Chapter III (3)

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.=