposit is absent.
But even if, by a preliminary removal of the nitrate of silver, the danger
of blackening be in a great measure obviated, yet the print must not be
taken out of the fixing bath too speedily, or some appearance of brown
patches, visible by transmitted light, may occur.
Each atom of nitrate of silver requires _three_ atoms of hyposulphite of
soda to form the _sweet and soluble double salt_, and hence, if the action
be not continued sufficiently long, another compound will be formed almost
tasteless and insoluble. Even immersion in a new bath of hyposulphite of
soda does not fix the print when once the yellow stage of decomposition
has been established. This yellow salt is insoluble in hyposulphite of
soda, and consequently remains in the paper.
Sugar of Milk. (_See_ Milk.)
Sulphuretted Hydrogen. (_See_ Hydrosulphuric Acid.)
Sulphuric Acid.
Symbol, SO{3}. Atomic weight, 40.
Sulphuric acid maybe formed by oxidizing sulphur with boiling nitric
acid; but this plan would be too expensive to be adopted on a large
scale. The commercial process for the manufacture of sulphuric acid is
exceedingly ingenious and beautiful, but it involves reactions which are
too complicated to admit of a superficial explanation. The sulphur is
first burnt into gaseous sulphurous acid (SO{2}), and then, by the agency
of binoxide of nitrogen gas, an additional atom of oxygen is imparted from
the atmosphere, so as to convert the SO{2} into SO{3}, or sulphuric acid.
_Properties._--Anhydrous sulphuric acid is a white crystalline solid. The
strongest liquid acid always contains one atom of water, which is closely
associated with it, and cannot be driven off by the application of heat.
This _mono-hydrated_ sulphuric acid, represented by the formula HO SO{3},
is a dense fluid, having a specific gravity of about 1·845; boils at
620°, and distils without decomposition. It is not volatile at common
temperatures, and therefore does not _fume_ in the same manner as nitric
or hydrochloric acid. The concentrated acid may be cooled down even to
zero without solidifying; but a weaker compound, containing twice the
quantity of water, and termed _glacial_ sulphuric acid, crystallizes at
40° Fahr. Sulphuric acid is intensely acid and caustic, but it does not
destroy the skin or dissolve metals so readily as nitric acid. It has an
energetic attraction for water, and when the two are mixed, condensation
ensues, and much heat is evolved; four parts of acid and one of water
produce a temperature equal to that of boiling water. Mixed with aqueous
nitric acid, it forms the compound known as nitro-sulphuric acid.
Sulphuric acid possesses intense chemical powers, and displaces the
greater number of ordinary acids from their salts. It _chars_ organic
substances, by removing the elements of water, and converts alcohol into
ether in a similar manner. The _strength_ of a given sample of sulphuric
acid may generally be calculated from its specific gravity, and a table is
given by Dr. Ure for that purpose.
_Impurities of Commercial Sulphuric Acid._--The liquid acid sold as _oil
of vitriol_ is tolerably constant in composition, and seems to be as well
adapted for photographic use as the _pure_ sulphuric acid, which is far
more expensive. The specific gravity should be about 1·836 at 60°. If
a drop, evaporated upon platinum foil, gives a fixed residue, probably
bisulphate of potash is present. A milkiness, on dilution, indicates
sulphate of lead.
_Test for Sulphuric Acid._--If the presence of sulphuric acid, or a
soluble sulphate, be suspected in any liquid, it is tested for by adding a
few drops of dilute solution of chloride of barium, or nitrate of baryta.
A white precipitate, _insoluble in nitric acid_, indicates sulphuric acid.
If the liquid to be tested is very acid, from nitric or hydrochloric acid,
it must be largely diluted before testing, or a crystalline precipitate
will form, caused by the sparing solubility of the chloride of barium
itself in acid solutions.
Sulphurous Acid.
Symbol, SO{2}. Atomic weight, 32.
This is a gaseous compound, formed by burning sulphur in atmospheric air
or oxygen gas; also by heating oil of vitriol in contact with metallic
copper, or with charcoal.
When an acid of any kind is added to hyposulphite of soda, sulphurous acid
is formed as a product of the decomposition of hyposulphurous acid, but it
afterwards disappears from the liquid by a secondary reaction, resulting
in the production of trithionate and tetrathionate of soda.
_Properties._--Sulphurous acid possesses a peculiar and suffocating odor,
familiar to all in the fumes of burning sulphur. It is a feeble acid, and
escapes with effervescence, like carbonic acid, when its salts are treated
with oil of vitriol. It is soluble in water.
_Water._
Symbol, H{2}O. Atomic weight, 9.
Water is an oxide of hydrogen, containing single atoms of each of the
gases.
_Distilled water_ is water which has been vaporized and again condensed:
by this means it is freed from earthy and saline impurities, which, not
being volatile, are left in the body of the retort. _Pure_ distilled water
leaves no residue on evaporation, and should remain perfectly clear on the
addition of nitrate of silver, _even when exposed to the light_; it should
also be neutral to test-paper.
The condensed water of steam-boilers sold as distilled water is apt to be
contaminated with oily and empyreumatic matter, which discolors nitrate of
silver, and is therefore injurious.
_Rain-water_, having undergone a natural process of distillation, is
free from inorganic salts, but it usually contains a minute portion of
_ammonia_, which gives it an alkaline reaction to test-paper. It is very
good for photographic purposes if collected in clean vessels, but when
taken from a common rain-water tank should always be examined, and if much
organic matter be present, tinging it of a brown color and imparting an
unpleasant smell, it must be rejected.
_Spring_ or _river_ water, commonly known as "hard water," usually
contains sulphate of lime, and carbonate of lime dissolved in carbonic
acid: also chloride of sodium in greater or less quantity. On boiling
the water, the carbonic acid gas is evolved, and the greater part of
the carbonate of lime (if any is present) deposits, forming an earthy
incrustation on the boiler.
In testing water for sulphates and chlorides, acidify a portion with a few
drops of _pure_ nitric acid, free from chlorine (if this is not at hand,
use pure acetic acid); then divide it into two parts, and add to the first
a _dilute_ solution of chloride of barium, and to the second nitrate of
silver,--a milkiness indicates the presence of sulphates in the first case
or of chlorides in the second. The _photographic nitrate bath_ cannot be
used as a test, since the iodide of silver it contains is precipitated
on dilution, giving a milkiness which might be mistaken for chloride of
silver.
Common hard water can often be used for making a nitrate bath when nothing
better is at hand. The chlorides it contains are precipitated by the
nitrate of silver, leaving soluble _nitrates_ in solution, which are not
injurious. The carbonate of lime, if any is present, neutralizes free
nitric acid, rendering the bath alkaline in the same manner as carbonate
of soda. Sulphate of lime, usually present in well water, is said to
exercise a retarding action upon the sensitive silver salts, but on this
point the writer is unable to give certain information.
Hard water is not often sufficiently pure for the developing fluids. The
chloride of sodium it contains decomposes the nitrate of silver upon the
film, and the image cannot be brought out perfectly. The New River water,
however supplied to many parts of London, is almost free from chlorides
and answers very well. In other cases a few drops of nitrate of silver
solution may be added to separate the chlorine, taking care not to use a
large excess.
Black Varnish.
_Asphaltum, dissolved in Spirits or Oil of Turpentine._--The asphaltum may
be coarsely pulverized and put into a bottle containing the turpentine,
and in a few hours, if it be occasionally shaken, it will be dissolved and
ready for use. It should be of about the consistency of thick paste.
I use the above, but will now give two more compositions, for any who may
wish to adopt them:
_Black Japan._--Boil together a gallon of boiled linseed oil, 8 ounces of
amber, and 3 ounces of asphaltum. When sufficiently cool, thin it with oil
of turpentine.
_Brunswick Black._--Melt 4 lbs. of asphaltum, add 2 lbs. of hot boiled
linseed oil, and when sufficiently cool, add a gallon of oil of turpentine.
The following is from _Humphrey's Journal_, Vol. viii, number 16.
_Black Varnish._--I generally purchase this from the dealer; but I have
made an article which answered the purpose well, by dissolving pulverized
asphaltum in spirits of turpentine. Any of the black varnishes can be
improved by the addition of a little bees'-wax to it. It is less liable to
crack and gives an improved gloss.
Before closing this chapter, it has been thought advisable to remark,
that one of the most important departments of Photography is the practice
of its chemistry. Many of the annoying failures experienced by those who
are just engaging in the practice of the art, arise from the want of good
and pure chemical agents, and the most certain way to avoid this, is
to purchase them only from persons who thoroughly understand both their
nature and mode of application. As many who may read this work might wish
to know the prices of the various articles employed in the practice of the
processes given, they can be informed by addressing the author, who will
furnish them with a printed Price List.
PRACTICAL DETAILS
OF THE
POSITIVE
OR
=AMBROTYPE PROCESS.=