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Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XIV.

As already remarked in a preceding chapter, the actinic impression of an object on the prepared collodion film is invisible or latent; it is like the impression of the finger on a plate of copper, or of a warm piece of metal on a glass mirror; after the removal of the finger, or of the metal, the eye can not distinguish the spot where the impression was made; but, as Moeser first illustrated, breathing upon the glass will make the impression manifest, will show that the image was there in a latent or invisible condition. In like manner a plate of polished silver may be substituted for the glass mirror, and excised metallic figures be placed when warm on its surface; the impression is quite invisible, but becomes visible when the silver plate is exposed to the vapor of mercury.

Furthermore, if the glass mirror, or the polished metallic plate be exposed in the camera before an object, and the former be breathed upon, and the latter exposed to the vapor of mercury, in either case the picture becomes visible; but the picture in either case is a mere breath, an evanescent shadow. It gives us, however, a distinct idea of what is meant by a developer, it is the prototype of a reducing agent. In chemistry is understood by a reducing agent, a substance, which, when applied to a combination, properly speaking of a metal, will decompose the compound in such a way as to leave the metal in the reguline condition, isolated from the other combining materials. Hydrogen and carbon are the best chemical reducing agents. Pass a current of hydrogen through a glass tube containing oxide of copper heated to redness; in this state the hydrogen has more affinity for the oxygen of the oxide than the copper possesses; the two metalloids therefore pair and pass off in combination as the vapor of water, leaving the copper reduced to the metallic state. A solution of nitrate of silver, impressed by blocks upon silk, is reduced to a bright film of silver when exposed to hydrogen gas. Heat a mixture of charcoal and oxide of lead in a crucible, carbonic acid results from the combination of charcoal and oxygen, whilst the metal lead is reduced. ]Electricity, Heat and Light are all reducing agents. Fill a tumbler with the solution of chloride of silver in cyanide of potassium, just above mentioned. Next take two copper wires, to the end of one solder a quarter of a dollar, to the other attach on a hook any clean and well-polished article of brass or copper; the other end of the latter wire is now fastened to the negative or zinc side of a galvanic battery, whilst the end of the other copper wire is fixed on the positive or platinum side of the battery. Insert the piece of silver and the brass, etc., object in the tumbler, but not in contact; the silver in the solution will immediately begin to be reduced, and by the electrical current, will be carried to the negative side and deposited on the object to be plated.

By heat alone several of the oxides are reduced to the metallic state, as for instance, oxide of mercury, of silver, etc. Some are reduced by light, as those of gold.

Many of the salts of the metals are reduced by the superior affinity of other metals. Immerse a piece of copper wire in a solution of nitrate of mercury; nitrate of copper will be formed and mercury precipitated on the copper. Mercury precipitates silver from nitrate of silver; zinc precipitates lead from the acetate of this metal, and iron precipitates copper from its nitrate.

Potassium and sodium by their very superior attraction for oxygen are regarded as among the best reducing agents; cyanide of potassium unites the properties of carbon and potassium in the way of reduction. The protosalts of iron are easily changed into the persalts when brought into contact with oxides in which the oxygen has been loosened in its affinities, or when in contact with chlorine or nitric acid; and the metallic base is precipitated. Tannic acid, gallic acid, pyrogallic acid and formic acid are all excellent reducers. The last substances enumerated are those in general use as reducers or developers in photography; but the substance reduced or precipitated by them is not always a pure metal; in some instances it appears pure and metallic, in others black and free from metallic lustre, as if it were mixed with organic material. The act of reduction in photography consists in reducing a silver compound; this reduction is, aided by the presence of nitric acid or a nitrate; without nitric acid or a nitrate the development in question seems impossible, and it is equally impossible without the previous action of light. Now let us see what the action of the protosulphate of iron is upon the oxide of silver in solution, as also of nitric arid upon the protosulphate of iron. In the first place dissolve a crystal of green vitriol in a drop or two of nitric acid: decomposition ensues; the nitric acid is broken up into parts, fumes of the peroxide of nitrogen are liberated, and a reddish colored persulphate of iron is produced from the absorption of oxygen. Secondly, dissolve a small quantity of the oxide of silver in nitrate of ammonia, and add solution of the protosulphate of iron to the ammonio-nitrate. The mixed solution becomes colored and turbid, and a deposit subsides, which is found to be pure silver.

By experience we know that the film on a collodion plate, after development with protosulphate of iron, is also pure silver, soluble in nitric acid. Now coupling the two facts together that both light and nitric acid are required before: the reduction can take place, and also that there must be present the oxide of silver in solution, (for the reduction is ineffectual with the iodide of silver,) it seems as if we were indicated to believe that the action, of light produced an oxide in all those parts where. it struck, or loosened the oxide of the nitrate of silver present on the film, wherever the actinic rays made an impression. This loosening of the oxide of silver from its connection with the acid may be effectuated by the conjoint action of light and iodine or bromine, whereby a double decomposition is instituted the very reverse of that which ordinarily takes place, that is, iodide of silver and nitrate of potassa are reconverted by light into iodide of potassium and nitrate of the oxide of silver in the act of formation, or properly speaking, into nitric acid and oxide of silver, held in abeyance by some power (light or electricity) which prevents their union. If this were so, it seems to me, we have an assemblage of materials in the right condition for producing the effects which in reality take place. With such circumstances and conditions it is easy to see how a solution of protosulphate of iron would reduce the oxide of silver into a film of pure silver, whose thickness would vary as the intensity of the actino-chemical action. There is no absurdity in supposing the possibility of the inversion alluded to. The vapor of water, by passing through an iron or porcelain tube heated to a white heat, is decomposed into its elements; whereas if the heat of flame be applied to a mixture of these gases, they recombine instantaneously anal reproduce the vapor of water. Other analogous inversions of chemical affinity are known to the chemist.

Iron Developer.

Iron.--Symbol, Fe. Combining Proportion., 28. Spec. Grav., 7.8.
Protoxide of iron.--Symbol, FeO. Combining Proportion 36.
Sesquioxide of iron.--Symbol, Fe2 O3. Combining Proportion 80.

With iron, as with some metals, we have two classes or salts, the protosalts and the persalts that is, the salts of the protoxide and the salts of the peroxide. The two classes are not equally permanent, sometimes the protosalts being the stable salts, and sometimes the other. Those salts which are not stable are liable to part with their oxygen, or to take up more oxygen, according to their condition of stability. Thus it happens with the iron compounds. The protonitrate, for instance, is clanged by boiling into a salt of the sesquioxide; and the proto-sulphate is apt to undergo decomposition and assume a coppery appearance, by changing into the persalt. This property in salts and acids of communicating to, or of abstracting oxygen from other chemical substances in contact with them is made available in various reactions; as, for instance, in toxicological investigations, arsenic acid is reduced by sulphurous acid into arsenious acid; on this account sulphurous acid is properly called a reducing agent. In photography, as already remarked, the sulphate of the protoxide of iron passes easily into the sesquisalt, by abstracting oxygen from somewhere, whereby a picture on the collodion filth becomes visible.

Nitrate of the Protoxide of Iron.

Symbol, FeO, N O5.

This substance is obtained best by decomposing the sulphate by means of nitrate of baryta. The solution has a green color, like all the protosalts; it can not easily be crystallized, because a high temperature decomposes it into a sesquisalt.

Sulphate of the Protoxide of Iron.

Symbol, Fe O, S O3, H O + 6 Aq. Combining Proportion, 139.

Sulphate of iron is obtained by dissolving iron to saturation in a dilute solution of sulphuric acid, decanting the supernatant liquid, evaporating and setting aside for crystallization. These crystals have a slightly bluish-green color. When exposed to the air the crystals become colored of a brick-red color, by decomposition; and if the crystals be exposed to a temperature of 212° Fahr., or a little upwards, they part with the six equivalents of the water of crystallization, and crumble into a grayish-white powder; at a higher temperature the remaining equivalent of water may be expelled. It is from the anhydrous salt now left that anhydrous sulphuric acid is obtained, or at least the very strong and fuming sulphuric acid of Nordhausen. In the preparation of this acid from the residual salt above mentioned, a high temperature is required, by which the affinity of the acid for the base is destroyed, and is expelled, leaving in the retort a pulverulent red mass, the colcothar of the alchemists, or sesquioxide of iron. Sulphate of iron is soluble in two parts of cold water and three fourths of a part of boiling water; the solution is neutral. This salt is not soluble in alcohol; if alcohol be added to a solution of sulphate of iron, the salt is precipitated in a white granular form, which is very convenient for photographic purposes; by this process it is purified from any superfluous acid which it may contain.

Double Sulphate of Iron and Ammonia.

It has been proposed by Meynier to substitute this double salt for the protosulphate of iron, because of its permanency when exposed to the air, or its less liability to decomposition. This double salt was described by Mitscherlich.


Take equivalent proportions of sulphate of iron and sulphate of ammonia, that is, 139 parts of the former to 75 of the latter, and dissolve the salts in four or five parts of water; when the solution is complete, filter and evaporate, and afterward set aside to crystallize. The solution for photographic purposes can be prepared in quantity, and it keeps well without undergoing much change. The formula for development with this double salt does not differ from the simple protosulphate; it contains alcohol, water, and acetic acid.

Sulphide of Iron.

Symbol, Fe S. Combining Proportion, 44.

This substance is not used directly in any photographic operation; but for the chemist and experimental photographer it has great value, because it assists in the formation of hydrosulphuric acid, which is by far the most valuable reagent in chemistry.


Heat a bar of iron in a blacksmith's forge to a welding heat, and then rub it on a stick of sulphur; combination will take place very, vividly, and the new compound will drop off like melted wax. When cool it has a dark gray, color and metallic appearance. Pulverized and thrown into dilute sulphuric acid, it gives rise to hydro-sulphuric acid, which may be collected or used immediately by passing it through a given fluid, as for instance, an old hyposulphite bath, in order to reduce the silver in the form of the sulphide of silver.

Tannic Acid-Gallic Acid-Pyrogallic Acid.

The first substance exists in the vegetable kingdom, and is obtained from the astringent materials in various plants, but especially from oak bark and nutgalls, which are excrescences on the leaves of an oak (quercus hafectoria) produced by an insect. The second does not exist naturally, or at least in very minute quantity, but is rather a production arising from tannic acid when exposed to moisture and the atmosphere; and the third is obtained from the second by sublimation at a given temperature. The peculiar property of the astringent principle in various barks, is to occasion a precipitate in solutions of gelatine, and in several metallic salts. It produces in solutions of the persalts of iron a dark blue or dingy green color, according to the bark from which it is extracted. From the property of acting upon gelatine, by which skins are converted into leather, it is denominated tannin; and from its power of combining with metallic bases, and forming precipitates, etc., it is regarded as an acid, and termed tannic acid.

The tannin extracted from the wood, the bark, the leaves and the galls of oak, the twigs of the black currant and of the sumac, the petals of the pomegranate, etc., and from the roots of several plants, produces in solutions of the sesquisalts of iron, a deep blue color, the foundation of writing-ink.

Whereas the tannin from horse-chestnuts, the different varieties of tea, from catechu and kino, cinchona bark, cinnamon, cassia, etc., yields a green precipitate with solutions of the persalts of iron.

Tannic Acid.-- Symbol, C54H22O34.
Gallic Acid.-- Symbol, C14H6O10
Pyrogallic Acid.-- Symbol, C6H3O3.

Preparation of Tannic Acid.

Tannic acid is prepared by a process suggested by Pelouze. Take an elongated glass funnel, terminating at the upper orifice like a bottle, which can be closed by a cork. The lower orifice is loosely closed by a plug of cotton-wool, or a piece of sponge; the body of the funnel is then half filled with powdered nutgalls, over which is poured a quantity of commercial ether, so as to fill the remaining part of the funnel. The cork is then replaced loosely, admitting a little air as the filtration proceeds. The liquid that passes through the funnel, and accumulates beneath, forms two layers; the upper one light and very fluid, and the lower heavier and of a yellowish tinge. Ether is added above the galls, from time to time, until the lower stratum of the filtrate no longer increases in depth. The funnel is then removed from the vessel beneath, and the lower stratum is separated by means of a glass syringe inserted to the bottom; or the whole contents can be placed in a funnel, of which the lower aperture is closed by the finger. In this way the dense fluid is allowed to flow off, and when the whole has been thus removed, the aperture is again closed with the finger, and the light fluid is poured into a retort, and distilled at a gentle heat. It consists principally of ether. The dense fluid is then washed with concentrated ether, from which it is separated as before, and afterward evaporated at a low temperature to dryness. The resulting substance is light and spongy, of an ochreous color. It is pure tannin or tannic acid, in quantity about thirty-five per cent of the galls employed. It has a slightly acid reaction, is very astringent, not bitter. It is soluble in water and alcohol, but sparingly soluble in ether. With mineral acids, albumen, gelatine, salts of the alkaloids, mineral bases, it forms precipitates. Salts of the protoxide of iron are not changed by tannic acid; but those of the sesquioxide give a deep bluish-black precipitate.

Tannic acid is used extensively in photography in the preparation of the dry plates by the Tannin Process of Major Russell. This process is fully described in a subsequent chapter.

Preparation of Gallic Acid.

As before observed, gallic acid exists in minute quantity in nutgalls; but it is rather a product of the decomposition of tannin, than a naturally existing substance. Mix powdered nutgalls into a thin paste, and expose it to the air for two or three months, taking care to replace the water as it evaporates. The mass becomes mouldy, and darker in color by this exposure; it is then pressed in a cloth; afterward, the residue is boiled in water and filtered whilst hot. On cooling, crystals of gallic acid are deposited, which are purified by boiling in eight parts of water and one fifth of their weight of animal charcoal. After filtration and cooling, pure crystals of gallic acid are deposited, in the form of long silky needles. During exposure to the atmosphere, moist tannic acid absorbs oxygen, and liberates carbonic acid, so that gallic acid is altogether a definite and distinct compound. When quite purified, it has no effect upon a solution of gelatine; it has an acid and astringent taste. The solution is soon decomposed. Gallic acid is soluble in one hundred parts of cold water, and in three of boiling water. It has no effect upon the solution of salts of the protoxide of iron, but upon those of the sesquioxide, it produces a deep bluish-black precipitate, which disappears when the liquid is heated, the sesquioxide being converted into the protoxide by the decomposition of the gallic acid. Gallic acid meets with an extensive application in photography, in various processes, as in the Tannin Process of Major Russell, the Dry Process of Taupenot, etc., and in the process of Positive Printing by Development.

Preparation of Pyrogallic Acid.

The etymology of the word indicates the origin of this substance. When gallic acid is heated to the temperature of 410° Fahrenheit, and kept at this temperature, in an oil-bath, a volatile substance sublimes of a beautiful white color, in crystalline plates. This is pyrogallic acid, which is soluble in water, alcohol, and ether. The solution of pyrogallic acid soon turns brown when exposed to the air, by becoming oxidized. It communicates a blackish-blue color to the solutions of the salts of the protoxide of iron, and reduces those of the sesquioxide to the state of the protoxide. When mixed with an alkaline solution, it absorbs a large quantity of oxygen from the atmosphere, and has been used in the analysis of air for this special purpose. When gallic acid is raised to a higher temperature than 410° Fahrenheit, that is, to 480° Fahrenheit, it is decomposed into carbonic acid, water, and a new substance denominated metagallic acid, being the black shining residue left in the retort. Pyrogallic acid, at the proper temperature, is in like manner decomposed into metagallic acid and water.

Owing to the property possessed by pyrogallic acid of absorbing oxygen from bodies with which it is in contact, it is as yet the second best developer of the latent image in the collodion process; and taking into consideration the nature of the image produced, where the time of exposure is not important, it certainly is the most easy and reliable developer. There is no doubt that a solution of protosulphate of iron acts more quickly; or, what is meant, requires a much shorter time of exposure. From the experiments in ordinary landscape photography, I have frequently observed a difference of three to one in the time in favor of the sulphate of the protoxide of iron.

Acids in Developing Solutions.

The solution of protosulphate of iron, or of pyrogallic acid, is frequently much more energetic in reduction than is manageable, and proceeds, after the image has been thoroughly developed, to act upon those parts on which the actinic influence has been but very feeble or almost imperceptible. The difficulty in such a case is two-fold. It consists in flowing the plate uniformly grad instantaneously; otherwise lines of demarkation will be quite visible at those edges where the fluid was momentarily retarded; and secondly, in stopping the progress of development uniformly and instantaneously. Many excellent negatives have been ruined by the misfortunes arising from the difficulties alluded to; and yet Instantaneous Photography has to search in this direction for the surest means of success, rather than upon any fortuitous advantages in the collodion. The operation of light is, practically speaking, instantaneous, because its velocity is greater than conception. A certain time always elapses between the opening and closing of the shutter, before the lenses, in the operation of instantaneity; and in this time light has traveled thousands of miles, or rushed with its thousands of miles' momentum on the sensitized plate. The picture, therefore, is already there; because the impression has been made. It remains, consequently, to find a reducing agent so refined and energetic as to effectuate the proper reduction. With the ordinary quantity of acids in our developers, we can scarcely hope for success; but with their diminution, and a proportionate increase of velocity in the manipulation of flowing the plates, and of stopping the further advance of reduction, instantaneous photography has, in my opinion, to seek a clue for its reliable performance. As a general rule in practice, the photographer requires less acid in the developer according as the time of exposure is less; consequently, the positive on glass, or prepared iron plate, called the ambrotype and the melainotype, requires a much less acid developer than the negative, where the time of exposure is much longer. In like manner, two photographers may be in the habit of operating, the one with short exposures, and the other with long exposures; but it will be found that the developer of the former is much less acid than that of the latter. Now it may be asked: What is the reason that the same developer can not be used for the two kinds of pictures ? Because, in the case of ambrotypes, if the developer be acid as is the case for negatives, the reduction will be very slow, and most likely ineffectual; whilst in the case of a negative, the non-acidified developer would be too rapid and too unmanageable.

The temperature is a very influential item in modifying the operation of development. The higher the temperature the greater the quantity of acid required to preserve the exact equilibrium between fogging on the one hand and deficiency of development on the other.

The principal acids used for this special purpose are acetic acid, tartaric acid, citric acid, and formic acid. The latter may be regarded at the same time a developer from its power of reducing metallic salts, and from its analogy to acetic acid as a check upon development.

Acetic Acid.

Symbol, C, H3 O3 HO. Combining Proportion, 60. Specific Gravity, 1.063.

Acetic acid belongs to a small group of which acetyle is the base or compound radical derivative from ethyle by the oxidation of two equivalents of its hydrogen in the formation of water. When alcohol and ether burn in the air the products of combustion are carbonic acid and water. But sometimes the oxidation of the hydrogen alone takes place, and water only is formed, together with a small series of new bodies containing the wine number of equivalents of carbon. Some of the substances arise from the decomposition of collodion, such as aldehyde, etc. This acid may be formed directly from the oxidation of alcohol or by substituting two equivalents of oxygen in the place of two of hydrogen. Platinum-black acting upon the vapor of alcohol will produce this reaction; or a small quantity of yeast, or almost any other nitrogenized organic material undergoing putrefactive decomposition, added to dilute alcohol and exposed to the air induces the same reaction. In this manner vinegar and alecar arise from the slow acetic fermentation, as it is denominated, of weak wines and beer. When hard dry wood or twigs, or oak, beech, etc., are submitted to destructive distillation at a red heat, acetic acid is one of the products of the distillate. The first part of the sour liquor which distills over by a second operation is not acetic acid; the Second, however, contains the acid, but is impure. It is now saturated with hydrate of lime or carbonate of lime, by which process acetate of lime is formed. Sulphate of soda is then added in solution to the acetate of lime as long as any precipitate of sulphate of lime falls. The resulting acetate of soda is filtered from the lime salt, and evaporated to its crystallizing point and then set aside until crystals are formed. The latter are drained as much as possible from the water and adhering tarry liquor, and then heated cautiously to fusion, by which the tar is decomposed and expelled. The fused mass is again dissolved and crystallized. By decomposing this salt by means of an equivalent of sulphuric acid and by distillation we obtain strong acetic acid, which, by rectification over red oxide of lead, can be concentrated so as to yield crystals at a low temperature. This is denominated glacial acetic acid, and melts into a colorless liquid above 60° Fahr. It boils at a temperature of 240°; its vapor is inflammable. It mixes in all proportions with water, alcohol, and ether. The acetates are very numerous; all of them are soluble; those of silver and mercury the least so.

Its photographic uses are, as above described, to check the vehemence of reduction by the developers; it is used also to acidify the nitrate of silver bath in connection sometimes with acetate of soda, and with this connection it is said to yield much sensitiveness and intensity with a plain iodized collodion.

Formic Acid.

Symbol, C2 HO3 HO. Combining Proportion, 46. Specific Gravity, 1.235.

This acid is so called because it is found in ants, from the Latin of which the word is derived. It bears the same relation in the methyle group as acetic acid does in the ethyle series; acetic acid being formed by the substitution of two equivalents of oxygen for two of hydrogen in the formula for alcohol, whilst formic acid arises from the substitution of two equivalents of oxygen for two of hydrogen in the formula for wood-spirit, a substance very analogous to alcohol. This acid can be obtained by distilling ants in water. It is an organic acid, however, which can be formed artificially by heating organic substances, such as sugar, starch, etc., with oxidizing agents. Thus: mix one part of starch or sugar or tartaric acid with four of the binoxide of manganese, four of water, and four of sulphuric acid. By this mixture carbonic acid will be liberated with effervescence. As soon as this is over the materials are subjected to distillation until four parts and a half have passed over. The acid liquor thus obtained is impure formic acid, which is purified by neutralizing it with carbonate of soda, and evaporating the solution so as to obtain formiate of soda in crystals which may be freed from all impurities in the same manner as acetate of soda in the preceding paragraphs. From the pure formiate of soda, any other formiate, or formic acid, may be obtained by neutralizing the formiate with sulphuric acid and by distillation. Hydrated formic acid is a limpid, colorless fluid, of an intensely pungent odor; it fumes slightly; at a temperature below 32° Fahr. it crystallizes in brilliant plates; it boils at 212°. It produces a blister on the skin when concentrated. In very many respects it is very similar to acetic acid, but may be distinguished from the latter by its comportment with oxide of silver or mercury, in which, when heated, it reduces the metal after a while and liberates carbonic acid. This acid is obtained, and perhaps most easily, by the decomposition of oxalic acid in contact with glycerine and by distillation.

Photographic Uses of Formic Acid.

From the similarity between acetic and formic acid it may easily be inferred that either might be substituted for the other in the developer, but the reader will have remarked a decided difference in their action on silver salts; and it is just on these salts that the acid is brought into action; it is in fact an excellent reducing agent, and when heated is used by several distinguished photographers in their developing solutions, of which the formula will be given in the proper place.

Citric Acid.

Symbol, C12 H5 O11 + 3 130 + 2 Aq.

This acid is obtained from the juice of limes, lemons, orange, currant, quince, cranberry, red whortleberry, and other fruits. The juice is imported in the liquid state from the West-Indies, and being in connection with much mucilage and other organic impurities, it is liable to undergo decomposition on the way, and to yield in the preparation of citric acid other acids endowed with different properties. On this account it is advisable in many instances for the photographer to prepare his own citric acid.


Take ten ounces of expressed lemon juice; boil the juice for a few minutes, then add to it after it is cool the whites of three eggs, and stir the mixture so that the albumen is intimately broken up and mixed with the juice. Boil the mixture again, stirring it all the while, and allow the coagulum to settle. When cool, filter the sour liquor and boil it again, adding to it gradually powdered chalk as long as effervescence is produced; citrate of lime is formed, which is but sparingly soluble in water. The dark-colored mucilaginous liquor is filtered off; the residue is well washed, and afterward decomposed by a quantity of sulphuric acid equal in weight to the chalk employed in the previous decomposition. The sulphuric acid is diluted with about seven times its weight of water; and the mixture is stirred about for some time until the citrate of lime is completely decomposed. By filtration the citric acid is separated from the insoluble sulphate of lime, and is afterward evaporated until a pellicle forms on its surface; it is then set aside to crystallize. The dark-colored crystals are removed from the supernatant liquid by a strainer and again dissolved in pure water; the liquid is again evaporated as before, until the formation of a pellicle takes place, and is again set aside to crystallize. By repeating the operation several times the crystals become quite clean and purified. Citric acid has an agreeably sour taste; like phosphoric acid it is tribasic, and gives rise to three classes of citrates. It is soluble in less than its own weight of cold water, and in half its weight of boiling water; it is not very soluble in alcohol.

Citrate of Soda.

This salt is prepared by dissolving citric acid in pure water and throwing into the solution, by degrees, pulverized carbonate of soda as long as effervescence is produced. The liquid is afterward evaporated to a crystallizing consistency and then set aside. In this case, as well as in the preceding, the mother-liquor can be made to yield new crops of crystals by further evaporation or by a repeated decomposition and a repetition of the other proceedings arising out of it.

Photographic Uses of Citric Acid.

This acid is frequently mixed with pyrogallic acid in proper quantity for solution in water instead of acetic acid. It is used as a check on the too rapid action of pyrogallic acid, and as a reducing agent. A frequent impurity in this substance is malic acid, and sometimes aconitic acid. Citric acid is recognized by its producing in a diluted state no immediate precipitate with Chloride of Calcium; but an immediate precipitate is formed when the solution is boiled.

Tartaric Acid.

Symbol, C8 H4 O10+2 Aq.

This acid exists in combination with potassa in most kinds of fruit, and sometimes in a free state. Its combinations in fruit are cream of tartar and tartrate of lime. The former exists in abundance in grape juice, and is denominated, in the crude state, Argol or Tartar, which is either red or white according to the wine from which it is deposited during fermentation.

Preparation of Tartaric Acid.

This acid is obtained from argol, or from cream of tartar, which is a bitartrate of potassa, by two processes; one consists in abstracting One equivalent of tartaric acid from the bitartrate, and the other in decomposing the residual tartrate in the solution. Following the formula of the London College, and using the imperial gallon, which contains ten pounds of water, the method stands thus: take of bitartrate of potassa four pounds; boiling distilled water, two gallons and a half; prepared chalk, twenty-five ounces and six drachms; diluted sulphuric acid, seven pints and seventeen fluid ounces; hydrochloric acid, twenty-six fluid ounces and a half, or as much as may be sufficient. Boil the bitartrate of potassa with two gallons of the distilled water, and add, by degrees, the half of the chalk; when the effervescence is over, add the remainder of the chalk, previously dissolved in the hydrochloric acid, diluted with four pints of the distilled water. Then set aside until the tartrate subsides; after which pour off the liquor, and wash the tartrate of lime frequently with distilled water as long as it has any taste. Next pour on the diluted sulphuric acid, and boil for a quarter of an hour. Having filtered the liquor from the insoluble sulphate of lime, evaporate it by a gentle heat until a pellicle is formed on its surface; then set it aside to crystallize. By dissolving the crystals in pure water, filtering, and recrystallizing, and by repeating these three operations several times, pure tartaric acid may be obtained.

Tartaric acid is not volatile; when heated it leaves an abundant coaly residue. It is soluble in half its weight of water; it dissolves also in alcohol. The salt itself undergoes no change when exposed to the atmosphere; but its solution, when long exposed, absorbs oxygen and forms acetic and carbonic acid. When boiled over an excess of oxide of silver, the same decomposition is produced, and metallic silver is liberated. When fused with potassa it is decomposed into acetic and oxalic acid; whilst with binoxide of manganese and sulphuric acid, it gives rise to carbonic and formic acid. Concentrated sulphuric acid, when heated with the crystals of tartaric acid, decomposes it and separates carbon, which renders the mixture black; and carbonic oxide is evolved at the same time, which burns with a blue flame.

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