A good arrangement for this purpose may be made by taking an ordinary

Portrait Camera, and prolonging it in front by a deal box blackened inside

and with a double body, to' admit of being lengthened out as required; or,

more simply, by adding a framework of wood covered in with black cloth. A

groove in front carries the Negative, or receives the slide containing the

sensitive layer, as the case may be.

In _reducing_ Photographs, the Negative is placed in front of the lens, in

the position ordinarily occupied by the object; but in making an enlarged

copy, it must be fixed _behind_ the lens, or, which is equivalent, the

lens must be turned round, so that the rays of light transmitted by the

Negative enter the back glass of the combination, and pass out at the

front. This point should be attended to in order to avoid indistinctness

of image from spherical aberration.

A Portrait combination of lenses of 2-1/2 or 3-1/4 inches diameter

is the best form to use, and the actinic and luminous foci should

accurately correspond, as any difference between them would be increased

by enlarging. A stop of an inch or an inch and a half aperture placed

_between_ the lenses obviates to some extent the loss of sharp outline

usually following enlargement of the image.

The light may be admitted through the Negative by pointing the Camera

towards the sky; or direct sunlight may be used, thrown upon the Negative

by a plane reflector. A common swing looking-glass, if clear and free from

specks, does very well; it should be so placed that the centre on which it

turns is on a level with the axis of the lens.

The best Negatives for printing enlarged Positives are those which are

distinct and clear; and it is important to use a _small_ Negative, which

strains the lens less and gives a better result than one of larger size.

In printing by a 2-1/4 lens for instance, prepare the Negative upon a

plate about two inches square, and afterwards enlarge it four diameters.

Paper containing Chloride of Silver is not sufficiently sensitive to

receive the image, and the Print should be formed upon Collodion, or on

iodized paper developed by Gallic Acid (see p. 263).

The exposure required will vary not only with the intensity of the light

and the sensibility of the surface used, but also _with the degree of

reduction or enlargement of the image_.

In printing upon Collodion the resulting picture is Positive by

transmitted light; it should be backed up with white varnish, and then

becomes Positive by reflected light. The tone of the blacks is improved

by treating the plate first with Bichloride of Mercury, and then with

Ammonia, in the manner described at pages 113 and 207.

Mr. Wenham, who has written a paper on the mode of obtaining Positives

of the life size, operates in the following way:--he places the Camera,

with the slide containing the Negative, in a dark room, and reflects the

sunlight in through a hole in the shutter, so as to pass first through the

Negative and then through the lens; the image is received upon iodized

paper, and developed by Gallic Acid, in the mode described in the second

Section of this Chapter (p. 263).

_On printing Collodion transparencies for the Stereoscope._--This may

be done by using the Camera to form an image of the Negative in the

mode described in the last page; but more simply by the following

process:--Coat the glass, upon which the Print is to be formed, with

Collodio-Iodide of Silver in the usual way; then lay it upon a piece of

black cloth, Collodion side uppermost, and place two strips of paper of

about the thickness of cardboard and one-fourth of an inch broad, along

the two opposite edges, to prevent the Negative being soiled by contact

with the film. Both glasses must be _perfectly flat_, and even then it may

happen that the Negative is unavoidably wetted; if so, wash it immediately

with water, and if it be properly varnished, no harm will result.

A little ingenuity will suggest a simple framework of wood, on which the

Negative and sensitive plate are retained, separated only by the thickness

of a sheet of paper; and the use of this will be better than holding the

combination in the hand.

The printing is conducted by the light of gas, or of a camphine or

moderator lamp; diffused daylight would be too powerful.

The employment of a concave reflector, which may be purchased for a few

shillings, ensures parallelism of rays, and is a great improvement.

The lamp is placed in the focus of the mirror, which may at once

be ascertained by moving it backwards and forwards until an evenly

illuminated circle is thrown upon a white screen held in front. This in

fact is one of the disadvantages of printing by a naked flame--that the

light falls most powerfully upon the central part, and less so upon the

edges, of the Negative.

The picture must be exposed for a longer or shorter time (about ten

seconds will be an average) according to its behaviour during development

(see p, 224); this process, as well as the fixing, is conducted in the

same manner as for Collodion pictures generally.

Some adopt the plan of whitening by Corrosive Sublimate, and again

blackening by dilute Ammonia, as an improvement to the colour of the dark

shadows (see p. 113).

If this mode of printing upon Collodion be conducted with care, the

Negative being separated from the film by the smallest interval only, the

loss of distinctness in outline will scarcely be perceived.

Stereoscopic transparencies may also be printed by the dry Collodion

process described in Chapter VI., or by the Collodio-Albumen process. Mr.

Llewellyn recommends the employment of a solution of Oxymel, so dilute

that the plate becomes nearly dry, and may be laid in contact with the

Negative without fear of injury (see the footnote at page 302).

CHAPTER V.

CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS.

Section I.--Imperfections in Collodion Photographs.

Section II.--Imperfections in Paper Positives.

SECTION I.

_Imperfections in Negative and Positive Collodion Photographs._

The following may be mentioned:--fogging--spots-- markings, etc.

CAUSES OF FOGGING OF COLLODION PLATES.

1. _Over-exposure of the Plate._--This is likely to happen when using the

full aperture of a double combination lens for distant objects brightly

illuminated, the Collodion being highly sensitive. Also from the film

being very blue and transparent, with too little Iodide of Silver (p. 114).

2. _Diffused Light._--_a._ In the developing room. This is a frequent

cause of fogging, and especially so when the common yellow calico

is employed, which is apt to fade. Use a treble thickness, or

procure the waterproof material, in which the pores are stopped with

gutta-percha.--_b._ In the Camera. The slide may not fit accurately, or

the door does not shut close. Throw a black cloth over the Camera during

the exposure of the plate.--_c._ From direct rays of the sun or the light

of the sky falling upon the lens. With the full aperture of a double

combination Lens, a portion of sky included in the field (as for instance

to form the background of a portrait) is apt to cause fogging. The

portrait will probably be more brilliant if a funnel-shaped canvas bag, or

a curtain with an oblong aperture admitting only the rays proceeding from

the sitter, be placed in front of the Camera.

3. _Alkalinity of the Bath._--This condition, explained at page 88, may

be due to one of the following causes:--_a._ The use of Nitrate of Silver

which has been too strongly fused (p. 13).--_b._ Constant employment of a

Collodion containing free Ammonia or Carbonate of Ammonia (p. 89).--_c._

Addition of Potash, Ammonia, or Carbonate of Soda to the Nitrate Bath, in

order to remove free Nitric Acid (p. 89).--_d._ Use of rain-water or hard

water for making the Nitrate Bath (rain-water usually contains traces of

Ammonia; hard water often abounds with Carbonate of Lime).

In either case the alkalinity may easily be removed by the addition of

Acetic Acid, one drop to four ounces of the solution. The proper mode of

testing for alkalinity is described at p. 89.

4. _Decomposition of the Nitrate Bath._--_a._ By constant exposure to

light (the injurious effects of this will be mostly seen when Positives

are taken).--_b._ By organic matter: this is sometimes present in Nitrate

of Silver which has been prepared from the residues of old Baths; or

it may be introduced by floating papers for the printing process upon

the Bath, or by dissolving the crystals of Nitrate of Silver in putrid

rain-water, or in impure distilled water collected from the condensed

water of steam-boilers and contaminated with oily matter.--_c._

Decomposition of the Bath by contact with metallic iron or copper, or with

a fixing agent, or a developing agent (p. 90).

5. _Faults of the developing solution._--a. Brown and decomposed solution

of Pyrogallic Acid; this may sometimes be used with impunity, but it

tends, as a rule, to facilitate irregular reduction of Silver.--h. Impure

Acetic Acid having a smell of Garlic and which probably contains Sulphur

in organic combination.--c. Omission of the Acetic Acid in the developer:

this will produce a universal blackness.

6. _Sundry other causes of fogging._--_a._ Vapour of Ammonia or

Hydrosulphate of Ammonia, or the products of the combustion of coal-gas,

escaping into the developing room.--_b._ Development of the image by

immersion in solution of Sulphate of Iron: this is a safe plan when the

films are formed in an acid Nitrate Bath; but with pale films formed in a

chemically neutral Bath it is better to pour the fluid over the plate, and

not to use the same portion twice.--_c._ Redipping the plate in the Bath

before development: this is apt to give a foggy picture when using an old

Bath, and is not recommended.

_Systematic plan of proceeding to detect the cause of the fogging._--If

the amateur has had but little experience in the Collodion process, and is

using Collodion of moderate sensitiveness and a new Bath, the probability

is that the fogging is caused by over-exposure. Having obviated this,

proceed to test the Bath; _if it is made from pure materials, and does not

restore the blue colour of a piece of litmus-paper previously reddened

by holding it over the mouth of a glacial Acetic Acid bottle_, it may be

considered in working order.

Next prepare a sensitive plate, and after draining it for two or three

minutes in a dark place, pour on the developer: wash, fix, and bring out

to the light; if any mistiness is perceptible, either the developing room

is not sufficiently dark, or the Bath was prepared with a bad sample of

Nitrate of Silver, or with impure Alcohol, or impure water.

On the other hand, if the plate remains absolutely clear under these

circumstances, _the cause of error may be in the Camera_;--therefore

prepare another sensitive film, place it in the Camera, and proceed

exactly as if taking a picture, with the exception of not removing the

brass cap of the lens: allow to remain for two or three minutes, and then

remove and develope as usual.

If no indication of the cause of the fogging is obtained in either of

these ways, there is every reason to suppose that it is due to diffused

Light gaining entrance through the lens. This cause of error may often

be detected by looking into the Camera from the front, when an irregular

reflection will be seen upon the glass.

SPOTS UPON COLLODION PLATES.

Spots are of two kinds: spots of opacity, which appear black by

transmitted light, and white by reflected light; and spots of

transparency, the reverse of the others, being white when seen upon

Negatives, and black on Positives.

Opaque Spots are referable to an excess of development at the point where

the spot is seen; they may be caused by--

1. _The use of Collodion holding small particles in suspension._--Each

particle becomes a centre of chemical action, and produces a speck, or a

speck with a tail to it. The Collodion should be placed aside to settle

for several hours, after which the upper portion may be poured off.

2. _Turbidity of the Nitrate solution._--_a._ From flakes of Iodide of

Silver having fallen away into the solution, by use of an over-iodized

Collodion.--_b._ From a deposit formed by degrees upon the sides of the

gutta-percha trough.--_c._ From the inside of the trough being dusty at

the time of pouring in the solution.

In order to obviate these inconveniences, it is well to make at least half

as much again of the Nitrate solution as is necessary, and to keep it in a

stock-bottle, from which the upper part may be poured off when required.

The frequent filtration of Silver Baths is unadvisable, since the paper

employed may be contaminated with impurities.

3. _Dust upon the surface of the glass at the time of pouring on the

Collodion._--Perfectly clean glasses, if set aside for a few minutes,

acquire small particles of dust; each plate should therefore be gently

wiped with a silk handkerchief immediately before being used.

4. _Faults of the Slide._--Sometimes a small hole exists, which admits a

pencil of light, and produces a spot, known by its being always in the

same part of the plate; occasionally the door works too tightly so that

small particles of wood, etc., are scraped off, and projected against the

plate when it is raised. Or perhaps the operator, after the exposure is

finished, shuts down the door with a jerk, and so causes a splash in the

liquid which has drained down and accumulated in the groove below; this

cause, although not a common one, may sometimes occur.

5. _Insoluble particles in the Pyrogallic Acid._--The solution of

Pyrogallic Acid will not usually require filtering, but if specks of

Metagallic Acid are present, the developer should be passed through

blotting paper before use.

Spots of Transparency may generally be traced to some cause

_which renders the Iodide of Silver insensible to light at particular

points_, so that on the application of the developer no reduction takes

place.

1. _Concentration of the Nitrate of Silver on the surface of the film by

evaporation._--When the film becomes too dry after removal from the Bath,

the solvent power of the Nitrate increases so much that it eats away the

Iodide and produces spots.

2. _Small particles of undissolved Iodide of Potassium in the

Collodion._--These are likely to occur when Anhydrous Ether and Alcohol

are employed. They produce transparent specks at every part of the plate.

Allow the Collodion to settle, or add a drop of water, which will dissolve

the Iodide.

3. _Alcohol or Ether containing too much water_.--This causes a

reticulated appearance of the film, which is rotten and full of holes.

4. _Use of glasses improperly cleaned._--This cause is perhaps the most

frequent of all, when the film of Pyroxyline is very thin and the Bath

neutral. After glasses have been long used it is often difficult to clean

them so thoroughly that the breath lies smoothly; but the use of Potash

gives the best chance.

MARKINGS OF VARIOUS KINDS ON COLLODION PLATES.

1. _A reticulated appearance on the film after developing._--When this is

universal, it often depends upon the employment of Collodion containing

water. Or, if not due to this cause, the plate may have been immersed too

quickly in the Bath, and the soluble Pyroxyline partially precipitated.

2. _Oily spots or lines._--_a._ From raising the plate out of the Nitrate

Bath before it has been immersed sufficiently long to have become

thoroughly wetted.--_b._ Removal of the plate from the Bath before the

Ether upon the surface has been washed away.--_c._ Redipping the plate in

the Nitrate Bath after exposure to light, and pouring on the developer

_immediately_; if a few minutes be not allowed to drain off the excess of

Nitrate, the Pyrogallic Acid will not amalgamate readily with the surface

of the film.--_d._ From the Nitrate Bath being covered with an oily scum,

which is carried down by the plate. Draw a slip of blotting-paper gently

along the surface of the liquid before using it.

3. _Straight lines traversing the film horizontally._--From a check having

been made in immersing the plate in the Bath.

4. _Curved lines of over-development._--By employing the developer too

concentrated; or by not pouring it on sufficiently quickly to cover the

surface before the action begins; or by using too little Acetic Acid, and

omitting the Alcohol. The addition of Alcohol to the developer will not be

required as a rule when the Bath is newly made; but when much Ether has

accumulated in it, the developer has a tendency to run into oily lines,

unless containing Alcohol.

5. _Stains from too small a quantity of fluid having been employed to

develope the image._--In this case, the whole plate not being thoroughly

covered during the development, the action does not always proceed with

regularity.

6. _Irregular striæ._--From fragments of dried Collodion accumulating in

the neck of the bottle, and being washed on the film; to avoid this, the

finger should be passed gently round the inside of the neck before use.

7. _Markings like those represented in the woodcut._--They are caused by

using an inferior sample of Pyroxyline made from too hot acids, and are

most seen when using an old Bath.

[Illustration]

8. _Stains on the upper part of the plate, from using a dirty slide._--To

avoid these, place, if necessary, strips of blotting-paper between the

supports and the glass.

9. _Wavy marks at the lower parts of the plate._--_a._ If the Collodion

is becoming thick and glutinous from constant use, dilute it with a

little Ether containing an eighth part of Alcohol.--_b._ From reversing

the direction of the plate after its removal from the Bath, so that the

Nitrate of Silver flows back again over the surface and causes a stain on

the application of Pyrogallic Acid.--_c._ Impurities on the woodwork of

the frame ascending the film by capillary attraction. This is a frequent

source of stains.

10. _Marks from the developer not running up to the edge of the film_ (p.

212). Remedy this as far as possible by allowing the Collodion to set a

little more firmly before dipping the plate in the Bath.

IMPERFECTIONS IN COLLODION NEGATIVES.

1. _A want of Intensity._--a. From the development not having been

sufficiently pushed (p. 224).--_b._ From the Collodion film being too

blue and transparent for Negatives.--_c._ The Collodion newly made from

pure materials (p. 114).--_d._ The plate kept too long between exciting

and development (p. 100).--_e._ The Bath newly prepared from commercial

crystallized Nitrate of Silver (p. 101).--_f._ The light too feeble, as on

very dark wintry days, or in copying interiors, etc.

2. _Inferior half-tones, with great intensity of the high Lights._--_a._

From the plate being insufficiently exposed.--_b._ The Collodion of

inferior quality, either too strongly tinted with Iodine or made

from impure materials.--_c._ The Nitrate Bath old and partially

decomposed.--_d._ The light reflected too strongly from the object. When

the light is unusually bright, a feeble Collodion and a newly mixed

Nitrate Bath will be found to give better definition in the high lights

than an intense Collodion, which may produce chalky Negatives.

3. _The image pale and misty._--The plate is over-exposed (if so, the

image will probably be a reddish-brown colour by transmitted light), or

there is diffused light in the Camera or developing room. The presence of

Bromides or Chlorides in the Collodion may occasionally produce the same

effect.

4. _The high lights of the image are solarized._--A change of colour to

a light brown or red tint by transmitted light, with a dark shade by

reflected light, is favoured by over-exposure of the plate, by organic

decomposition of the Collodion, and by Acetate of Silver and other organic

bodies in the Bath.

5. _The image dissolves off on applying the Cyanide of Potassium._--The

Collodion is probably over-iodized. The same thing may also happen in

the Honey preservative process, when the plates have been long kept and

the indurated layer of syrup not properly removed before applying the

developer.

6. _The developer does not run up to the edge of the film._--This is

likely to occur when using Collodion nearly anhydrous; and particularly

so with a new Bath not containing much Alcohol. The film will be less

repellent, if a longer time be allowed before dipping in the Bath.

7. _The film does not stick to the glass._--Clean the plates very

carefully, and make the Collodion a little thinner if required. Allow

a longer time before dipping in the Bath. A very effectual plan is to

roughen the surface of the plates, about an eighth of an inch round the

edges.

IMPERFECTIONS IN COLLODION POSITIVES.

The principal difficulty in the production of Negatives is to ascertain

the right time of exposure to light and the proper point to which to carry

the development of the image. A minor amount of fogging, stains, etc., is

of less consequence, and will scarcely be noticed in the printing.

With direct Positives however the case is different. The beauty of these

pictures depends entirely upon their being clean and brilliant, without

fogging, specks, or imperfections of any kind. On the other hand, the

exposure and development of Positives is comparatively simple and easily

ascertained.

1. _The shadows dark and heavy._--The plate has not received sufficient

exposure in the Camera;--or the film being very transparent and the Silver

solution weak, Nitric Acid is present in the Bath, or the Collodion is

brown from free Iodine; in the latter case make the Collodion a little

thicker, and develope with Sulphate of Iron in preference to Pyrogallic

Acid.

2. _The shadows good, but the lights overdone._--The developing fluid may

have been kept on too long; or the object is not properly illuminated (p.

220); or the Collodion is not adapted for Positives.

3. _The high lights pale and flat, the shadows misty._--The plate is

over-exposed. Indistinctness of outline caused by over-exposure is

distinguished from that produced by fogging by holding the plate up to the

light; in the former case the image shows as a Negative.

If the Collodion is colourless, clearer shadows will probably be obtained

by dropping in Tincture of Iodine until a yellow colour is produced.

4. _The picture developes slowly; spangles of metallic Silver are

formed._--Too much Nitric Acid is present in proportion to the strength

of the Bath, to the amount of Iodide in the film, and to the quantity of

Protosalt of Iron in the developer (p. 112).

5. _Circular spots of a black colour after hacking up with the

varnish._--These are often caused by lifting the plate too quickly out of

the Bath; or by pouring on the developer at one spot, so as to wash away

the Nitrate of Silver; or by the use of glasses imperfectly cleaned.

6. _The image becomes metallic on drying._--If Sulphate of Iron is

employed, the solution is too weak, or free Nitric Acid has been added in

excess. If Pyrogallic Acid is used to develope, the proportion of Nitric

Acid is too great.

7. _A green or blue tint in certain parts of the image._--This is

caused by the deposit of Silver being too scanty, which may happen from

over-action of the light, or from the film of Pyroxyline being _very

thin_;--if the Collodion is diluted down beyond a certain point, the same

quantity of free Nitrate of Silver is not retained upon the surface of the

film. Add a few drops of the Bath to the developer before pouring it on

the plate.

8. _Vertical lines, and mistiness, on the image._--If the Bath has been

much used, add to it a third part of a simple solution of Nitrate of

Silver in water, without any Alcohol or Iodide. Also prepare the developer

with addition of Alcohol, to make it flow more readily (p. 211).

SECTION II.

_Imperfections in Paper Positives._

1. _The Print marbled and spotty._--The quality of the paper is often

inferior, which causes it to imbibe liquids unevenly at different points;

or the amount of Silver in the Nitrate Bath is insufficient. In this case

the spots are often absent at the lower and most depending part of the

sheet, where the excess of liquid drains off.

2. _The Print clean on the surface, but spotted when held up to the

light._--In this case the spots are probably due to imperfect fixation

(see p. 129).

3. _The Print becomes pale in the Hyposulphite Bath, and has a cold and

faded appearance when finished._--The Chloride of Silver in the paper may

have been in excess with regard to the free Nitrate of Silver; which is

especially likely if no bronzing could be obtained by prolonged action of

the light, or if a weak solution of Nitrate of Silver was laid on with a

brush, or by a glass rod. Prints formed on paper which has been kept too

long after sensitizing present the same appearance, the free Nitrate of

Silver having entered into combination with the organic matter.

4. _Yellowness of the light parts of the proof._--The following causes

are likely to produce yellowness:--acidity of the fixing and toning Bath

(p. 139),--its action continued for too long a time,--the first washings

of the proof not performed quickly,--the toning Bath laid aside until it

had become decomposed and nearly useless,--the paper kept for several days

after sensitizing.

A creamy yellowness is also common in Prints toned by Sel d'or, when the

Hydrochloric Acid has been omitted from the formula; the proof exposed to

light during the toning and fixing process; or too long a time allowed to

elapse between the toning and fixing. It is also more frequently met with

on albuminized paper.

5. _Intense bronzing of the deep shadows._--In this case the Negative

is in fault; remedy the evil as far as possible by printing on paper

containing but little salt.

6. _The definition of the Print imperfect, the Negative being a good

one._--Much will depend upon the quality of the paper. Towgood's Positive

gives good definition. The use of Albumen will be a great advantage.

Citrate of Soda (p. 246) will also improve the definition on plain paper.

7. _Markings of a yellow tint in the dark portions of the

Positive._--These are common on Prints toned without Gold; care should

be taken not to handle the paper too much, either before or after

sensitizing; to wash the prints in a clean vessel; and not to lay them

down whilst wet on a wooden table or in contact with anything likely to

communicate impurities.

8. _Small specks and spots of different hinds._--These, when not

corresponding to similar marks upon the Negative, are usually due to

metallic specks in the paper; or to insoluble particles floating in the

bath.

9. _Markings of the brush in Ammonio-Nitrate pictures._--In this case

there is probably an excess of Ammonia, which dissolves the Chloride of

Silver. Add a little fresh Nitrate of Silver, or use the Oxide of Silver

dissolved in Nitrate of Ammonia (p. 249).

10. _Marbled stains on the surface of the Print._--Draw a strip of

blotting-paper gently over the surface of the Nitrate Bath before

sensitizing the paper; and see that the sheet does not touch the bottom of

the dish.

11. _Streaks on Albuminized paper._--Apply the Albumen more rapidly and

evenly to the paper. If this does not succeed, add a little Ox-Gall (p.

243).

12. _Removal of the Albumen from the paper during sensitizing._--The

Nitrate Bath is probably alkaline (see page 89).

CHAPTER VI.

LANDSCAPE PHOTOGRAPHY ON PRESERVED COLLODION AND COLLODIO-ALBUMEN.

The Collodion process may be applied with success to landscape

Photography; but as the plates become dry and lose their sensitiveness

shortly after their removal from the Bath, the operator will require to

provide himself with a yellow tent or some portable vehicle in which the

operations of sensitizing and developing can be conducted. As it is a

point of great importance in the Collodion process that the plate should

receive exactly the right amount of exposure in the Camera,--a few seconds

more or less sufficing to affect the character of the picture,--many will

submit to much trouble and inconvenience in order to have the apparatus

complete upon the spot at which the view is taken.

The object of the "Collodion Preservative Processes" is to maintain the

sensitiveness of the film for a certain length of time after it has been

excited in the Bath. There is some difficulty in doing this, because if

the plate be allowed to dry spontaneously, the solution of free Nitrate of

Silver upon the surface, becoming concentrated by evaporation, eats away

the Iodide of Silver, and produces transparent spots.

Some operators have attempted to use a second plate of glass in such a way

as to enclose the sensitive film with an intervening stratum of liquid.

The difficulty however of separating the glasses again without tearing the

film, is considerable.

In the process of Messrs. Spiller and Crookes, the property possessed

by certain saline substances of remaining for a long time in a moist

condition was turned to account. Such salts are termed "deliquescent," and

many of them have so great an attraction for water that they absorb it

eagerly from the air: the solution having been formed, the water cannot

entirely be driven off except by the application of a considerable heat.

More recently, Honey has been employed by Mr. Shadbolt.[52] This

substance can scarcely be termed deliquescent, but it possesses, like

other uncrystallizable sugars, the property of remaining moist and sticky

for a long time. Honey is, according to the Author's views, superior

to inorganic deliquescent salts as a preservative agent, from its

possessing an affinity for Oxides of Silver, and thus acting chemically in

communicating organic intensity to the image.--Collodion plates when kept

long in a moist and sensitive state often give a pale and blue image, even

although the Nitrate of Silver be left upon the film; and neither Nitrate

of Magnesia nor Glycerine appears capable of supplying the deficient

element, both being nearly or quite indifferent to the Salts of Silver.

[52] A claim has lately been advanced by Mr. Maxwell Lyte to be considered

as the discoverer of the Honey Process. This gentleman appears to have

worked simultaneously with Mr. Shadbolt, and to have anticipated him in

publishing; but the object of Mr. Lyte's process was rather to increase

the sensibility of the plates than to confer upon them keeping qualities.

THE HONEY AND OXYMEL KEEPING PROCESSES.

When the weather is cool, Collodion plates may be preserved with tolerable

certainty for a few hours, by simply applying Honey to them in the state

in which they are taken from the Nitrate of Silver Bath.

The best pure Virgin Honey should be obtained by dripping it immediately

from the comb. This point is of importance, since if the sample of honey

be of inferior quality, or adulterated, the process may not succeed. The

quantity of water to be added will vary with the consistence of the honey,

from about an equal bulk to two parts: it should be sufficient to make the

preservative solution pass slowly through filtering-paper.

After the plate is removed from the Nitrate Bath, it is to be drained and

wiped on the back in the usual way. The Honey is then poured along the

edge in such a manner as to form a broad wave which forces the Nitrate of

Silver solution before it and covers the film. Next drain the plate into a

measure and pour on a second portion of Honey as before. This second dose

may be used again for the first application to the succeeding plate.

Lastly, stand the glass on blotting-paper in a dark place for about a

quarter of an hour or twenty minutes, and wipe the lower edge before

putting it into the plate box.

The exposure required will probably be about four or five times as long

as that for new and sensitive Collodion, or twice as long as the exposure

required for old and brown Collodion.

Before applying the developer, immerse the plate in a Bath of rain-water

for five minutes, moving it about occasionally to soften the honey. This

will probably be sufficient for plates which have not been kept longer

than four hours, and beyond that time the process is not considered

certain, since the Honey exercises a slow reducing action upon the Nitrate

of Silver.

The solution of Pyrogallic Acid may be used of the ordinary strength,

with a full dose of Acetic Acid. Only a faint image comes out at first,

but on pouring over the plate a fresh portion of the developer with two

or three drops of the Nitrate Bath added to each fluid drachm, it may be

intensified to any extent.

Fix with Hyposulphite of Soda, and wash in the usual way.

When the process fails, from heat of the weather or other causes, the

image will probably be feeble and red by transmitted light, and the

shadows defective and misty. This is especially likely to happen when the

Nitrate Bath is very old and contains much Acetate of Silver; or when the

same portion of Honey is used more than once, and has undergone partial

decomposition by the action of the Nitrate of Silver. The use of _pure_

Honey, free from mouldiness and fermentation, will, _in cool weather_,

almost certainly ensure success.

_A modification of the process when the plates are to be kept over four

hours._--In this case the whole, or the greater part of the Nitrate of

Silver must be removed before applying the preservative agent. Wash the

sensitive plate in water in the manner described for the Oxymel process

in the next page. Then apply the syrup as before, using it as thick as

possible. Honeyed plates, free from Nitrate of Silver, may commonly

be kept for five or six days; often much longer. Dr. Mansell, who has

employed this process with great success, speaks of _temperature_ as

a point to be attended to. In hot weather the same length of keeping

properties will not be attained.

_Use of Oxymel for preserving Collodion plates._--The principal difficulty

in the employment of Honey in Photography, is its disposition to ferment,

or to become mouldy. Fermentation occurs most readily in a dilute

solution, and will be obviated by using the syrup as thick and free from

water as possible. Mr. Llewellyn employs "Oxymel," which is a mixture of

Honey and Vinegar, as a preservative agent. This substance will keep even

in dilute solution for a long time without decomposition; and, being very

readily removed from the plates, does not interfere with the development

of the image. The preparation of Oxymel is described in the Vocabulary,

Part III.; it must be diluted with three or four parts of water, and

filtered.

Certain facts to which attention has been lately drawn by Dr. Norris and

Mr. Barnes in working with dry Collodion, may be advantageously borne in

mind when using Oxymel; the preservative solution of which is employed

in so dilute a state that the process resembles to a great extent a dry

Collodion process. The observations above referred to relate to the

quality of the Collodion best adapted for the purpose, and will be found

at page 298, to which the reader is referred.

The manipulation of the Oxymel process is very simple. Two flat

gutta-percha dishes are provided, the one containing common water and the

other diluted and filtered Oxymel. The Collodion plate, on its removal

from the Bath, is placed in the first dish, which is gently tilted up and

down, to wash away the free Nitrate of Silver. In a few seconds, when

the liquid is rendered milky, it is poured away, and fresh water being

introduced, the process is repeated _until the oily lines disappear, and

the surface of the film becomes smooth and glassy_. The plate is then,

after a slight draining, removed to the second tray, and the Oxymel waved

backwards and forwards for about half a minute, after which the glass is

lifted out and placed vertically on blotting-paper, which must be renewed

when it becomes wet and saturated.

The plates may be used any time within a fortnight from the date of their

preparation, and it is not necessary to develope immediately after the

exposure. The sensitiveness will be considerably less than that of fresh

Collodion: from two to five minutes may be allowed with a Stereoscopic

view lens having a quarter-inch diaphragm.

Before developing, the film should be gently washed for a few seconds with

common water. Solution of Pyrogallic Acid, of the ordinary strength, but

previously mixed with a portion of the Nitrate Bath solution, one or two

drops to each drachm, may then be poured on in the ordinary way. Use less

Nitrate of Silver and more Acetic Acid in hot weather. When discoloration

of the developer occurs, mix a fresh portion and proceed as before.

PRECAUTIONS TO BE OBSERVED IN KEEPING PROCESSES.

The plates must be roughened at the edges, and also upon the surface, to

make the film adhere.

It is advisable to use a tolerably thick Collodion, giving a yellow film;

the pale opalescent films being more easily affected by markings on the

glass, and not retaining so much of syrup or Nitrate of Silver upon the

surface.

The room in which the plates are prepared must be carefully guarded

from scattered pencils of white light; the films are exposed to injury

from this cause during the whole of the time occupied in applying the

preservative syrup; and hence anything short of absolute chemical darkness

will be likely to cause fogging; especially so when free Nitrate of Silver

is left upon the film.

The water used for washing away the free Nitrate of Silver before applying

the preserving liquid, need not be distilled. Common hard water containing

Carbonates and Chlorides, and producing _milkiness_ with Nitrate of

Silver, will often suffice. The water of the New River and of the River

Thames, with which many parts of London are supplied, may certainly be

used; but in the case of a very _hard_ water, containing much Sulphate of

Lime, it might perhaps be advisable to substitute clean rain-water, free

from brown organic discoloration.

The preservative Oxymel must be carefully filtered, and kept _covered_,

in order to protect it from dust. It will also be necessary occasionally,

before using it, to run it through a piece of white cambric, to stop back

suspended particles, which, if allowed to remain, would be a source of

spots. If it becomes mouldy, or discoloured by Silver, or ferments and

evolves gas, throw it away.

After the syrup is applied and the plates are drained, stow them in a

grooved box perfectly protected from light; or place them in slides,

which must be kept scrupulously clean, since any trace of impurity would

be likely to produce a stain when the plate was left a long time in the

slide. If the preserved plates are kept in a cupboard or box, see that no

volatile matter, such as Ammonia, coal-gas, etc., can find entrance.

In changing the plates after the exposure in the Camera, use a large bag

made of _several thicknesses_ of black calico, with a square of yellow

calico let in at the top; an elastic band securing it round the waist.

THE COLLODIO-ALBUMEN PROCESS.

This process, the theory of which has been briefly explained at page 181,

is more sensitive than the one last described, and has the additional

advantage of giving _dry_ plates, which do not attract dust, and are

less liable to injury. The details of manipulation are complex, but this

inconvenience is not so much felt when preparing a large number of plates.

_Cleaning the Glasses._--Success will greatly depend upon the mode in

which this part of the process is performed. The layer of Albumen which is

applied to the Collodion film tends to swell and to raise the latter in

blisters; the most effectual mode of obviating which will be to clean the

glass so that the film adheres with unusual tenacity.

The Liquor Potassæ of the Druggists, diluted with three or four parts of

water, and rubbed on the glass by a roll of flannel (page 214), is very

effectual. A mixture of Tripoli-water and Nitric Acid may however, if

desired, be substituted:--

Tripoli 1 drachm.

Nitric Acid 30 minims.

Water 1 ounce.

Lay the glass flat on a cloth, and rub the surface carefully with a tuft

of cotton-wool dipped in the Tripoli; then, before the cream dries, wipe

it off with a second tuft, and polish with a third. Lastly, breathe upon

the glass, and having ascertained that it is chemically clean, apply the

Collodion.

_Coating with Collodion._--Choose a rather thin Collodion which adheres

tightly to the glass. A preparation which has been kept a long time after

iodizing will usually answer the purpose very well, and, as a rule, a

non-contractile, structureless Collodion is better than one which is

glutinous and wavy. The degree of sensibility of the Collodion is not

thought to have much influence upon the result.

_Coating the Plate._--Apply the Collodion in the usual manner, and allow

it full time to set perfectly, before dipping in the Bath, in order to

favour its adherence to the glass. With Collodion prepared from anhydrous

spirits, about half a minute may be given in cool weather.

_The Nitrate Bath._--Take of

Fused Nitrate of Silver 40 grains.

Glacial Acetic Acid 30 minims.

Alcohol 20 minims.

Water 1 fluid ounce.

Saturate with Iodide of Silver as described at page 204, and filter.

An immersion of one minute will be sufficient; after which, give the

plate an up-and-down movement, and wash it in plain water, in the manner

advised for the Oxymel preservative process, at page 292. Then stand it on

blotting-paper, to drain for a minute or two, wipe the back of the glass,

and pour on the Albumen.

This Bath may become discoloured after a time; continue to use it until it

is of a dark sherry-colour, and then treat it with "Kaolin," in the manner

and with the precautions advised at pages 91 and 245.

_The Iodized Albumen._--Procure eggs, fresh laid, or not more than two or

three days old. Separate the whites in the same way as for Albuminized

paper (p. 241), and mix by the following Formula:--

Albumen 9 fluid ounces.

Water 3 fluid ounces.

Liquor Ammoniæ 2 fluid drachms.

Iodide of Potassium 48 grains.

Bromide of Potassium 12 grains.

The Iodide and Bromide should be free from Carbonate of Potash, which is

said to cause pin-holes in the Negatives. To ensure the absence of this

salt, dissolve the total quantity of both Iodide and Bromide in the three

ounces of water advised in the formula; then, previously to adding the

Ammonia and Albumen, introduce _an excessively minute particle of Iodine_,

enough barely to colour the liquid. The Iodine decomposes the Carbonate

of Potash, but it must not be used in excess, since free Iodine possesses

the property of coagulating Albumen. Iodide of Cadmium also coagulates

Albumen, so that the Iodides of Potassium and Ammonium are the best.

Having mixed the ingredients in the order above given, introduce them into

a bottle, and shake it violently until they have thoroughly amalgamated.

Then transfer to a tall narrow jar; allow to settle for twenty-four hours,

and draw off the upper clear portion for use. Particulars of this part of

the process have already been given under the head of Albuminized Paper,

to which the reader is referred (p. 241).

The ammoniacal solution of Albumen may be kept for some time in a

stoppered bottle without much decomposition. If mucous threads form in it,

filter through fine linen.

_Mode of applying the Albumen._--Cover the moist film with the Albumen

in the same way as advised for Collodion (p. 216), pouring on at once a

sufficient quantity to cause it to spread in an even and undivided sheet;

otherwise a veined appearance may be produced, which will show in the

development. Return the excess of Albumen into the bottle, and pour it

once again upon the plate: the film will remain clear and transparent, if

the whole of the Nitrate of Silver has been properly washed away from the

Collodion. Lastly, stand the plate nearly vertically on blotting-paper to

dry. This will occupy five or six hours; but the process may be hastened

by artificial heat.

After the Albumen solution has been used to coat a number of plates

successively, it becomes diluted with water; the result of which is, that

unequal intensity of image is produced at the upper and lower edge of the

film.

The iodized Albumen plates are at this stage of the process nearly or

quite insensitive to light, and may be preserved unchanged for many weeks.

_Sensitizing the Albumen film._--When the plate has become thoroughly

dry, it is again introduced into the Bath of Aceto-Nitrate of Silver,

and allowed to remain for one minute: then washed with water in the same

manner as before, but with even greater care, in order to obviate clouding

in the development. If blisters should form on drying, it will be found

useful to hasten the process by holding the plates to the fire--or a hot

iron may be placed in the centre of a covered box and the glasses reared

up round the sides. They will thus dry quickly, and there will not be time

for the Albumen to swell much by imbibition.

_Exposure in the Camera._--This may be performed at any period within a

few weeks from the date of preparation of the plates. For a landscape view

with a small Stereoscopic single lens, allow about three minutes in the

winter, or one minute and a half in the summer.

_Development of the image._--This can be deferred as long as fourteen days

after the exposure, with successful results. Pour water over the plate

until the film is thoroughly wetted; then cover it with a solution of

Pyrogallic Acid containing one grain of the acid to the ounce of water,

and twenty minims of Glacial Acetic Acid. Two drops of a neutral solution

of Nitrate of Silver made with forty grains of Nitrate to the ounce of

water must be previously added to each fluid drachm of the Pyrogallic.

The development, in the case of a landscape view taken with sunlight,

commences almost immediately, and may be completed in about ten minutes,

but the time occupied in developing will vary greatly with the length of

exposure, the quantity of Nitrate of Silver, and the nature of the subject

copied--a badly lighted interior, for instance, often taking an hour or

longer to appear in all its details. If the developer should discolour

before the proper intensity has been obtained, pour it off and mix a fresh

quantity.

_Fixing the image._--Hyposulphite of Soda (one ounce to four of water)

will be found preferable to the Cyanide of Potassium, as the latter has a

solvent effect upon the Albumen. An unusually long time will be required,

as the fixing agent must penetrate the Albumen, to reach the Collodion

beneath.

Careful washing in water for five or ten minutes removes the excess of

Hyposulphite, and the plate may then be varnished in the usual way.

THE DRY COLLODION PROCESS.

The earlier attempts to employ sensitive Collodion plates in a desiccated

condition were unsuccessful. The film of Pyroxyline shrinks on drying, and

becomes almost impervious to moisture: hence, the developing solution not

penetrating properly, density cannot easily be obtained. We are indebted

to Dr. Hill Norris, of Birmingham, for establishing the theory of the

subject upon a more correct basis. He has pointed out the importance of

distinguishing two different conditions of the Collodion surface,[53]

viz. the _contractile_, common in newly-mixed Collodion,--and the _short_

or _powdery_, in Collodion which has been iodized with the alkaline

Iodides, and kept until much Iodine has been set free. The latter is the

most suitable condition for the dry process; and the practical mode of

distinguishing between them is by sensitizing a plate and passing the

finger across it; if it can be easily pushed away in a firm and connected

skin, it will be unfit for the purpose required. In order still further

to preserve the film in a condition permeable by the developer, it is

recommended to coat it whilst moist with a solution of Gelatine.

[53] See these states of the film more fully described at page 83.

The dry Collodion process, although less sensitive, is more simple than

that on Collodio-Albumen, and possesses many of its advantages; but it is

less universally applicable, since it depends entirely for success upon

the peculiar state of the Collodion, resembling in this respect the Oxymel

process already described.

_Mode of preparing the plates._--The glasses are coated with the Collodion

in the usual way. Blistering during development being liable to happen in

this process as in the last, every care must be taken to make the films

adhere with the greatest possible tenacity, both by cleaning the glasses

with extra care (see p. 294), and also by allowing the Collodion to set

firmly before dipping in the Bath. The plate may be held from twenty to

thirty seconds previous to immersion, or even longer, provided the film,

when lifted out of the Bath, appears of uniform thickness throughout (see

page 218).

The sensitizing having been completed, wash the plates with plain water,

exactly in the same way as for Oxymel (p. 292). If Nitrate of Silver

be left, clouding will take place in the process of development. After

washing, drain for a few seconds, and immerse in the solution of Gelatine.

To prepare this Bath, take of

Nelson's patent Gelatine 128 grains.

Distilled water 14 ounces.

Alcohol 2-1/2 ounces.

Put the Gelatine in the cold water, and allow it a quarter of an hour

to soften and swell; it will then readily dissolve on applying a gentle

heat. This may be done in a glazed saucepan or a pipkin of earthenware,

taking care not to scorch the bottom part by too strong a heat. Next

clarify the solution by adding to it, whilst barely warm, a tea-spoonful

of white of egg (previously beaten up with a silver fork), and afterwards

heating nearly to the boiling point. The Alcohol must now be added, to

facilitate the coagulation of the Albumen. When this takes place and the

liquid becomes clear, filter through a clean piece of cambric folded

three or four times. If a hot water filtering apparatus can be obtained,

the solution may be made to pass through _paper_; but as it tends to

gelatinize on cooling, the ordinary mode of filtration commonly fails.

The quantity of Alcohol in the above formula is greater than is usually

recommended, allowance having been made for a partial evaporation of the

spirit.

The filtered liquid may be poured into a flat porcelain dish, or a

vertical trough, but in either case it will be necessary to stand the

vessel in warm water, in order to prevent gelatinization.

The Collodion plate, thoroughly washed, is to be immersed in this solution

and moved up and down for two or three minutes. It is then removed,

drained on blotting-paper, and dried. The use of artificial heat in drying

will be found a great advantage; it prevents the gelatine from settling

unequally upon the plate. Those who possess an apparatus made purposely

for drying plates by hot air, will experience no difficulty, but an

ordinary deal trunk may be made to answer, with a little management. Cover

the bottom of the box with blotting-paper, and having heated one or two

"flat irons," place them in the centre: then range the glasses side by

side, with the coated surface looking inwards; in a quarter of an hour,

or from that to twenty minutes, the desiccation will be complete. If the

Collodion plates are prepared in a room containing a fire, they may be

reared up side by side at a distance of two or three feet, and in that

way may be safely dried without fear of injury, provided white light be

excluded.

When dry they can be stowed away in a box; all the precautions given at

page 293 being observed. The sensibility remains good for many days,

possibly for weeks or months in cold weather.

_Exposure in the Camera._--Allow from four to eight times the exposure

of the most sensitive moist Collodion. On a clear summer's day, a sun-lit

view may require one minute or a minute and a half, with a short focus

Stereoscopic lens, having a diaphragm of a quarter of an inch diameter.

The average time however with the same lens would be about twice as much,

viz. three minutes.

_Development of the Image._--Make a saturated solution of Gallic Acid in

water by the directions given at page 261. Then dissolve forty grains

of pure Nitrate of Silver in one ounce of distilled water. Pour into a

flat porcelain dish a sufficient quantity of the Gallic Acid solution to

flood the plate readily. Then measure it, and to each fluid ounce add

_ten minims_ of the solution of Silver, or five minims in hot weather. It

is important that no discoloration should occur on mixing these liquids

together, to obviate which, observe the following precautions:--Clean the

porcelain vessel very carefully with Nitric Acid or Cyanide before use.

Employ a pure solution of Nitrate of Silver; and mix it with the Gallic

Acid, in preference to adding the Gallic Acid to the Silver solution (read

the remarks at p. 179).

The picture may be expected to appear in five or ten minutes, and in

one hour, or from that to four hours (p. 298), the development will be

complete. It will not be necessary to keep the plates in motion, but

simply to lay them side by side in the solution of the Gallic Acid. If

in spite of all precautions the developer begins to blacken before the

intensity has reached the proper point, it must be poured off and a fresh

mixture prepared. This however will not often happen.

Lastly, when a full amount of opacity has been obtained, wash the plate

with water, and fix it in a solution of Hyposulphite of Soda, or dilute

solution of Cyanide of Potassium.

_Failures in the process._--Stains in the development may arise from

using dirty dishes, or glasses which have been left in Gallo-Nitrate

of Silver and improperly cleaned. It must be borne in mind that these

impurities are not visible to the eye, although they produce the effect of

discolouring the developer. A thorough cleansing with strong Nitric Acid

or Potash will prove a remedy.

Blisters, unless of large size, may often be disregarded, as they

disappear on drying. General cloudiness may depend upon the film having

been imperfectly washed. Irregular reduction at certain parts may be due

to the Gelatine setting before the plate has become dry, or to stains

produced by the finger applied to the upper edge of the plate.[54]

[54] Since the above was written, Mr. Maxwell Lyte has communicated to the

'Photographic Journal' (vol. iii.) a dry process in which a _modified_

Gelatine is used. The change is produced by boiling a solution of gelatine

with dilute Sulphuric Acid, which is afterwards neutralized and removed by

means of chalk. The result is to destroy the gelatinizing property of the

animal substance; the solution retains its fluidity on cooling, and the

necessity of employing artificial heat in drying the plates is avoided.

PART III.

OUTLINES or GENERAL CHEMISTRY.

OUTLINES or GENERAL CHEMISTRY.

[Illustration]

CHAPTER I.

THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS.

The limits of the present Work allow only of a simple sketch of the

subjects which it is proposed to treat in this Chapter. Our attention

therefore must be confined to an explanation of certain points which

are alluded to in the First Part of the Work, and without a proper

understanding of which it will be impossible for the reader to make

progress.

The following division may be adopted:--The more important Elementary

Bodies, with their symbols and atomic weights; the Compounds formed

by their union; the class of Salts; illustrations of the nature of

Chemical Affinity; Chemical Nomenclature; Symbolic Notation; the laws of

Combination; the Atomic Theory; the Chemistry of Organic Bodies.

THE CHEMICAL ELEMENTS, WITH THEIR SYMBOLS AND ATOMIC WEIGHTS.

The class of elementary bodies embraces all those substances which cannot,

in the present state of our knowledge, be resolved into simpler forms of

matter.

The chemical elements are divided into "metallic" and "non-metallic,"

according to the possession of certain general characters.

The following are some of the principal non-metallic elements, with the

symbols employed to designate them, and their atomic weights:[55]--

Symbol. Atomic Wt.

Gases. { Oxygen O 8

{ Hydrogen H 1

{ Nitrogen N 14

{ Chlorine Cl 36

Solids. { Iodine I 126

{ Carbon C 6

{ Sulphur S 16

{ Phosphorus P 32

Liquid. Bromine Br 78

Unknown. Fluorine F 19

The metallic elements are more numerous. The following list includes only

those which are commonly known:--

Symbol. Atomic Wt.

Metals of the { Potassium K 40

Alkalies. { Sodium Na 24

Metals of the { Barium Ba 69

Alkaline { Calcium Ca 20

Earths { Magnesium Mg 12

Metals { Iron Fe 28

Proper. { Zinc Zn 32

{ Cadmium Cd 56

{ Copper Cu 32

{ Lead Pb 104

{ Tin Sn 59

{ Arsenic As 75

{ Antimony Sb 129

Nobel { Mercury Hg 202

Metals. { Silver Ag 108

{ Gold Au 197

{ Platinum Pt 99

[55] The atomic weights, with the exception of that of Gold, are taken

from the last edition of Brande's 'Manual of Chemistry.'

ON THE BINARY COMPOUNDS OF THE ELEMENTS.

Many of the elementary bodies exhibit a strong tendency to combine with

each other, and to form compounds, which differ in properties from either

of their constituent elements. This attraction, which is termed "Chemical

Affinity," is exerted principally between bodies which are opposed to each

other in their general characters. Thus, taking for example the elements

Chlorine and Iodine--they are analogous in their reactions, and therefore

there is but little attraction between them, whereas either of the two

combines eagerly with Silver, which is an element of a different class.

So, again. Sulphur unites with the metals, but two metallic elements are

comparatively indifferent to each other.

Oxygen is by far the most important in the list of chemical elements.

It combines with all the others, with the single exception, perhaps, of

Fluorine. The attraction, or chemical affinity, however, which is exerted,

varies much, in different cases. The metals, as a class, are easily

oxidized; whilst many of the non-metallic elements, such as Chlorine,

Iodine, Bromine, etc., exhibit but little affinity for Oxygen. Nitrogen is

also a peculiarly negative element, showing little or no tendency to unite

with the others.

Classification of binary compounds containing Oxygen.--When one simple

element unites with another, the product is termed a "binary" compound.

There are three distinct classes of binary compounds of Oxygen:--Neutral

Oxides, basic Oxides, and acid Oxides.

Neutral and basic Oxides.--Take as examples--the Oxide of Hydrogen, or

Water, a neutral Oxide; the Oxide of Potassium, or Potash, a basic Oxide.

Water is termed a neutral oxide, because its affinities are low, and it is

comparatively indifferent to other bodies. Potash and Oxide of Silver are

examples of basic oxides; but there is a great difference between the two

in chemical energy, the former belonging to a superior class of bases,

viz. the alkaline.

By studying the properties of an alkali (such as Potash or Soda) which

are familiar to all, we gain a correct notion of the whole class of basic

oxides. An alkali is a substance readily soluble in water, and yielding a

solution which has a slimy feel from its solvent action upon the skin. It

immediately restores the blue colour of reddened litmus, and changes the

blue infusion of cabbage to green. Lastly, it is neutralized and loses all

its characteristic properties upon the addition of an acid.

The _weaker bases_ are, as a rule, sparingly or not at all soluble in

water, neither have they the same caustic and solvent action upon the

skin; but they restore the colour of reddened litmus, and neutralize acids

in the same manner as the more powerful bases, or alkalies.

_The_ Acid _Oxides._--This class, taking the stronger acids

as the type, may be described as follows:--very soluble in water, the

solution possessing an intensely sour taste, and a _corroding_ rather than

a solvent action upon the skin; changes the blue colour of litmus and

other vegetable substances to red, and neutralizes the alkalies and basic

oxides generally.

Observe however that these properties are possessed in very various

degrees by different acids. Prussic Acid and Carbonic Acid, for instance,

are not sour to the taste, and being feeble in their reactions, redden

litmus scarcely or not at all. All acids however, without any exception,

tend to combine with bases and to neutralize themselves; so that this may

be said to be the most characteristic property of the class.

_Chemical composition of Acid and Basic Oxides contrasted._--It is a law

commonly observed, although with many exceptions, that bases are formed

by the union of Oxygen with _metals_; and acids, by Oxygen uniting with

_non-metallic elements_. Thus, Sulphuric Acid is a compound of Sulphur and

Oxygen; Nitric Acid, of Nitrogen and Oxygen. But the alkali, Potash, is

an oxide of the _metal_ Potassium; and the oxides of Iron, Silver, Zinc,

etc. are bases, and not acids.

Again, the composition of acids and bases is different in another respect;

the former invariably contain more Oxygen in proportion to the other

element than the latter. Taking the same examples as before, the two

classes may be represented thus:--

Acids { Oil of Vitriol, Sulphur 1 atom, Oxygen 3 atoms.

{ Aqua-fortis, Nitrogen 1 " Oxygen 5 "

Bases { Oxide of Silver, Silver 1 atom. Oxygen 1 atom.

{ Oxide of Iron, Iron 1 " Oxygen 1 "

The class of Hydrogen Acids.--Oxygen is so essentially the element which

forms the acidifying principle of acids, that its very name is derived

from that fact (οξυς, acid, and γενναω, to generate). Still there are

exceptions to this rule, and in some acids _Hydrogen_ appears to play the

same part; the _Hydracids_, as they are termed, are formed principally by

Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine,

etc. Thus, Muriatic or Hydrochloric Acid contains Chlorine and Hydrogen;

Hydriodic Acid contains Iodine and Hydrogen.

Observe, however, that the position held by the Hydrogen in these

compounds, is different from that of the Oxygen in the "Oxyacids," as

regards the number of atoms usually present; thus--

Aqua-fortis = Nitrogen 1 atom, Oxygen 5 atoms,

Muriatic Acid = Chlorine 1 " Hydrogen 1 atom;

so that the composition of the Hydracids is analogous to the _basic_

oxides, in containing a single atom of each constituent.

THE TERNARY COMPOUNDS OF THE ELEMENTS.

As the various elementary substances unite with each other to form Binary

Compounds, so these binary compounds again unite and form _Ternary_

Compounds.

Compound bodies however do not, as a rule, unite with simple elements. In

illustration, take the action of Nitric Acid upon Silver, described at

page 12. No effect is produced upon the metal until _Oxygen_ is imparted;

then the Oxide of Silver so formed dissolves in the Nitric Acid. In other

words, it is necessary that a binary compound should be first formed,

before the solution can take place. The mutual attraction or chemical

affinity exhibited by compound bodies is, as in the case of elements, most

strongly marked when the two substances are opposed to each other in their

general properties.

Thus, _acids_ do not unite with other acids, but they combine instantly

with _alkalies_; the two mutually neutralizing each other and forming "a

salt."

_Salts_ therefore are ternary compounds produced by the union of acids and

bases; common Salt, formed by neutralizing Muriatic Acid with Soda, being

taken as the type of the whole class.

_General characters of the Salts._--An aqueous solution of Chloride of

Sodium, or common Salt, possesses those characters which are usually

termed saline; it is neither sour nor corrosive, but, on the other hand,

has a cooling agreeable taste. It produces no effect upon litmus and other

vegetable colours, and is wanting in those energetic reactions which are

characteristic of both acids and alkalies; hence, although formed by the

union of two binary compounds, it differs essentially in properties from

both.

All salts however do not correspond to this description of the properties

of Chloride of Sodium. The Carbonate of Potash, for instance, is an

acrid and alkaline salt, and the Nitrate of Iron reddens litmus-paper.

A perfectly neutral salt is formed when a strong acid unites with an

energetic base; but if, of the two constituents, one is more powerful than

the other, the properties of that one are often seen in the resulting

salt. Thus the Carbonate of Potash is _alkaline_ to test-paper, because

the Carbonic Acid is feeble in its reactions; but if _Nitric Acid_ and

_Potash_ are brought together, then a Nitrate of Potash is produced, which

is neutral in every sense of the term.

The Chloride of Sodium and salts of a similar kind are freely soluble in

water, but all salts are not so. Some dissolve only sparingly, and others

not at all. The Chloride and Iodide of Silver are examples of the latter

class; they are not bitter and caustic like the Nitrate of Silver, but are

perfectly tasteless from being insoluble in the fluids of the mouth.

It is seen therefore from these examples, and many others which might

be adduced, that the popular notion of a saline body is far from being

correct, and that, in the language of strict definition, any substance

is a salt which is produced by the union of an acid with an alkali,

independent of the properties it may possess.

Thus, _Cyanide of Potassium_ is a true salt, although highly poisonous;

Nitrate of Silver is a salt; the green Sulphate of Iron is a salt; so also

is Chalk or Carbonate of Lime, which has neither taste, colour, nor smell.

_On the "Hydracid" class of Salts._--The distinction between Oxyacids

and Hydracids has already been pointed out (p. 309), the latter having

been shown to consist of Hydrogen united with elements analogous in their

reactions to Chlorine, Iodine, Bromine, etc.

In a salt formed by an Oxygen Acid, both the basic and acid elements

appear. Thus the common Nitre, which is a Nitrate of Potash, is found

by analysis to contain Oxide of Potassium as a base, in a state of

combination with Nitric Acid. But if a salt be formed by neutralizing an

alkali with a _Hydrogen Acid_, the product in that case does not contain

all the elements. This is seen from the following example:--

Hydrochloric Acid + Soda

= Chloride of Sodium + Water;

or, stated more at length,--

(Chlorine Hydrogen) + (Oxygen Sodium)

= (Chlorine Sodium) + (Oxygen Hydrogen).

Observe that the Hydrogen and Oxygen, being present in the correct

proportions, unite to form Water, which is an Oxide of Hydrogen. This

water passes off when the solution is evaporated, and leaves the dry

crystals of salt. On the other hand, with the Oxyacid Salts, the

elementary Hydrogen being absent, no water is formed, and the Oxygen

remains.

It must therefore be borne in mind that salts like the Chlorides,

Bromides, Iodides, etc. contain only _two_ elements; but that in the

Oxyacid Salts, such as Sulphates, Nitrates, Acetates, _three_ are present.

Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but

Chloride of Silver contains simply Chlorine and metallic Silver united,

without Oxygen.

The Hydracid salts however, when decomposed, yield products similar to

the Oxyacid salts. For instance, if Iodide of Potassium be dissolved in

water, and dilute Sulphuric Acid added, this acid, being powerful in its

chemical affinities, tends to appropriate to itself the alkali; but it

does not remove _Potassium_ and liberate _Iodine_, but takes the _Oxide_

of Potassium and sets free _Hydriodic Acid_, In other words, as an atom of

water is produced during the _formation_ of a Hydracid Salt, so is an atom

destroyed and made to yield up its elements in the _decomposition_ of a

Hydracid Salt.

The reaction of dilute Sulphuric Acid upon Iodide of Potassium may be

stated thus:--

Sulphuric Acid _plus_ (Iodine Potassium) _plus_ (Hydrogen Oxygen)

_equals_ (Sulphuric Acid, Oxygen Potassium) or Sulphate of Potash,

_and_ (Hydrogen Iodine) or Hydriodic Acid.

THE NATURE OF CHEMICAL AFFINITY FURTHER ILLUSTRATED.

_Illustration from the Non-metallic Elements._--If a stream of Chlorine

gas be passed into a solution containing the same salt as before

mentioned, viz. the Iodide of Potassium, the result is to liberate a

certain portion of Iodine, which dissolves in the liquid, and tinges it

of a brown colour. The element Chlorine, possessing a degree of chemical

energy superior to that of Iodine, prevails over it, and removes the

Potassium with which the Iodine was previously combined.

Chlorine + Iodide of Potassium

= Iodine + Chloride of Potassium.

_The same Law illustrated by the Metals._--A strip of Iron dipped in

solution of Nitrate of Silver becomes immediately coated with metallic

Silver; but a piece of Silver-foil may be left for any length of time in

Sulphate of Iron without undergoing change: the difference depends upon

the fact, that metallic Iron has a greater attraction for Oxygen than

Silver, and hence it displaces it from its solution.

Iron + Nitrate of Silver

= Silver + Nitrate of Iron.

_Illustrations amongst Binary Compounds._--If a few drops of solution

of Potash be added to solution of Nitrate of Silver, a brown deposit is

formed, which is the Oxide of Silver, sparingly soluble in water. That is

to say, as a stronger metal displaces _metallic Silver_, so does an oxide

of the same metal displace _Oxide of Silver_. Therefore bases like the

alkalies, alkaline earths, etc. cannot exist in a free state in solutions

of the salts of weaker bases,--a liquid containing Nitrate of Silver could

not also contain free Potash or Ammonia.

In the list given at page 306, the metallic elements are arranged

principally in the order of their chemical affinities; those of Potassium,

Sodium, Barium, etc. being the most marked.

As the alkalies displace the weaker bases from their combination with

acids, so the strong _acids_ displace weak acids from their combination

with bases. Thus, as

Oxide of Potassium + Acetate of Silver

= Oxide of Silver + Acetate of Potash;

So

Nitric Acid + Acetate of Silver

= Acetic Acid + Nitrate of Silver.

In the list of acids. Sulphuric Acid is usually placed first as being the

strongest, and Carbonic Acid, which is a gaseous substance, last. The

vegetable acids, such as Acetic, Tartaric, etc., are _intermediate_, being

weaker than the mineral acids, but stronger than Carbonic, or Hydrocyanic

Acid.

_The order of decompositions affected by the insolubility or the

volatility of the products which may be formed._--It might be inferred

from remarks already made, that on mixing saline solutions, a gradual

interchange of elements would take place, until the strongest acids

were associated with the strongest bases, and _vice versâ_. There are

many causes however which interfere to prevent this; one of which is

_volatility_.---

The violent effervescence which takes place on treating a _Carbonate_ of

any kind with an acid is due to the _gaseous_ nature of Carbonic Acid and

its escape in that form, which greatly facilitates the decomposition.

_Insolubility_ is also a cause which exercises a great influence on the

result which will follow in mixing solutions. If the formation of an

insoluble substance is possible by any interchange of elements, it will

take place. A solution of Chloride of Sodium added to Nitrate of Silver

invariably produces Chloride of Silver; the _insolubility_ of Chloride of

Silver being the cause which determines its formation.

So again, Sulphate of Lead and Protonitrate of Iron are produced by mixing

Nitrate of Lead with Sulphate of Iron; but if Nitrate of _Potash_ be

substituted for Nitrate of Lead, the result is uncertain, because there

are no elements present which can, by interchanging, form an insoluble

salt; Sulphate of Potash, although _sparingly_ soluble in water, not being

_insoluble_, like the Sulphate of Lead or the Sulphate of Baryta.

ON CHEMICAL NOMENCLATURE.

The nomenclature of the chemical _elements_ is mostly independent of any

rule; but an attempt has been made to obviate this in the case of those of

later discovery. Thus the names of the newly-found _metals_ usually end

in _um_, as Potassium, Sodium, Barium, Calcium, etc.; and those elements

which possess analogous characters have corresponding terminations

assigned to them, as Chlorine, Bromine, Iodine, Fluorine, etc.

_Nomenclature of Binary Compounds._--These are often named by attaching

the termination _ide_ to the more important element of the two; as, the

Ox_ide_ of Hydrogen, or Water; the Chlor_ide_ of Silver; the Sulph_ide_

of Silver. Binary compounds of Sulphur however are sometimes termed

Sulphurets, as the _Sulphuret_ or the _Sulphide_ of Silver indifferently.

When the same body combines with Oxygen, or the corresponding element, in

more than one proportion, the prefix _proto_ is applied to that containing

the least Oxygen; _sesqui_ to that with once and a half as much as the

_proto_; _bi_ or _bin_ to that with twice as much; and _per_ to the one

containing the most Oxygen of all. As examples, take the following:--The

Protoxide of Iron; the Sesquioxide of Iron: the Protochloride of Mercury;

the Bichloride of Mercury. In these examples the Sesquioxide of Iron

is also a _Per_oxide, because no higher simple oxide is known, and the

Bichloride of Mercury is a _Per_chloride for a similar reason.

When an inferior compound is discovered, it is often termed _sub_; as the

Suboxide of Silver, the Subchloride of Silver. These bodies contain the

least known quantity of Oxygen and Chlorine respectively, and are hence

entitled to the prefix _proto_; but being of minor importance, they are

excepted from the general rule.

The combinations of metallic elements with each other are termed "alloys;"

or if containing Mercury, "amalgams."

_Nomenclature of binary Compounds possessing acid properties._--These

are named on a different principle. The termination _ic_ is applied to

one element. Thus, taking as an illustration the liquid known as "Oil of

Vitriol," it is truly an _Oxide_ of Sulphur, but as it possesses strong

acid properties it is termed Sulphur_ic_ Acid. So Nitric Acid is an Oxide

of Nitrogen; Carbonic Acid is an Oxide of Carbon, etc. When there are two

oxides of the same element, both possessing acid properties, the most

important has the termination _ic_, and the other _ous_; as Sulphuric

Acid, Sulphur_ous_ Acid; Nitric Acid, Nitr_ous_ Acid.

_Nomenclature of the Hydracids._--The Hydrogen Acids are distinguished

from Oxyacids by retaining the names of both constituents, the termination

_ic_ being annexed as usual. Thus, _Hydro_chloric Acid, or the Chloride of

Hydrogen; _Hydr_iodic Acid, or the Iodide of Hydrogen.

_Further illustrations of the nomenclature of Binary Compounds._--The

Oxides of Nitrogen, and also of Sulphur, afford an interesting

illustration of the principles of nomenclature. The former are as

follows:--

Nitrogen. Oxygen.

Protoxide of Nitrogen 1 atom. 1 atom.

Binoxide of Nitrogen 1 " 2 "

Nitrous Acid 1 " 3 "

Peroxide of Nitrogen 1 " 4 "

Nitric Acid 1 " 5 "

Observe, that two only out of the five possess acid properties, the others

being simple oxides. Nitric Acid is, strictly speaking, the "Peroxide,"

but as it belongs to the class of acids, that term naturally falls to the

compound below.

The binary compounds of Sulphur with Oxygen all possess acid properties;

they may be represented (in part) as follows:--

Sulphur. Oxygen.

Hyposulphurous Acid 2 atoms. 2 atoms.

Sulphurous Acid 1 " 2 "

Hyposulphuric Acid 2 " 5 "

Sulphuric Acid 1 " 3 "

In this case the Sulphuric and Sulphurous Acids had become familiarly

known before the others, intermediate in composition, were discovered.

Hence, to avoid the confusion which would result from changing

the nomenclature, the new bodies are termed _Hypo_sulphuric and

_Hypo_sulphurous (from ὑπο, _under_).

_Nomenclature of Salts._--Salts are named according to the acid they

contain; the termination _ic_ being changed into _ate_, and _ous_ into

_ite_. Thus, Sulphuric Acid forms Sulph_ates_; Nitric Acid, Nitr_ates_;

but Sulphur_ous_ Acid forms Sulph_ites_, and Nitrous Acid, Nitr_ites_.

In naming a salt, the base is always placed _after_ the acid, the term

_oxide_ being omitted; thus. _Nitrate of Oxide of Silver_ is more shortly

known as "Nitrate of Silver," the presence of Oxygen being understood.

When there are two oxides of the same base, both of which are

_salifiable_,--in naming the salts, the term _proto_ is prefixed to the

acid of the salt formed by the lowest, and per to that of the higher

oxide; as, the _Proto_sulphate of Iron, or Sulphate of the Protoxide; the

_Per_sulphate of Iron, or Sulphate of the Peroxide.

Many salts contain more than one atom of acid to each atom of base. In

that case, the usual prefixes expressive of quantity are adopted: thus,

the _Bi_sulphate of Potash contains twice as much Sulphuric Acid as the

neutral Sulphate, etc.

On the other hand, there are salts in which the base is in excess with

regard to the acid, and which are usually known as "basic salts;" thus,

the red powder which deposits from solution of Sulphate of Iron, is a

_basic_ Persulphate of Iron, or a Sulphate of the Peroxide of Iron with

more than the normal proportion of oxide.

_Nomenclature of the Hydracid Salts._--The composition of these salts

being different from those formed by Oxygen Acids, the nomenclature varies

also. Thus, in neutralizing Hydrochloric Acid with Soda, the product

formed is not known as Hydrochlorate of Soda, but as _Chloride of Sodium_;

this salt, and others of a similar constitution, being _binary_, and not

_ternary_, compounds. The salt produced by Hydrochloric Acid and _Ammonia_

however is often called "Muriate or Hydrochlorate of Ammonia," although

more strictly it should be the _Chloride of Ammonium_.

ON SYMBOLIC NOTATION.

The list of symbols employed to represent the various elementary bodies is

given at page 306.--Commonly the initial letter of the Latin name is used,

a second or smaller letter being added when two elements correspond in

their initials: thus C stands for Carbon, Cl for Chlorine, Cd for Cadmium,

and Cu for Copper.

The chemical symbol however does not simply represent a particular

element; it denotes also a definite weight, or equivalent proportion, of

that element. This will be explained more fully in the succeeding pages,

when speaking of the Laws of Combination.

_Formulæ of Compounds._--In the _nomenclature_ of compounds it is usual

to place the Oxygen or analogous element _first_ in the case of binary

compounds, and the acid before the base in the ternary compounds, or

salts; but in representing them _symbolically_ this order is reversed:

thus, Oxide of Silver is written AgO, and never as OAg; Nitrate of Silver

as AgO NO{5}, not NO{5}AgO.

The juxtaposition of symbols expresses combination; thus, FeO is a

compound of one proportion of Iron with one of Oxygen, or the "Protoxide

of Iron," If more than one equivalent be present, small figures are

placed below the symbols: thus, Fe{2}O{3} represents two equivalents of

Iron united with three of Oxygen, or the "Peroxide of Iron;" SO{3}, one

equivalent of Sulphur with three of Oxygen, or Sulphuric Acid.

Larger figures placed before and in the same line with the symbols, affect

the _whole compound_ which the symbols express: thus, 2 SO{3} means two

equivalents of Sulphuric Acid; 3 NO{5}, three equivalents of Nitric Acid.

The interposition of a comma prevents the influence of the large figure

from extending further. Thus, the double Hyposulphite of Soda and Silver

is represented as follows:--

2 NaO S{2}O{2}, AgO S{2}O{2},

or _two_ equivalents of Hyposulphite of Soda with one of Hyposulphite of

Silver; the large figure referring only to the first half of the formula.

Sometimes brackets, etc. are employed, in order to render a complicated

formula more plain. For example, the formula for the double Hyposulphite

of Gold and Soda, or "Sel d'or," may be written thus;--

3 (NaO S{2}O{2}) AuO S{2}O{2} + 4 HO.

In this formula, the _plus sign_ (+) denotes that the four atoms of water

which follow, are less intimately united with the framework of the salt

than the other constituents.

The use of a plus sign is commonly adopted in representing salts which

contain water of crystallization. Thus, the formula for the crystallized

Protosulphate of Iron is written as follows:--

FeO SO{3} + 7 HO.

These atoms of water are driven off by the application of heat, leaving a

white substance, which is the Anhydrous salt, and would be written simply

as FeO SO{3}.

The _plus_ sign however is often employed in token of simple _addition_,

no combination of any kind being intended. Thus the decomposition which

follows on mixing Chloride of Sodium with Nitrate of Silver may be

written as follows:--

NaCl + AgO NO{5} = AgCl + NaO NO{5};

that is,--

Chloride of Sodium _added to_ Nitrate of Silver.

= Chloride of Silver _and_ Nitrate of Soda.

ON EQUIVALENT PROPORTIONS.

When elementary or compound bodies enter into chemical union with each

other, they do not combine in indefinite proportions, as in the case of a

mixture of two liquids, or the solution of a saline body in water. On the

other hand, a certain definite weight of the one unites with an equally

definite weight of the other; and if an excess of either be present, it

remains free and uncombined.

Thus, if we take a _single grain_ of the element Hydrogen--to convert that

grain into Water there will be required exactly 8 grains of Oxygen; and

if a larger quantity than this were added, as for instance _ten grains_,

then two grains would be over and above. So, to form Hydrochloric Acid, 1

grain of Hydrogen takes 36 grains of Chlorine:--for the _Hydriodic Acid_,

1 grain of Hydrogen unites with 126 grains of Iodine.

Again, if separate portions of metallic Silver, of 108 grains each, are

weighed out,--in order to convert them into Oxide, Chloride, and Iodide of

Silver respectively, there would be required

Oxygen 8 grains.

Chlorine 36 "

Iodine 126 "

Therefore it appears that 8 grains of Oxygen are _equivalent_ to 36 grains

of Chlorine and to 126 grains of Iodine, seeing that these quantities all

play the same part in combining; and so it is with regard to the other

elements,--to every one of them a figure can be assigned which represents

the number of parts by weight in which that element unites with others.

These figures are the "equivalents" or "combining proportions," and they

are denoted by the _symbol_ of the element. A symbol does not stand as

a simple representative of an element, but as a representative of _one

equivalent_ of an element. Thus "O" indicates 8 parts by weight of Oxygen;

"Cl" one equivalent, or 36 parts by weight, of Chlorine; and so with the

rest.

Observe however that these figures, termed "equivalents," do not refer

to the _actual number_ of parts by weight, but only to the _ratio_ which

exists between them: if Oxygen is 8, then Chlorine is 36; but if we term

Oxygen 100, as some have proposed, then Chlorine would be 442·65.

In the scale of equivalents now usually adopted, Hydrogen, as being the

lowest of all, is taken as unity, and the others are related to it.

_Equivalents of Compounds._--The law of equivalent proportions applies

to compounds as well as to simple bodies, the combining proportion of a

compound being always the sum of the equivalents of its constituents. Thus

Sulphur is 16, and Oxygen 8, therefore Sulphuric Acid, or SO{3}, equals

40. The equivalent of Nitrogen is 14, that of Nitric Acid, or NO{5}, is 54.

The same rule applies with regard to salts. Take for instance the Nitrate

of Silver: it contains

Equivalent.

Nitrogen 14

6 Oxygen 48

Silver 108

---

Total of equivalents, or equivalent } 170

of the Nitrate of Silver }

_Practical application of the Laws of Combination ._--The utility of being

acquainted with the law of combining proportions is obvious when their

nature is understood. As bodies both unite with and replace each other

in equivalents, a simple calculation shows at once how much of each

element or compound will be required in a given reaction. Thus, supposing

it be desired to convert 100 grains of Nitrate of Silver into _Chloride_

of Silver, the weight of Chloride of Sodium which will be necessary is

deduced thus:--one equivalent, or 170 parts, of Nitrate of Silver, is

decomposed by an equivalent, or 60 parts, of Chloride of Sodium. Therefore

as 170 : 60 :: 100 : 35·2;

that is, 35·2 grains of Salt will precipitate, in the state of Chloride,

the whole of the Silver contained in 100 grains of Nitrate.

So again, in order to form the Iodide of Silver, the proportions in which

the two salts should be mixed is thus shown. The equivalent of Iodide of

Potassium is 166, and that of Nitrate of Silver is 170. These numbers so

nearly correspond, that it is common to direct that equal weights of the

two salts should be taken.

One more illustration will suffice. Supposing it be required to form 20

grains of Iodide of Silver--how much Iodide of Potassium and Nitrate of

Silver must be used? One equivalent, or 166 parts, of Iodide of Potassium,

will yield an equivalent, or 234 parts, of Iodide of Silver; therefore

as 234 : 166 :: 20 : 14·2.

Hence, if 14·2 grains of the Iodide of Potassium be dissolved in water,

and an equivalent quantity, viz. 14·5 grains, of the Nitrate of Silver

added, the yellow precipitate, when washed and dried, will weigh precisely

20 grains.

ON THE ATOMIC THEORY.

The atomic theory, originally proposed by Dalton, so much facilitates the

comprehension of chemical reactions generally, that it may be useful to

give a short sketch of it.

It is supposed that all matter is made up of an infinite number of minute

atoms, which are elementary, and do not admit of further division. Each

of these atoms possesses an actual weight, although inappreciable by our

present methods of investigation. Simple atoms, by uniting with each

other, form _compound atoms_; and when these compounds are broken up, the

elementary constituent atoms are not destroyed, but separate from each

other, in possession of all their original properties.

In representing the simple atomic structure of bodies, _circles_ may be

used, as in the following diagram.

[Illustration: Fig. 1. Fig. 2. Fig. 3.]

Fig. 1 is a compound atom of Sulphuric Acid, consisting of an atom of

Sulphur united intimately with three of Oxygen; fig. 2 is an atom of

Peroxide of Nitrogen, NO{4}; and fig. 3, an atom of Nitric Acid, composed

of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NO{5}.

_The term "atomic weight" substituted for equivalent proportion._--If

we suppose that the simple atoms of different kinds of matter _differ

in weight_, and that this difference is expressed by their equivalent

numbers, the whole laws of combination follow by the simplest reasoning.

It is easy to understand that an atom of one element, or compound, would

displace, or be substituted for, a single atom of another; therefore,

taking as the illustration the decomposition of Iodide of Potassium by

Chlorine,--the weight of the latter element required to liberate 126

grains of Iodine is 36 grains, because _the weights of the atoms of those

two elementary bodies are as 36 to 126_. So again, in the reaction

between Chloride of Sodium and Nitrate of Silver, a compound atom of the

former, represented by the weight 60, reacts upon a compound atom of the

latter, which equals 170.

Therefore in place of the term "equivalent" or "combining proportion," it

is more usual to employ that of "atomic weight." Thus the atomic weight

of Oxygen is 8, represented by the symbol O; that of Sulphur is 16; hence

the atomic weight of the compound atom of Sulphuric Acid, or SO{3}, is

necessarily equal to the combined weights of the four simple atoms; _id

est_, 16 + 24 = 40.

ON THE CHEMISTRY OF ORGANIC SUBSTANCES.

By "organic" substances are meant those which have possessed _life_, with

definite organs and tissues, in contra-distinction to the various forms of

dead inorganic matter, in which no structural organization of that kind is

found.

The term organic however is also applied to substances which are obtained

by chemical processes from the vegetable and animal kingdoms, although

they cannot themselves be said to be living bodies; thus Acetic Acid,

procured by the distillation of woody fibre, and Alcohol, by fermentation

from sugar, are strictly organic substances.

The class of organic bodies embraces a great variety of products; which,

like inorganic Oxides, may be divided into neutral, acid, and basic.

The organic _acids_ are numerous, including Acetic Acid, Tartaric, Citric,

and a variety of others.

The _neutral substances_ cannot easily be assimilated to any class of

inorganic compounds; as examples, take Starch, Sugar, Lignine, etc.

The _bases_ are also a large class. They are mostly rare substances, not

familiarly known: Morphia, obtained from Opium; Quinia, from Quinine;

Nicotine, from Tobacco, are illustrations.

_Composition of organic and inorganic bodies contrasted._--There are more

than fifty elementary substances found in the inorganic kingdom, but

only _four_, commonly speaking, in the organic: these four are Carbon,

Hydrogen, Nitrogen, and Oxygen.

Some organic bodies,--oil of turpentine, naphtha, etc., contain only

Carbon and Hydrogen; many others, such as sugar, gum, alcohol, fats,

vegetable acids--Carbon, Hydrogen, and Oxygen. The _Nitrogenous bodies_,

so called, containing Nitrogen in addition to the other elements, are

principally substances derived from animal and vegetable tissues, such as

Albumen, Caseine, Gelatine, etc.; Sulphur and Phosphorus are also present

in many of the Nitrogenous bodies, but only to a small extent.

Organic substances, although simple as regards the _number_ of elements

involved in their formation, are often highly complex in the arrangement

of the atoms; this may be illustrated by the following formulæ:--

Starch C{24}H{20}O{20}

Lignine C{24}H{20}O{20}

Cane Sugar C{24}H{22}O{22}

Grape Sugar C{24}H{28}O{28}

Inorganic bodies, as already shown, unite _in pairs_,--two elements join

to form a binary compound; two binary compounds produce a salt; two salts

associated together form a double salt. With organic bodies however the

arrangement is different,--the elementary atoms are all grouped equally

in one compound atom, which is highly complex in structure, and cannot be

split up into binary products.

Observe also, as characteristic of Organic Chemistry, the apparent

similarity in composition between bodies which differ widely in

properties. As examples take _Lignine_, or cotton fibre, and Starch,--each

of which contains the three elements united as C{24}H{20}O{20}.

_Mode of distinguishing between Organic and Inorganic matter._--A simple

means of doing this is as follows:-- place the suspected substance upon

a piece of Platinum-foil, and heat it to redness with a spirit-lamp: if

it first _blackens_, and then burns completely away, it is probably of

organic origin. This test depends upon the fact, that the constituent

elements of organic bodies are all either themselves volatile, or capable

of forming volatile combinations with Oxygen. Inorganic substances, on the

other hand, are often unaffected by heat, or, if volatile, are dissipated

without previous charring.

The action of heat upon organic matter may further be illustrated by the

combustion of coal or wood in an ordinary furnace;--first, an escape

of Carbon and Hydrogen, united in the form of volatile gaseous matter,

takes place, leaving behind a black cinder, which consists of Carbon and

inorganic matter combined; afterwards this Carbon burns away into Carbonic

Acid, and a grey ash is left which is composed of inorganic salts, and is

indestructible by heat.

CHAPTER II.

VOCABULARY OF PHOTOGRAPHIC CHEMICALS.

ACETIC ACID.

Symbol, C{4}H{3}O{3} + HO. Atomic weight, 60.

Acetic Acid is a product of the _oxidation_ of Alcohol. Spirituous

liquids, when perfectly pure, are not affected by exposure to air; but if

a portion of yeast, or Nitrogenous organic matter of any kind, be added,

it soon acts as a _ferment_, and causes the spirit to unite with oxygen

derived from the atmosphere, and so to become sour from formation of

Acetic Acid, or "vinegar."

Acetic Acid is also produced on a large scale by heating _wood_ in close

vessels: a substance distils over which is Acetic Acid contaminated with

empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much

used in commerce.

The most concentrated Acetic Acid may be obtained by neutralizing common

vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda

so formed; this Acetate of Soda is then distilled with Sulphuric Acid,

which removes the Soda and liberates Acetic Acid: the Acetic Acid being

volatile, distils over, and may be condensed.

_Properties of Acetic Acid._--The strongest acid contains only a single

atom of water; it is sold under the name of "Glacial Acetic Acid," so

called from its property of solidifying at a moderately low temperature.

At about 50° the crystals melt, and form a limpid liquid of pungent odour

and a density nearly corresponding to that of water; the specific gravity

of Acetic Acid however is no test of its real strength, which can only be

estimated by analysis.

The commercial _Glacial_ Acetic Acid is often diluted with water, which

may be suspected if it does not solidify during the cold winter months.

Sulphurous and Hydrochloric Acids are also common impurities. They are

injurious in Photographic Processes, from their property of precipitating

Nitrate of Silver. To detect them proceed as follows:--dissolve a small

crystal of Nitrate of Silver in a few drops of water, and add to it about

half a drachm of the Glacial Acid; the mixture should remain quite clear

even when exposed to the light. Hydrochloric and Sulphurous Acid produce

a white deposit of Chloride or Sulphite of Silver; and if _Aldehyde_ or

volatile tarry matter be present in the Acetic Acid, the mixture with

Nitrate of Silver, although clear at first, becomes discoloured by the

action of light.

Glacial Acetic Acid sometimes has a smell of garlic. In this state it

probably contains an organic Sulphur Acid, and is unfit for use.

Many employ a cheaper form of Acetic Acid, sold by druggists as

"Beaufoy's" acid; it should be of the strength of the Acetic Acid fortiss.

of the London Pharmacopœia, containing 30 per cent, real acid. It will be

advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other

impurities, before use.

ACETATE OF SILVER. _See_ Silver, Acetate of.

ALBUMEN.

Albumen is an organic principle found both in the animal and vegetable

kingdom. Its properties are best studied in the _white of egg_, which is

a very pure form of Albumen.

Albumen is capable of existing in two states; in one of which it is

soluble, in the other insoluble, in water. The aqueous solution of the

soluble variety gives a slightly alkaline reaction to test-paper; it is

somewhat thick and glutinous, but becomes more fluid on the addition of a

small quantity of an alkali, such as Potash or Ammonia.

Soluble Albumen may be converted into the _insoluble_ form in the

following ways:--

1. _By the application of heat._--A moderately strong solution of

Albumen becomes opalescent and coagulates on being heated to about 150°

Fahrenheit, but a temperature of 212° is required if the liquid is very

dilute. A layer of _dried_ Albumen cannot easily be coagulated by the mere

application of heat.

2. _By addition of strong acids._--Nitric Acid coagulates Albumen

perfectly without the aid of heat. Acetic Acid however acts differently,

appearing to enter into combination with the Albumen, and forming a

compound soluble in warm water acidified by Acetic Acid.

3. _By the action of metallic salts._--Many of the salts of the metals

coagulate Albumen completely. Nitrate of Silver does so; also the

Bichloride of Mercury. Ammoniacal Oxide of Silver however does not

coagulate Albumen.

The white precipitate formed on mixing Albumen with Nitrate of Silver

is a chemical compound of the animal matter with Protoxide of Silver.

This substance, which has been termed Albuminate of Silver, is soluble

in Ammonia and Hyposulphite of Soda; but after exposure to light, or

heating in a current of Hydrogen gas, it assumes a brick-red colour, being

probably reduced to the condition of an organic compound of a _Suboxide_

of Silver. It is then almost insoluble in Ammonia, but enough dissolves

to tinge the liquid wine-red. The _red coloration_ of solution of Nitrate

of Silver employed in sensitizing the Albuminized photographic paper is

probably produced by the same compound, although, often referred to the

presence of Sulphuret of Silver.

Albumen also combines with Lime and Baryta. When Chloride of Barium is

used with Albumen, a white precipitate of this kind usually forms.

_Chemical composition of Albumen._--Albumen belongs to the _Nitrogenous_

class of organic substances (see page 325). It also contains small

quantities of Sulphur and Phosphorus.

ALCOHOL.

Symbol, C{4}H{6}O{2}. Atomic weight, 46.

Alcohol is obtained by the careful distillation of any spirituous or

fermented liquor. If wine or beer be placed in a retort, and heat applied,

the Alcohol, being more volatile than water, rises first, and is condensed

in an appropriate receiver; a portion of the vapour of water however

passes over with the Alcohol, and dilutes it to a certain extent, forming

what is termed "Spirits of Wine." Much of this water may be removed by

redistillation from Carbonate of Potash, in the manner described at

page 196 of this work; but in order to render the Alcohol thoroughly

_anhydrous_, it is necessary to employ _Quicklime_, which possesses a

still greater attraction for water. An equal weight of this powdered lime

is mixed with strong Alcohol of ·823, and the two are distilled together.

_Properties of Alcohol._--Pure anhydrous Alcohol is a limpid liquid, of an

agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour

of water, and becomes diluted by exposure to damp air; boils at 173° Fahr.

It has never been frozen.

Alcohol distilled from Carbonate of Potash has a specific gravity of ·815

to ·823, and contains 90 to 93 per cent, of real spirit.

The specific gravity of ordinary rectified Spirits of Wine is usually

about ·840, and it contains 80 to 83 per cent, of absolute Alcohol.

AMMONIA.

Symbol, NH{3} or NH{4}O. Atomic weight, 17.

The liquid known by this name is an aqueous solution of the volatile gas

Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three

of Hydrogen: these two elementary bodies exhibit no affinity for each

other, but they can be made to unite under certain circumstances, and the

result is Ammonia.

_Properties of Ammonia._--Ammoniacal gas is soluble in water to a large

extent; the solution possessing those properties which are termed alkaline

(see page 308). Ammonia however differs from the other alkalies in one

important particular--it is volatile: hence the original colour of

turmeric-paper affected by Ammonia is restored on the application of heat.

Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is

converted into Carbonate of Ammonia; it should therefore be preserved in

stoppered bottles. Besides Carbonate, commercial Ammonia often contains

Chloride of Ammonium, recognized by the white precipitate given by Nitrate

of Silver after acidifying with pure Nitric Acid.

The strength of commercial Ammonia varies greatly; that sold for

pharmaceutical purposes under the name of Liquor Ammoniæ, contains about

10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes

with the proportion of Ammonia present, the Liquor Ammoniæ being usually

about ·936.

Ammonia, although forming a large class of salts, appears at first sight

to contrast strongly in composition with the alkalies proper, such as

Potash and Soda. Mineral bases generally are _protoxides of metals_, as

already shown at page 308, but Ammonia consists simply of Nitrogen and

Hydrogen united without Oxygen. The following remarks may perhaps tend

somewhat to elucidate the difficulty:--

_Theory of Ammonium._--This theory supposes the existence of a substance

possessing the properties of a _metal_, but differing from metallic

bodies generally in being _compound_ in structure: the formula assigned

to it is NH{4}, one atom of Nitrogen united with four of Hydrogen. This

hypothetical metal is termed "Ammonium;" and Ammonia, associated with an

atom of water, may be viewed as its _Oxide_, for NH{3} + HO plainly equals

NH{4}O. Thus, as Potash is the Oxide of _Potassium_, so Ammonia is the

Oxide of _Ammonium_.

The composition of the _salts_ of Ammonia is on this view assimilated to

those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of

the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride

of Ammonium, etc.

AMMONIO-NITRATE OF SILVER. _See_ Silver, Ammonio-Nitrate of.

AQUA-REGIA. _See_ Nitro-Hydrochloric Acid.

BARYTA, NITRATE OF. _See_ Nitrate of Baryta.

BICHLORIDE OF MERCURY. _See_ Mercury, Bichloride of.

BROMINE.

Symbol, Br. Atomic weight, 78.

This elementary substance is obtained from the uncrystallizable residuum

of sea-water, termed _bittern_. It exists in the water in very minute

proportion, combined with Magnesium in the form of a soluble Bromide of

Magnesium.

_Properties._--Bromine is a deep reddish-brown liquid of a disagreeable

odour, and fuming strongly at common temperatures; sparingly soluble

in water (1 part in 23, Löwig), but more abundantly so in Alcohol, and

especially in Ether. It is very heavy, having a specific gravity of 3·0.

Bromine is closely analogous to Chlorine and Iodine in its chemical

properties. It stands on the list intermediately between the two; its

affinities being stronger than those of Iodine, but weaker than Chlorine

(see Chlorine).

It forms a large class of salts, of which the Bromides of Potassium,

Cadmium, and Silver are the most familiar to Photographers.

BROMIDE OF POTASSIUM.

Symbol, KBr. Atomic weight, 118.

Bromide of Potassium is prepared by adding Bromine to Caustic Potash,

and heating the product, which is a mixture of Bromide of Potassium and

Bromate of Potash, to redness, in order to drive off the Oxygen from the

latter salt. It crystallizes in anhydrous cubes, like the Chloride and

Iodide of Potassium; it is easily soluble in water, but more sparingly so

in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric

Acid.

BROMIDE OF SILVER. _See_ Silver, Bromide of.

CARBONATE OF SODA.

Symbol, NaO CO{2} + 10 Aq.

This salt was formerly obtained from the ashes of seaweeds, but is

now more economically manufactured on a large scale from common salt.

The Chloride of Sodium is first converted into Sulphate of Soda, and

afterwards the Sulphate into Carbonate of Soda.

_Properties._--The perfect crystals contain ten atoms of water, which

are driven off by the application of heat, leaving a white powder--the

anhydrous Carbonate. _Common Washing Soda_ is a neutral Carbonate,

contaminated to a certain extent with Chloride of Sodium and Sulphate of

Soda. The Carbonate used for effervescing draughts is either a Bicarbonate

with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent,

of real alkali; it is therefore nearly double as strong as the washing

Carbonate, which contains about 22 per cent, of Soda. Carbonate of Soda is

soluble in twice its weight of water at 60°, the solution being strongly

alkaline.

CARBONATE OF POTASH. See Potash, Carbonate of.

CASEINE. _See_ Milk.

CHARCOAL, ANIMAL.

Animal Charcoal is obtained by heating animal substances, such as bones,

dried blood, horns, etc., to redness, in close vessels, until all

volatile empyreumatic matters have been driven off, and a residue of

Carbon remains. When prepared from bones it contains a large quantity of

inorganic matter in the shape of Carbonate and Phosphate of Lime, the

former of which produces _alkalinity_ in reacting upon Nitrate of Silver

(see p. 89). Animal Charcoal is freed from these earthy salts by repeated

digestion in Hydrochloric Acid; but unless very carefully washed it is apt

to retain an acid reaction, and so to liberate free Nitric Acid when added

to solution of Nitrate of Silver.

_Properties._--Animal Charcoal, when pure, consists, solely of Carbon, and

burns away in the air without leaving any residue: it is remarkable for

its property of decolorizing solutions; the organic colouring substance

being separated, but not actually _destroyed_, as it is by _Chlorine_

employed as a bleaching agent. This power of absorbing colouring matter is

not possessed in an equal degree by all varieties of Charcoal, but is in

great measure peculiar to those derived from the animal kingdom.

CHINA CLAY, OR KAOLIN.

This is prepared, by careful levigation, from mouldering granite and

other disintegrated felspathic rocks. It consists of the _Silicate of

Alumina_,--that is, of Silicic Acid or _Flint_, which is an Oxide of

Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is

perfectly insoluble in water and acids, and produces no decomposition

in solution of Nitrate of Silver. It is employed by Photographers to

decolorize solutions of Nitrate of Silver which have become brown from the

action of Albumen or other organic matters.

Commercial Kaolin may contain chalk, in which state it produces

alkalinity in solution of Nitrate of Silver. The impurity, detected by

its effervescence with acids, is removed by washing the Kaolin in diluted

vinegar and subsequently in water.

CHLORINE.

Symbol, Cl. Atomic weight, 36.

Chlorine is a chemical element found abundantly in nature, combined with

metallic Sodium in the form of Chloride of Sodium, or Sea-salt.

_Preparation._--By distilling common Salt with Sulphuric Acid, Sulphate of

Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine

combined with Hydrogen; by the action of nascent Oxygen (see Oxygen), the

Hydrogen may be removed in the form of water, and the Chlorine left alone.

_Properties._--Chlorine is a greenish-yellow gas, of a pungent and

suffocating odour; soluble to a considerable extent in water, the solution

possessing the odour and colour of the gas. It is nearly 2-1/2 times as

heavy as a corresponding bulk of atmospheric air.

_Chemical properties._--Chlorine belongs to a small natural group of

elements which contains also Bromine, Iodine, and Fluorine. They are

characterized by having a strong affinity for Hydrogen, and also for

the metals; but are comparatively indifferent to Oxygen. Many metallic

substances actually undergo _combustion_ when projected into an atmosphere

of Chlorine, the union between the two taking place with extreme violence.

The characteristic bleaching properties of Chlorine gas are explained in

the same manner:--Hydrogen is removed from the organic substance, and in

that way the structure is broken up and the colour destroyed.

Chlorine is more powerful in its affinities than either Bromine or

Iodine. The salts formed by these three elements are closely analogous

in composition and often in properties. Those of the Alkalies, Alkaline

Earths, and many of the Metals, are soluble in water; but the Silver salts

are insoluble; the Lead salts sparingly so.

The combinations of Chlorine, Bromine, Iodine, and Fluorine, with

Hydrogen, are acids, and neutralize Alkalies in the usual, manner, with

formation of Alkaline Chloride and water (see page 311).

The test by which the presence of Chlorine is detected, either free or in

combination with bases, is _Nitrate of Silver_; it gives a white curdy

precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble

in Ammonia. The solution of Nitrate of Silver employed as the test must

not contain Iodide of Silver, as this compound is precipitated by dilution.

CHLORIDE OF AMMONIUM.

Symbol, NH{4}Cl. Atomic weight, 54.

This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in

commerce in the form of colourless and translucent masses, which are

procured by _sublimation_, the dry salt being volatile when strongly

heated. It dissolves in an equal weight of boiling, or in three parts of

cold water. It contains more Chlorine in proportion to the weight used

than Chloride of Sodium, the atomic weights of the two being as 54 to 60.

CHLORIDE OF BARIUM.

Symbol, BaCl + 2 HO. Atomic weight, 123.

Barium is a metallic element very closely allied to Calcium, the

elementary basis of Lime. The Chloride of Barium is commonly employed as

a test for Sulphuric Acid, with which it forms an insoluble precipitate

of Sulphate of Baryta. It is also said to affect the colour of the

Photographic image when used in preparing Positive paper, which may

possibly be due to a chemical combination of Baryta with Albumen; but

it must be remembered that this Chloride, from its high atomic weight,

contains less Chlorine than the alkaline Chlorides (see page 124).

_Properties of Chloride of Barium._--Chloride of Barium occurs in the

form of white crystals, soluble in about two parts of water, at common

temperature. These crystals contain two atoms of water of crystallization,

which are expelled at 212°, leaving the anhydrous Chloride.

CHLORIDE OF GOLD. See Gold, Chloride of.

CHLORIDE OF SODIUM.

Symbol, NaCl. Atomic weight, 60.

Common Salt exists abundantly in nature, both in the form of solid

rock-salt and dissolved in the waters of the ocean.

Properties of the pure Salt.--Fusible without decomposition at low

redness, but sublimes at higher temperatures; the melted salt concretes

into a hard white mass on cooling. Nearly insoluble in absolute alcohol,

but dissolves in minute quantity in rectified spirit. Soluble in three

parts of water, both hot and cold. Crystallizes in cubes, which are

anhydrous.

_Impurities of Common Salt._--Table Salt often contains large quantities

of the Chlorides of Magnesium and Calcium, which, being deliquescent,

produce a dampness by absorption of atmospheric moisture: Sulphate of

Soda is also commonly present. The salt may be purified by repeated

re-crystallization, but it is more simple to prepare the pure compound

_directly_, by neutralizing Hydrochloric Acid with Carbonate of Soda.

CHLORIDE OF SILVER. _See_ Silver, Chloride of.

CITRIC ACID.

This acid is found abundantly in lemon-juice and in lime-juice. It occurs

in commerce in the form of large crystals, which are soluble in less than

their own weight of water at 60°.

Commercial Citric Acid is sometimes mixed with Tartaric Acid. The

adulteration may be discovered by making a concentrated solution of the

acid and adding _Acetate of Potash_; crystals of Bitartrate of Potash will

separate if Tartaric Acid be present.

Citric Acid is tribasic. It forms with Silver a white insoluble salt,

containing 3 atoms of Oxide of Silver to 1 atom of Citric Acid. When the

Citrate of Silver is heated in a current of Hydrogen gas, a part of the

acid is liberated and the salt is reduced to a Citrate of Suboxide of

Silver; which is of a red colour. The action of white light in reddening

Citrate of Silver is shown by the Author to be of a similar nature.

CYANIDE OF POTASSIUM.

Symbol, KC{2}N, or KCy. Atomic weight, 66.