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Carey Lea discovered the preparation of so-called "allotropic" and "intermediate" silver in 1889 while he was studying reductions of silver nitrate. "Allotropic" is however a misnomer, . In 1925, Dr. Richard Zsigmondy, Professor of Chemistry at the University of Göttingen, received the Nobel Prize in Chemistry for his study of Lea's "allotropic" silver under the ultramicrosope. Dr. Zsigmondy found that such silver actually was a monoatomic colloid of ordinary silver, not another isotope.

Lea determined that silver occurs in "allotropic", "intermediate", and ordinary forms. Ordinary silver is protean in nature. The aqueous solutions are colloidal monoatoms, and give perfectly clear solutions. The several forms of "allotropic" silver (a-Ag) dry with their particles in optical contact with each other, thus forming continuous films that are beautifully colored, perfect mirrors. Strong acids and pressure will convert a-Ag to the normal form. There are three forms of a-Ag, and all are unstable.

There is also a very stable "intermediate form" of silver (i-Ag) which is easy to prepare. It occurs as bright gold-yellow or green crystals with a metallic luster. Treatment with a very dilute solution of ferric chloride will enhance the appearance of its foliar structure, interpenetrating with plant-like ramifications, or fine acicular crystals up to 1 inch long.

Intermediate silver is hard, tough, and unaffected by pressure. It is nearly as indifferent to oxidizing and chlorizing agents as is normal silver. Intermediate silver can be formed from the allotropic varieties by light, heat, or chemical action.

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Excerpts from Journals --
   
Amer. J. Science ( 3 rd Series ) 37 ( 222 ) 476 - 491 ( June 1889 )
   
   
"... The form of allotropic silver which I have obtained may be classified as follows : --
   
A. Soluble, deep red in solution, mat lilac, blue, or green whilst moist, brilliant bluish metallic when dry.
   
B. Insoluble, derived from A, dark reddish brown whilst moist, when dry somewhat resembling A.
   
C. Gold, silver, dark broze whilst wet, when dry exactly resembling gold in burnished lumps. Of this form there is a variety which is copper-colord. Insoluble in water, appears to have no corresponding soluble form.
   
All these forms have several remarkable properties in common :
   
I. That of drying with their particles in optical contact and consequently, forming a continuous film...
   
II. The halogen reaction ... very beautiful color reactions are obtained...
   
III. The action of acids ... instantly convert the allotropic forms of silver into normal gray silver... absolutely without the separation of gas...
   
IV. Physical Condition ... All these allotropic forms of silver are easily reduced to an impalpable powder...
   
Preparation ---
   
A. Soluble Allotropic Silver
   
A solution of ferrous citrate added to one of a silver salt produces instantly a red liquid (Ferrous sulfate gives the same reaction but is less advantageous). These red solutions may either exhibit tolerable permanency or may decolorize, letting fall a black precipitate. It is not necessary to prepare the ferrous salt in an isolated form, a mixture of ferrous sulfate and sodium citrate answers perfectly.
   
When however concentrated solutions are used with a large excess of ferrous sulfate and a still larger one of alkaline citrate, the liquid turns almost completely black. It should be stirred very thoroughly for several minutes to makes sure that the whole of the precipitated silver citrate is acted upon by the iron. After standing for 10 or 15 minutes, the liquid may be decanted and will leave a large quantity of a heavy precipitate of fine lilac-blue color. It is best to adhere closely to certain proportions:
   
Of a 10% solution of silver, 200 cc may be placed in a precipitating jar. In another vessel are mixed 200 of a 30% solution of ferrous sulfate and 280 cc of a 40% solution of sodic citrate.( The same quantity of ferrous sulfate or of sodic citrate in a larger quantity of water will occasion much loss of the silver product). I think some advantage is gained by neutralizing the ferrous solution, which has a strong acid reaction, with the solution of sodium hydroxide: as much may be added as will not cause a permanent precipitate. To the quantities already given, about 50 cc of 10% soda solution. The reaction takes place equally well without the soda, but I think the product is a little more stable with it. ) The mixed solution is to be added at once to the silver solution.

The beautiful lilac shade of the precipitate is rather ephemeral. It remains for some time if the precipitate is left under the mother water, but when thrown upon a filter, it is scarcely uncovered before the lilac shade disappears and the precipitate takesa deep blue color, without losing its solubility. It may be washed either on a filter or by decantation, with any saline solution in which it is insoluble and which does not affect it too much. On the whole, ammonium nitrate does best, but sodic nitrate, citrate, or sulfate may be used, or the corresponding ammonia salts. Although in pure water the precipitate instantly dissolves with an intense blood red color, the presence of 5 or 10% of any of these salts renders it perfectly insoluble. I have usually proceeded by adding to the precipitate ( after decanting the mother water as completely as possible...), a moderateamount of water; for the above quantities about 150 cc. Much less would dissolve the precipitate but for the salts present: this much will dissolve the greater part but not the while, which is not necessary. A little of a saturated solution of ammonium nitrate is to be added, just enough to effect complete precipitation.

As the material appears continually to change, the amount of washing needed must depend upon the object in view. If wanted for analysis, the washing must be repeated many times until a ferric salt ceases to come away, but no amount of washing will eliminate it entirely. After 7 or 8 solutions in pure water and as many precipitations, the material is to be filtered and then the ammonium nitrate washed out with 95% alcohol until the filtrate leaves nothing on evaporation. The substance at this point is still soluble in water, though much less so than at first. During the washing the solubility slowly but steadily diminishes, a fact rendered noticeable by less and less ammonium nitrate being needed to precipitate it from its solution...

... The freshly precipitated material dissolves to a blood red liquid, by great dilution yellowish red. The purified substance gives a darker red solution, which with dilution remains still red ...

B. Insoluble Form of the Foregoing

The solution of the blue product just described is influenced in a remarkable way by the addition of almost any neutral substance. So far I hve not found any that does not precipitate it. Not only saline solutions do this, but even a solution of gum arabic.

Neutral salts may precipitate the silver in either a soluble or an insoluuble form. Alkaline sulfates, nitrates and citrates throw down the soluble form, magnesium sulfate, cupric sulfate, ferrous sulfate, nickel sulfate, potassium bichromate and ferrocyanide, barium nitrate, even silver nitrate and other salts throw down a perfectly insoluble form. The soluble form cosntitutes a bluish or bluish black precipitate; the insoluble, a purple brown, which by repeated washings continually darkens.

What is very curiosu is that the insoluble form may be made to return to the soluble condition. many substances are capable of effecting this change. Sodium borate does so, produing a brown solution, potassium and sodium sulfate produce a yellowish red solution and ammonium sulfate a red one. None of these solutions has the same blood-red color as the original solution; the form of silver seems to change with the slightest change of ondition.

The solutions used must be extremely dilute, otherwise the silver, though rendered soluble in pure water by them, will not dissolve in the solution itself, a singular complication of effects... The insoluble substance also is readily soluble in ammonia. The solution has a fine red color, and not the yellowish red of the sodium sulfate solution.

Most neutral salts act in one or another ways just described, precipitating the solution of the blue substance A in either the soluble or the insoluble form, the latter soluble in ammonia, but sodium nitrite is an exception; its solution effects an entire change and renders the substance wholly insoluble, probably reconverting it to normal silver...

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Amer. J. Sci. 138 ( 223 ) p. 47 ( 1889 )

... The two insoluble forms of allotropic silver which I have described as B and C; B, bluish green, C rich golden color, show the following curious reaction. A film of B, spread on glass and heated in a water stove to 100 C for a few minutes becomes superficially bright yellow. A similar film of the gold-colored substance C treated in the same way, acquires a blue bloom. In both case it is the surface only that changes.

Sensitiveness to Light -- All these forms of silver are acted upon by light. A and B acquire a brownish tinge after some hours' exposure to sunlight. With C the case is quite different, the color changes from that of red gold to that of pure yellow gold... Complete exhaustion of air and light is certainly favorable to permanence...

Specific gravity -- The allotropic forms of silver show a lower specific gravity that that of normal silver.

In determining the s.g. it was found essential to keep the s.g. bottle after placing the material in it for some hours under vauum. Films of air attach themselves obstiantely to the surfaces and esape but slowly even in vacuo.

... Blue substance B gave sp.gr. 9.58 and the yellow substance C, sp.gr. 8.51. The sp.gr. of normal silver after melting is 10.5. That of finely divided silver is 10.62 ...

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Amer. J. Sci. 42 ( 250 ) Oct., 1891.


Relations of the Yellow to the Blue Forms -- ... In previous papers there has been described a crystalline state intermediate between these active forms and ordinary silver, which intermediate condition, while retaining the bright yellow color of the active form is nearly as indiferrent to reagents as ordianry silver. Into this intermediate state both the yellow and blue forms are capable of passing, and apparently the intermediate states of both kinds of allotropic silver are identical: the intermediate form of blue silver is yellow. Thus when lumps of blue silver are heated in a test tube to about 180 C they assume a gold color and luster. The same changes take place at the same temperature when films of blue silver are placed in a hot air bath...

Blue silver can be converted into yellow at ordinary temperatures and consequently with retention of its active properties. This is accomplished through the delivery of sulfuric acid. When a solution of silver is obtained by the action of sodium hydroxide and dextrine on silver nitrate (*) it appears to contain the blue variety, for if allowed to precipitate spontaneously by long standing, or if precipitated by acetic acid, dilute nitric acid, or by many neutral substances, it gives a form of silver which is dark red while moist and dries with a blue surface color. (It is always a little difficult to characterize these substances by their colors since the surface color which they show when dry is mostly complementary to their color when wet. The surface color is much the more characteristic, I have adopted the course of naming them by that.

(*) 40 gr each of naOH and of yellow or brown dextrine (not white) are dissolved in 2 liters water and 28 gr Ag-Nitrate in solution are added in small quantities in turn, with frequent stirring, so that several hours shall lapse before the last portion is added. The solution is always slightly turbid when viewed by reflected light, by which it shows a beautiful deep green color. By transmitted light it is deep red, and when diluted, absolutely transparent. By diminishing the proportion of silver nitrate to one-half, a solution nearly or quite clear by reflected as well as by transmitted light is obtained.

The behavior of the red solution obtained by soda and dextrine with dilute sulfuric is very interesting and instructive. When 100 cc of solution are poured into 100 cc of water to which 3 cc sulfuric acid has been added, a dark red precipitate falls which, when dry, especially in films, is blue. The mixed liquid from which the precipitate is formed is acid. Increasing the proportion of acid to 4, 5 and 6 cc successivley, the substance obtained has a green surface color becoming more yellowish green in proportion as the acid is increased in quantity. With 7-1/2  the substance no longer dries green but yellow. Increased proportions of acid produces substances drying with a coppery shade.

It will be seen that from a single solution, and using one substnace only as a precipitant, we can obtain the whole range of different forms of allotropic silver, simply by varying the proportions of the precipitant.

That these forms of silver should subsist in the presence of sulfuric acid in excess is remarkable. For the most part the presence of this acid tends to quickly convert allotropic to ordinary silver. For example, bright yellow allotropic silver obtained with ferrous tartrate was washed on a filter with water containing 1/500 its volume of sulfuric acid: in two hours the entire mass was converted into gray ordinary silver.

...The substances precipitated with the least acid, have a very splendid luster, and this luster diminshes steadily as the proportion of acid is increased...

But we can also obtain the converse of this reaction. Just as the solution which naturally would yield the blue product, can be made to yield the yellow by the presence of excess of strong acid, so the solution which normally yields the yellwo substance, may be made to produce blue ( or rather green ) silver by adding alkali. Thus a mixture of dilute solutions of ferrous sulfate and of Rochelle salt added to mixed solutions of silver nitrate and of Rochelle salt rsults in the formation of fold-colored silver. But if we add a little sodium hydroxide to the iron solution or the silver mixture, we shall get a bluish green product, whose properties show that it belongs to the blue class and not to the yellow...

There is a well marked tendency of acids to give rise to the formation of the yellow product and of alkalies to the blue. But this is a tendency only. Both substances can be produced from neutral solutions, and slight changes are sufficient to alter the product formed. Thus, ferrous tartrate, in dilute solution acting on silver tartrate gives rise to the formation of the gold-colored substance, but when citrates are substituted, the blue substance is obtained.

Action of Light on Blue Silver -- This action differs with different varieties; it was more especially exmained with the form that is obtained from the  soda dextrine silver solution already described by pouring the solution into an equal bulk of water to which sulfuric acid had been added in the proportion of 4 cc per 100 cc water. This form was selected because it is easy to obtain with great constancy of result, and because it is one of the forms of blue silver most sensitive to light.

Exposed to light, this substance first becomes more distinctly blue, losing a slight greensih shade. With continued exposure it passes to a yellow-brown shade. and finally to a perfectly pure golden-yellow of great brilliancy and luster. The last is the intermediate or crystalline form.

The action of light on this form of silver is remarkable in this respect, that its first effect is to increase the sensitiveness to reagents.

This result was so unexpected and a priori so improbable, that it was subjected to the most careful verification before being accepted... Upon this form of silver light has a reversing action, first exalting its sensitiveness, then completely destroying it... [ like silver bromide ]

Three of the principal modes of formation of allotropic silver are:

(1) reduction of silver citrate or tartrate by ferrous nitrate or tartrate;
(2) acting on silver nitrae or oxide by dextrine and fixed alkaline hydroxide;
(3) acting on silver nitrate or carbonate by tannin and fixed alkali carbonates.

Now if in either of these cases we interrupt the action before it is complete by adding an excess of dilute hydrochloric acid we shall obtain a dark chestnut-brown or sometimes purple-brown substance which on examination proves to be a mixture of silver subchloride and protochloride. When, after complete removal of the excess hydrochloric by boiling with distilled water, the substance is treated with cold dilute nitric acid, that portion of the subchloride which is not combined with the normal chloride is broken up and there remains protochloride of a very rich and intense rose-color. It is perhaps the best means for obtaining silver protochloride... Hydrochloric acid, though without action on ordinary silver, is capable of forming a variable quantity of protochloride when placed in contact with allotropic silver.

I have not met with any exception to this general principle that when a reaction leading to the formation of allotropic silver is interrupted by the addition of hydrochloric acid, subchloride is abundantly formed as one of the products.

In all such cases the reduction is evidently indirect. The silver does not lose at once the whole of its oxygen, but appearently passes through an intermediate form, prodbably Ag4O, the reduction of which tends to the formation of allotropic silver.

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Amer.J. Science ( July 1889 )


The three forms of allotropic silver which were described in the June number of this Journal -- the blue soluble and the blue and the yellow insoluble -- are not to be understood as the only forms which exist, but as the best marked only. The substance is protean and exhibits other modification not yet studied. No other metal than silver appears to be capable of assuming such a remarkable variety of appearances. Every color is represented. I have obtained metallic silver blue, green ( many shades of both ), red, yellow and purple, In enumerating these colors I do not refer to interference colors produced superficially by reagents, also wonderfully brilliant, but to body colors....

Two of the insoluble forms of allotropic silver, the gold-colored and the blue, show in many respects a close relationship and almost identical reactions... Blue allotropic silver (dark red when moist, becoming blue in drying ) is very stable. It may be exposed for weeks in a moist state on a filter, or be placed in a pasty condition in a corked vial and so kept moist for months, without alteration.
 
The gold-colored form on the contrary tends constantly to revert to ordinary silver. This is especially the case when it is moist, so that from the time of its formation, it must be separated from its mother water and washed as rapidly as possible, otherwise it loses its brilliancy and purity of color and changes to a dark dull gray form of normal silver. On the filter, its proper color is black with a sort of yellow shimmer ( the gold color appearing as it dries ) often, especially if allowed to become uncovered by the water during washing, it will change superficially to gray (When well washed this form can be preserved for a time in the moist condition in a corked vial). But if the washing is done rapidly with the aid of a vacuum filter, the allotropic silver obtained, when allowed to dry in lumps, or brushed on paper or glass, is at least equal to pure gold in color and brilliancy...

When gold-colored allotropic silver is gently heated in a test tube it undergoes a remarkable change in cohesion. Before heating it is brittle and easily reduced to fine powder. After heating it has greatly increased in toughness and cannot be pulverized at all.

Both the gold-yellow and the blue forms resemble normal silver in disengaging oxygen from hydrogen peroxide.

Many substances which react little if at all with ordinary silver, attack the gold-colored and the blue allotropic silver with production of very beautiful colors due to the formation of thin films and resulting interference of two reflected rays. In my previous papers I called this the "halogen reaction" because first obtained by the action of halogens... But many other reagents will produce the same or similar effects. These are:

Sulfides -- Paper brushed over with either the gold, the copper-colored, or the bluish green substance exposed to the vapor of ammonium sulfide, or immersed in a dilute solution of it, assume beautiful hues, though less brilliant than those obtained in other ways.

Potassium permanganate -- in dilute solution produces blue, red and green colors.

Potassium ferricyanide -- in moderately strong solution gradually attacks allotropic silver with production of splendid blue, purple and green coloration.

Phosphorus acid -- produces gradually a rather dull coloration.

The color reaction is produced finely by substances which readily part with a halogen such as ferric and cupric chlorides, sodium hypochlorite, hydrochloric acid to which potassium bichromate has been added, and corresponding bromine and iodine compounds... I obtained effects of the same sort but in much weaker degree with alkaline haloids. But with purer products, the results have been different. There is at first some darkening, but no true color reaction and the allotropic silver appears to be gradually converted into normal, so that it is no longer capable of giving the brilliant color reaction with potassium ferricyanide, but, like normal silver, takes a pale and faint coloration only.

The perchlorides of platinum, gold, and tin do not give the color reaction, though by analogy one would expect that they should, since they can lose chlorine with formation of a lower chloride.

Action of Light -- in a previous paper was mentioned the remarkable fact that the gold- and copper-colored forms of allotropic silver can be converted first into yellow and finally into white normal silver by the continued action of light. Earlier specimens of the blue form became brown by exposure, but purer one since obtained are likewise converted into yellow by exposure, becoming continually lighter as the action is continued. The conversion from the darker shades to a bright yellow with full metallic luster is very easy, but [ unstable ]. Since then I have obtained the gold-colored silver in a more sensitive form, giving a perfectly white product by exposure for half that time.

The white silver thus obtained has all the character of ordinary silver and does not show the color reaction with ferric and cupric chloride, potassium ferrocyanide, etc. Just in proportion to the exposure to light, the ability to give this color reaction diminishes, so that after a day's exposure, when the exposed part has become bright yellow, the color reagents scarcely affect this yellow, whilst the protected part becomes intense blue, purple, or green. In this way it is easy to observe the gradual effect of light as it changes the allotropic silver into ordinary silver.

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Allotropic silver, in its moist and plastic state, may be bruished over paper and gives on drying a continuous and brilliant coating resembling metalic leaf. When a small crystal of iodine is placed on paper that has been this coated, rings of remarkable beauty are obtained. A funnel or beaker should be inverted over the paper to prevent distortion by currents of air [ unless desired -- controlled air flow produces beautiful patterns ]

That iodine is capable of producing interference rings ( Nobili's rings ) on metalic surfaces has long been known, and Robert Hunt has described their formation on surfaces of normal silver... The contrast between the pale and faded-looking products produced on normal silver, and the lustrous and glowing hues given by the allotropic, is very striking. One cannot help wishing that this splendid coloration could be made to do service for obtaining natural colors by photographic processes.

As to the durability of these products... protected from light and air they endure for several months at least. Both the bluish green insoluble silver B, and the gold-colored C produce these effects; the gold-colored is the better suited of the two...

The general properties of this substance can be much better observed in the thin films obtained by brushing the moist substance over paper than in lumps. The films thus obtained are bright metallic green, and this green evidently results from a mixture of blue and yellow... When the films are examined by normal light reflected from them at a large incidence with the normal and a Nicol's prism or an achromatized prism of calc-spar is interposed between the film and the eye, it becomes at once apparent that the blue and yellow light are oppositely polarized. The yellow light is polarized in the plane of incidence, and the blue light perpendicularly to that plane. All specimens show the yellow light, but the quantity of blue light is very variable and is directly connected with the amount of washing applied to the precipitate. The more it is washed the more yellow predominates. To see the blue form in its full beauty, a little of the red solution may be precipitated with a very little magnesium or aluminum sulfate and filtered. As soon as the liquid has drained off and without any washing, the deep bronze-colored substance is to be brushed on paper. On drying it has all the appearance of a bright blue metal with a remarkable luster. The mirrors obtained on glass are so beautiful and so perfect that it seems as if this property might have useful applications...

Crystalization -- On one occasion this substance was obtained in a crystaline form. Some crude red solution had been set aside in a corked vial. Some weeks after, it was noticed that the solution had become decolorized, with a crystaline deposit... [ consisting ] of short black needles and thin prisms. Evidently the saline matters present had balanced the silver so nearly as not to cause an immediate precipitation, but a very gradual one only. The mother water was decanted, and a few drops of pure water added. No sultion took pace: the crystals were therefore of material B, the insoluble form. The contact of pure water instantly destroyed the crystalization and the substance dried with a bright green metalic luster. Contact with pure water evidently tends always to bring this form of silver into the colloidal state, sometimes soluble and sometimes not; whilst the contact with certain neutral salts renders it crystaline...

To obtain the substance in a pure condition suitable for analysis, it is necessary to choose a precipitant not giving an insoluble product with either citric or sulfuric acid. Magnesium sulfate or nickel sulfate answers well. A very dilute solution is made of it and the red solution of A is to be filtered into it. The preipitate soon settles. A large quantity of water is to be poured on, and then washing by decantation can be continued to three decantations, after which the substance remains suspended. It can be made to subside by adding a very small quantity of magnesium sulfate ( 0.25 gr/liter is sufficient). The substance may then be filtered and washed....

C. Gold Yellow and Copper-Colored Silver

It has been long known that golden-yellow specks would occasionally show themselves in silver solutions, but could not be obtained at will and the quantity was infinitesimal. Probably this phenomenon has often led to a supposition that silver might be transmuted into gold. This yellow product, however, is only an allotropic form of silver, but it has all the color and brilliance of gold...

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Amer. J. Science [ 3 ] 51 ( 244 ) p 259-267 ( April 1891 )


... In the present case we have to consider three distinct forms (1) allotropic, (2) intermediate (3) ordinary silver. We notice that (1) can with the utmost facility and in several ways be converted into (2) and (3), and that (2) can always be converted into (3), but that these transformations can by no possibility be reversed. To convert ordinary silver into allotropic we must as a first step dissolve it in an acid: that is, convert it from a polymerized to an atomic form, and only from this atomic form can allotropic be obtained...

[ There may exist ] three possible moleular forms of silver, viz.: atomic, molecular and polymerized.,,

Silver may exist in three forms: 1st. Allotropic silver which is protean in its nature; may be soluble or insoluble in water, may be yellow, red, blue or green, or may have almost any color, but in all its insoluble varieties always exhibits plasticity, than is, if brushed in a pasty state upon a smooth surface its particles dry in contact, and with brilliant metallic luster. It is chemically active. 2nd. The intermediate form, which may be yellow, or green, always shows metallic luster, but is never plastic and is almost as indifferent chemically as white silver. 3d. Ordinary silver... Further, that alotropic silver can always be converted, either into the intermediate form, or directly into ordinary silver; that the intermediate form, or directly into ordinary silver; that the intermediate form, or directly into ordinary silver, that the intermediate form can always be converted into ordinary silver, but that these processes can never be reversed, so that to pass from ordinary silver to allotropic it must first be rendered atomi by combination, and then be brought back to the metallic form under condition which chek the atoms in uniting. Allotropic silver is affected by all forms of energy, and this effect is always in one direction, namely, towards condensation...

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When dextrine is dissolved in a solution of potassium or sodium hydroxide and silver nitrate is added, keeping the hydroxide in moderate excess, the silver is at first thrown down in the form of the well known brown oxide. This brown color presently changes to a reddish chocolate shade and at the same time the silver begins to dissolve. In a few minutes the whole has dissolved to a deep red color, so intense as to be almost black. A few drops poured into water give it a splendid red color of perfect transparency. Examination with the spectroscope leaves no doubt that we have to do with a true solution.

It is interesting to obseve that silver can be held in solution in neutral, acid and alkaline liquids. In the first process which I which I published, in which silver citrate is reduced by a mixture of sodic citrate and ferrous sulfate, the latter may be used either in acid solution or it may be first neutralized with alkaline hydroxide, so that that form of silver is held in solution in either a neutral or an acid liquid. The form that is obtained with the aid of dextrine dissolves most freely in the strongly alkaline liquid in which it is produced, and when dilute nitric or sulfuric acid is added the silver is precipitated. But with acetic the precipitation is very incomplete: the solution retains a brown color and contains silver. Even the addition of a large excess of strong acetic acid fails to throw down any more silver. it follows therefore that while this form of silver is most freely soluble in a strongly alkaline it is also soluble to some extent in one that is either neutral or acid.

The precipitate when once formed appears to be almost insoluble. A small portion of it stirred up with distilled water gives no indication of solution. But if a quantity is thrown is washed on a filter, as soon as the mother water is washed out the liquid runs through of a muddy red, and if this filtrate be allowed to stand it deposits an insoluble portion and then has a fine rose-red color and perfect transparency. Notwithstanding the beautiful color it contains a trace of silver only, so great is the coloring power of the metal. Sometimes if the alkaline stnads for a month or two the silver becomes spontaneously insoluble; most of it falls to the bottom as a deep red substance, but part remains in suspension with a bright brick red color. The difference between this and the true solution as originally formed is extremely well marked.

Dextrine is a very variable substance and different specimens act very differently. Common brown dextrine seems to do better than the purified forms.

Convenient proportions are as follows: in two liters of water 40 grams of sodium hydroxide may be dissolved and an equal quantity of dextrine, filtering if necessary. 28 grams of silver nitrate are to be dissolved in a small quantity of water and added by degrees at intervals. Complete solution readily takes place. Although the liquid contains less than 1% of metallic silver it appears absolutely black, when diluted, red, by great dilution yellowish. With some specimens of dextrine the solution remains clear, with others it soon becomes a little turbid.

Perhaps the most interesting reaction which this solution shows, is that with disodic phosphate. A little phosphate is sufficient to throw down the whole of the silver although both solutions are alkaline. When a gram of phosphate in solution is added to 100 cc of silver solution the color becomes bright red, sometimes scarlet, and the whole of the silver is presently on the filter has precipitated. This precipitate on the filter has a color like that of ruby copper, which color it retains during the first washing, but after a few hours washing with distilled water the color changes to a deep Nile green and at the same time it becomes slightly soluble, giving a port wine colored solution. With more washing this solubility may disappear.

It is a general fact that all these forms of silver, however various their color, have both a body and a surface color and these two colors tend always to be complementary. The body color is that shown by the precipitate while still moist; it is also visible when a thin coat is brushed over paper, a coat so thin that light passes through it, is reflected by the paper and returned again through the film. But when a thick and opaque film is applied, the body color disappears and only the complementary surface is visible...

Allotropic Silver obtained with Tannin and alkaline Carbonates

Tannin (gallotannic acid) in alkaline solution reduces silver nitrate to metallic silver in the allotropic form. Tannin acts more strongly than dextrine and therefore does best with carbonated alkali, dextrine best with alkaline hydroxide, although either substance will produce the reation with either form of alkali and, though less advantageously, with ammonia. Tannin with sodium carbonate gives a very perfect solution of silver, quite free from the turbidity that is apt to characterize the dextrine solution. The color of this solution is likewise very intense: one containing 1% of silver is quite black, by dilution deep yellowish red. It has very much the same characteristics as the preceding, bt is rather more stable. To obtain it, 24 grams of dry sodium carbonate may be dissolved in 1200 cc of water. A 4% solution of tannin is to be made and filtered, of this 72 cc are to be added to the solution just named; of silver nitrate, 24 grams dissolved in a little water are to be added by degrees. Solution takes place almost instantly as each successive portion is added. The solution after standing a day or two may be decanted or filtered from a small quantity of black precipitate.

When the solution is treated with a very dilute acid, as for example, nitric acid diluted with 20 volumes of water, allotropic silver is precipitated in the solid form. It dries with a brilliant metallic surface color of a shade different from the foregoing and somewhat dificult to exactly characterize, a sort of bluish-steel gray.

I do not find that blue allotropic ( in which is included the green and steel-gray varieties) can be reduced to any one definite type. On the contrary, its variations are endless. Slight differences in the conditions under which the solutions are formed or in the mode of precipitation give quite different products. For example, of ten products obtained give quite different products. For example, of ten products obtained with tannin and sodium carbonate in different proportions, several were easily and completely soluble in ammonia, some were slightly soluble and some not at all. Some specimens not at all soluble in water become so by moistening with dilute phosphoric acid: they did not dissolve in the acid but when it was removed they had beceome soluble in water. On other specimens phosphoric acid had no such effect. Some solutions are scarcely affected by acetic, others are partly precipitated, others almost but not quite wholly. The films spread on paper vary very much in their relations to light; some are readily converted into the yellow intermediate form, whilst others are very insensitive. The least sensitive specimens seemed to be those for which dilute nitric acid had been used as a precipitant. They had a steel-gray color. Precipitation by acetic acid seems to tend to a greenish metallic surface color and greater sensitiveness. Different specimens also vary very much as to permanency; this character is also affected by the amount of washing received: thorough washing tends to permanency.

In some ways the blue, gray and green forms seem more closely related to the black or dark gray forms of normal silver, for they tend in time to pass into them, while on the contrary,  gold-colored silver, if pure, tends with time to change to bright white normal silver on the surface, with dark or even black silver underneath.

Action of other Carbonates

Tannin is capable of producing allotropic silver, not only in the presence of the arbonates of potassium and sodium, but also with those of lithium and ammonia and also with the carbonates of calcium, magnesium, barium and strontium. The action of the last named carbonate has been more particularly examined. it yields allotropic silver of a dark red color while moist, drying with a rich bluish green metallic surface color in thick films, in very thin films transparent red. It is probable that the substances with which tannin produces these reactions would be further reduced by investigation...

Nature of the " intermediate substance"

It has been mentioned in previous papers that when allotropic silver is converted into normal silver by the action of heat it passes through a perfectly well marked intermediate state. In this state it retains the gold-yellow color and high luster but none of the other properties of the original form. Oxidizing and chlorizing agents show nearly the same indifference as with ordinary silver. While other allotropic silver is soft and easily reduced to powder the intermediate substnace is hard and tough. When a glass rod is drawn over a film of allotropic silver it leaves behind it a white trace of ordinary silver. The intermediate substance shows no such reaction: the trace of a glass rod does not differ from the rest of the film and even hard burnishing produces no change in the color. Continued exposure to sublight brings about the same alteration to the intermediate form and it takes place spontaneously with time.

At that time no explanation could be found as to the nature of the change. It proves however to be a passage into a crystalline form. Some films spread on paper were exposed to the action of a very dilute solution of ferric chloride. It chanced that one of these films had undergone a partial change into the intermediate; the unchanged portion was darkened by the ferric solution, while the portion that had passed into the intermediate form retained its bright gold-yellow color and luster rendering it thus distinguishable. The figures which it exhibited were strikingly crystaline. One portion showed a foliated struture such as if formed by interpenetrating crystals, other parts showed ramifications, with something of a plant-like form. Another part exhibited a sheaf of acicular cystals nearly parallel in direction, half an inch to an inch long and fine as hairs...

This change to the crystaline condition does not seem to be peculiar to gold-colored silver...
  
The stable "intermediate form" of silver (i-Ag) is easy to prepare. It occurs as bright gold-yellow or green crystals with a metallic luster. Treatment with a very dilute solution of ferric chloride will enhance the appearance of its foliar structure, interpenetrating with plant-like ramifications, or fine acicular crystals up to 1 inch long.

Intermediate silver is hard, tough, and unaffected by pressure. It is nearly as indifferent to oxidizing and chlorizing agents as is normal silver. Intermediate silver can be formed from the allotropic varieties by light, heat, or chemical action. The simplest preparation is as follows:

"...It is a little curious that its permanency seems to depend entirely on details in the mode of preparation. I have found many ways of obtaining it, but in a few months the specimens preserved changed spontaneously, to normal silver. This happened even in well closed tubes. The normal silver produced in this way is exquisitely beautiful. It has a pure and perfect white color like the finest frosted jewelers' silver, almost in fact exceeding the jeweler's best products. I found, however, one process by which a quite permanent result could be obtained...

In forming the blue product which I have called A, very concentrated solutions were necessary. C on the contrary is best obtained from very dilute ones. The following proportions give good results:

Two mixtures are required: No. 1 containing 200 cc of a 10% solution of silver nitrate, 200 cc of 20% solution of Rochelle Salt [Sodium potassium tartrate] and 800 cc of distilled water. No. 2, containing 107 cc of a 30% solution of ferrous sulfate, 200 cc of a 20% solution of Rochelle salt and 800 cc of distilled water. The second solution (which must be mixed immediately before using only) is poured into the first with constant stirring. A powder, at first glittering red, then changing back to black, falls, which on the filter has a beautiful bronze appearance. After washing it should be removed whilst in a pasty condition and spread over watch glasses or flat basins and allowed to dry spontaneously. It will be seen that this is a reduction of silver nitrate by ferrous sulfate. The metallic silver formed by reduction with ferrous citrate and ferrous tartrate is in an allotropic condition; with ferrous oxalate this result does not seem to be produced.

Although the gold-colored silver (into which the nitrate used is wholly converted) is very permanent when dry, it is less so when wet. In washing, the filter must be kept always full of water; this is essential. It dries into lumps exactly resembling highly polished gold... By brushing a thick paste of this substance evenly over clean glass, beautiful cold-colored mirrors are obtained; the film seems to be entirely continuous and the mirror is very perfect.

By continuous washing the precipitate changes somewhat, so that in drying it takes on a coppery rather than a golden color, and is rather less lustrous, though still bright and brilliant...

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Reactions --

The most characteristic reactions of gold-colored allotropic silver are those with the strong acids. When normal silver reduced with milk sugar and alkaline hydroxide is left in contact with strong hydrochloric acid even for several hours there is no action, and the silver after thorough washing dissolves in warm dilute nitric acid without residue. With allotropic silver similarly treated chloride is always formed. But strong hydrochloric acid instantly converts allotropic to ordinary silver and consequently only atrace of chloride is produced. By largely diluting the acid the conversion is retraded and the proportion of chloride is greatly increased. Thus for example when ordinary hydrochloric acid is diluted with 50 times its volume of water and is made to act on allotropic silver, about one-third is converted to chloride. Probably the whole would be but for the simultaneous conversion to normal silver. This double action is curious and strongly differentiates allotropic from ordinary silver. Even with the same acid diluted with 100 volumes of water, there is a gradual but complete conversion to white silver accompanied by the production of a not inconsiderable quantity of silver chloride.

Neutral chlorides also act strongly upon allotropic silver even when much diluted...

Sulfuric acid diluted with 50 volumes of water has no action upon n ormal silver. It quickly converts allotropic silver to normal but at the same time dissolves a little of it...

Ammonia seems to be without a converting action but dissolves a trace. It will be shown in a future paper that there exists a form of allotropic silver abundantly soluble in ammonia...

Intermediate Form

Allotropic silver presents itself in an almost endless variety of forms and colors... Most of these varieties seem to be capable of existing in two conditions, of which one is more active than the other.

If we coat a chemically clean glass plate with a film of gold-colored allotropic silver, let it dry, first in the air, then for an hour or two in a stove at 100 C, and then heat the middle of the plate carefully over a spirit lamp, we shall obtain with sufficient heat a circle of whitish gray with a bright, lustrous golden ring round it, somewhat lighter and brighter than the portion of the plate that has not been changed by heat. This ring consists of what I propose to call the "intermediate form".

With sulfuric acid diluted with four times its bulk of water and allowed to cool, an immersion of one or two seconds converts a film on glass or on pure paper wholly to the intermediate form...

Its properties are better seen by using a film formed on pure paper, one end of which is heated over a spirit lamp to a temperature just below that at which paper scorches. The change is sudden and passes over the heated portion like a flash. Examining the changed part, we find :

1st. That it has changed from a deep gold to a bright yellow gold color.

2nd. When subjected to a shearing stress it does not whiten or change color in the slightest degree.

3rd. It is much harder, as is readily perceived in burnishing it.

4th. It no longer shows the color reaction with potassium ferricyanide and ferric chloride, changing only by a slight deepening of color.

Of these characteristic changes the second is the most remarkable. The gold-colored silver in its original condition changes with singular facility to white silver; almost any touch, any friction or pressure effects the conversion... Heat effects the same change but with an intermediate stage at which pressure no longer produces any action.

The intermediate form is distinguished from normal silver almost solely by its bright yellow color and its higher luster. This last difference is very striking when a film on glass is heated in the same manner as above. The central parts in changing to white silver become wholly lusterless, while the circle of intermediate retains all its original luster. Its continuity is so complete, that if viewed through glass, it still acts as a mirror.

This change may be either molecular or depend on dehydration.

The latter seems doubtful for the change cannot be brought about by dessication...

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