EXPERIMENTS ON THE COLORED FILMS

FORMED BY IODINE, BROMINE, AND CHLORINE UPON VARIOUS METALS.

BY AUGUSTUS WALLER, M. D.

In a paper presented by me to the Academy of Sciences of Paris, an extract from which may be seen in the Comptes Rendus for October 5, 1840, I first demonstrated the error committed in ascribing to the iodide of silver alone the power of fixing the vapors of mercury, after it had been exposed to the action of light. Instead of this property being exclusively confined to a film of iodide of silver, as obtained in the process of M. Daguerre, I found that it existed in many other substances when presented to the action of light in the state of thin films, viz. by the bromide and chloride of silver; by the oxide, bromide, iodide and chloride of copper, and some others; all these, however, possessing less sensibility than the iodide of silver of Daguerre, and therefore less available for the reproduction of the images of the camera than the compound originally discovered by that gentleman. The iodide of Daguerre was found already too little sensitive to the influence of light in this climate, especially when applied to the reproduction of the image of animate objects, so that those films discovered by me seemed still less suitable to be employed for that purpose; this objection has, however, been completely removed by recent improvements, more particularly those of M. Claudet, who effected this principally by combining the original discovery of Daguerre with those mentioned above as having been subsequently made by myself. Pursuing the first stage of Daguerre's process, he obtained the film of iodide of silver, and added to this another film of bromide, either in a simple state,—as practised in my experiments published more than six months before—or after two of these substances had been combined together, as the chloride of iodine and the bromide of iodine, which he was the first to employ.

These colored films, however, merit attention independently of the purposes to which they may be applied in photography: the beauty of some of the phenomena themselves is peculiarly attractive; the numerous changes of color they undergo, either by a variation in the thickness of the film, or by the action of light, assign them a place among the most curious facts of science, and the extreme facility with which they are obtained adds to the interest they excite.

Impressed with these ideas, I was induced to pursue a train of investigation on this subject; among the results of which, one of the most interesting was a new method of making colored rings, like those generally known under the name of "Newton's colored rings," on many of the metals, by the same chemical process as that employed for forming the films of uniform thickness in photography. In order to procure these colored rings, and at the same time to show the identity of the origin of the colors with those of the ordinary transparent films, that is, as residing simply in the thickness of the lamina and not dependent on the ordinary cause of color, we have but to place a piece of iodine on a well-polished surface of silver or copper, and in a short time we find around the iodine a series of colored zones of the various tints of the spectrum, and approaching in a greater or less degree to the form of a circle, according as they have been more or less disturbed in their formation by currents of the surrounding air. In order that they may be perfectly regular, as large as possible, and with tints undisturbed by the action of light, it is necessary to place a piece of iodine in the centre of a well-polished plate, as before described; this is then to be shaded by an opake screen superimposed a few lines from the surface to cause the vapors which would otherwise ascend and partially escape, to expand over its silver surface. Colored rings may be formed in the same manner by bromine and chlorine and the various combinations of these bodies with each other, except that for those that are gaseous or liquid it is requisite to pay a little attention to the manner of disengaging them on the surface of the metal, either by passing them through a glass tube, or by some other contrivance easy to execute.

These rings correspond to those formed by reflected light in Newton's experiments, with this difference, however, that in the colored films of the soap bubble, and in those formed by the glass lenses, the thinest film is in the centre; whilst in these rings, obtained by chemical action, it exists at the circumference, as is the case with the colored rings of Nobili. In watching the formation of these phenomena, at first are seen two or three very small circles, which appear almost as soon as the iodine and the metal are placed in contact with each other; as the experiment continues, the circumference of these circles become gradually greater; whilst the external colors extend themselves over a great space, those of the centre grow fainter; red and green now only remain visible, and these at last, when the film has attained a certain thickness, in their turn also give place to a dull coating of brown.

The formation of these rings evidently depends on the vaporization of the iodine from the solid nucleus. The variety in color and extent of these zones is caused by the difference between the strength of the vapor at the centre and the circumference of the iodic atmosphere whilst expanding over so large a surface. In the metal thus combining with the vapor, we have to consider,—1, the force of the vapor of different distances from the centre; 2, the obstacle which a film of iodine, once formed, opposes to any further action between the iodine and the metal.

This experiment may be varied in different ways: two pieces of iodine of about the same size, placed at a small distance from each other on a silver plate, form separate colored circles, until these come in contact at their circumferences, when the two systems will slowly coalesce and produce one common outline of the form of an ellipsis.

As the colors formed on various metals by the above-mentioned agents are very similar to one another, it may be sufficient to examine in particular those produced on silver by iodine.

The external film of the iodide of silver rings, which corresponds to the central black spot in those of Newton, is completely invisible, it being impossible to perceive any difference between the parts so covered and those where the metal is intact; but by exposing half the plate to the influence of light, whilst the other part remains covered, the silver is then found darkened far beyond the limits of the external gold-colored zone, where previously the surface was perfectly clear. The dark film thus rendered apparent is now liable to be rubbed off by the slightest friction, whereas before it was very adherent to the subjacent surface. The first zone is of a pale gold color, which assumes a deeper tint as the thickness of the film increases: the second zone is blue, the third white: after these appear the different colors of the spectrum in regular succession, as in the films studied by Newton and others, viz. yellow, orange, red, blue green, yellow &c.

The presence of the golden-colored zone in the place mentioned is worthy of remark, as in the tables of Newton of the colors presented by films of various thicknesses, the blue is stated as immediately following the black. The same gold film is the first which appears on most metals when their surface is attacked in this manner. Chlorine and bromine on silver; oxygen on steel; chlorine and bromine on titanium, bismuth, &c., commence their colors in the same way. Copper, however, is in one respect an exception, this metal first becoming of a dark red, which increases to a ruddy brown and then changes into blue. This deviation is fully accounted for by the color of the copper itself. With this single particularity, this metal undergoes the same alterations as the others.

The action of light on the different colors of the iodide of silver is very interesting: the most correct way of studying this is to protect one half of a system of colored rings by an opake screen, while the other half is exposed for a short time to the influence of the solar rays. The golden zone undergoes the greatest change; at first it grows darker, then red, and at length is converted into a beautiful green. The blue film, which comes next in thickness, suffers considerable alteration in its tint, assuming a much deeper and more brilliant shade; the rest of the colors appear to be similarly affected by the action of light, though to a very slight degree, acquiring a trifling accession in their brilliancy. It has already been remarked that light destroys the adherence of the external invisible film; the same thing obtains with the second or gold-colored film, which turns green, but only to a certain depth of the film, as may be proved by slightly rubbing the part thus altered; the green color is then seen to disappear, and beneath the pulverulent portion thus removed is found the gold color, having almost the same appearance as before the plate had been exposed. As this experiment may be repeated several times with the same results, it shows to how inconceivably small a depth the light has acted to produce this effect.

To ascertain what would take place on augmenting the thickness of the portion turned green, and the adherence of which was destroy ed, a piece of iodine was placed on the plate so that its vapor, by expanding, might arrive upon the green, at the same time the whole being kept from the light; the result was that the additional film combined with the one already existing, producing a blue, being the color which would have resulted by the combination of the unaltered yellow films. I have found no chemical substance possessing the power of arresting, or in any way influencing these changes of color; strong adds, provided they do not attack the silver—for then, of course, the experiment would be destroyed,—and alkalies in concentrated solution, allow the action of light to go on as usual. The hyposulphate of soda, and ammonia in solution have no longer the power of dissolving the green film as they had before the action of light.

When the plate is left still longer exposed, after the changes above stated have taken place, the colors become more faint, and within the zone of green a white cloudy film is caused by the light, which, as it increases, veils the spectral colors beneath.

The knowledge we at present possess in chemistry of the affinities with which different bodies are endowed for combining with each other is but very imperfect, and the causes which complicate most chemical phenomena are so numerous, that it is scarcely possible to compare any two chemical actions to each other. Most of the facts upon which chemical science is founded, are acquired either by bringing the two bodies destined to act on each other into contact by dissolving them in a liquid, or by subjecting them to a temperature more or less elevated.

In the first of these methods, we are so far from being able to calculate the force of the chemical powers called into play, that Berthollet was induced to deny the existence of chemical power in the various phenomena of solution and precipitation of saline substances, and according to him what is called insolubility in a body is merely the result of its strength of cohesion, an entirely physical property.

When the intervention of caloric is required, the effects are still more complicated, as they vary according to the intensity of the heat employed, and the time its action is exerted; besides, the chemical action, when it does take place, is frequently so instantaneous that it is impossible in our present state of science to imagine any means by which it might be measured. In the combination of the three bodies, iodine, bromine and chlorine, with the metals, however, most of these objections cease to exist, or may be easily avoided. As their vapors combine with the metallic surfaces at the ordinary temperature, they are all of them in the same circumstances in that respect; and if the temperature should be required more elevated, the gasiform state of these substances, iodine not excepted, enables us to submit the metals to be experimented upon all at the same time to the same influence. If, therefore, it were possible to reduce the metallic substances into fine powders the particles of which were of the same dimensions, by acting upon them with either of these vapors, an idea might be formed of the affinities which produce their binary compounds by the increased weight acquired by the powders in this process; but the difference which exists in the physical properties of the various metals would preclude the possibility of any near approach to accuracy in this mode of proceeding; but by acting on the polished metallic surfaces, as in the preceding experiments, all the advantages offered by the process with the powders are included, whilst several of the difficulties are removed.

As the film of the compound augments, it undergoes the various changes of color which take place in all transparent films, thus affording a means of ascertaining the absolute thickness obtained in different circumstances, when it would be difficult to detect the slightest difference in weight by means of the most delicate balance. The depth of this coating may be ascertained when either the index of refraction of the compound itself is known, or if the angle of polarized light is given by means of the law discovered by Sir David Brewster, between the tangent of the angle of polarization, and the index of refraction. The most convenient way which occurred to me of performing these experiments, was the employment of a bell-glass within which some iodine is fixed at the top; this apparatus being placed over the metal to be acted on, the experiment may be watched in all its progress, and the action can be retarded or accelerated at pleasure by varying the interval of the iodine from the metal, or by interposing at some distance from its surface a disc of paper so as to cause the vapors of iodine to pass through it. Bromine may be made use of likewise by pouring a few drops of it over some carded cotton, and using it in a similar manner with the iodine. In respect to chlorine, it is most convenient to disengage it slowly by dropping a little sulphuric acid upon some chlorinated lime.

In illustration of the objects of this mode of experimenting, I will aduce some of the results it has given me with various metals. Some of the experiments below were performed before I had the idea of watching the progress of the combination through a transparent medium; they are therefore less exact than they might otherwise have been: but I have preferred stating them as I had inserted them in my note-book before I had conceived any idea as to their probable utility in the elucidation of chemical affinity, and when I intended them for other purposes, which I shall hereafter explain.

Iodine with Silver and Copper.

1st change. Silver—pale gold.

" Copper—assumes a darker red.

" Silver—blue.

2nd do. Copper—blue.

" Silver—white.

3rd do. Copper—white.

" Silver—yellow.

4th do. Copper—yellow more extended than on the silver.

" Silver—Orange.

5th do. Copper—Red.

" Silver—blue, bluish-red.

" Copper—red, with a tinge of green on some parts.

" Silver—greenish blue.

" Copper—red, tinged with green.

" Silver—green.

" Copper—orange.

" Silver—yellowish green.

" Copper—orange tending to red.

" Silver—yellowish green.

" Copper—orange-red.

" Silver—red.

" Copper—dull green.

" Silver—red.

" Copper—green.

" Silver—deep green.

" Copper—dull red.

Bromine with Silver and Copper.

5th change. Copper—sensibly darkened.

" Silver—unchanged.

" Copper—deep red.

" Silver—unchanged.

" Copper—red, blue.

" Silver—pale gold.

" Copper—white, orange of the 2d order.

" Silver—yellow.

" Copper—green of the 1st order, red 3rd order.

" Silver—blue.

Chlorine with Silver and Copper.

The affinity of chlorine with silver is much inferior to that which it possesses for copper.

Iodine with Titanium.

Iodine at the common temperature has no action upon this metal.

Bromine with Titanium.

Bromine, when the surface of this substance is perfectly dry, has no more action upon it than iodine; but if it have a slight coating of moisture, as is formed by merely condensing on it the vapor of the breath, the colored films are formed without difficulty by the vapors of bromine. Their appearance is the same as those of the iodide of silver, viz. gold, deep gold, blue, white, yellow, orange, red, &c.

Chlorine with Titanium and Copper.

Titanium has a stronger affinity than it has for either of the preceding vapors. The combination takes place when the metallic surface is either dry or moist.

Copper—much reddened. Titanium—not affected. Copper—passed through several of the spectral orders of red and green until it arrived at almost its last changes of colors.

Titanium under the same action received a dull film, which viewed obliquely showed red, green, yellow.

Silver, exposed to the same influence as the two former, had yellow in the centre and blue more externally.

Iodine with Bismuth and Silver.

Silver—pale gold. Bismuth—some parts yellow, others not attacked. Silver—blue, white, yellow-orange. Bismuth—blue, yellow, orange.

In the action of iodine on bismuth, the influence of the physical condition of metallic surface is very manifest. The crystaline texture of this metal may be perceived, and the difference of its hardness admits, to a certain point, of being measured by the difference of the color of the films that are formed on various points; while most parts are yellow, there exist others of an angular outline which remain still unattacked; the same difference is remarked in the other stages of the combination.

Iodine with Mercury.

It is impossible to estimate the affinity between mercury and iodine by means of the colored films, because, on combining, these two substances merely cause a dirty white appearance on the surface of the latter. Their combining affinity appears to be considerable, for when exposed together with silver the action produced with both was red at the edges, little altered in color; on the rest of its surface a dull white film, in the midst of which were seen several dark spots, where the metal was apparently unaltered.

Bromine with Mercury and Copper.

1st. Mercury—gold color. " Copper slightly darkened. 2nd. Mercury—blue. " Copper dark red. 3rd. Mercury—green on some parts. " Copper white.

After this the copper underwent its usual changes of color on prolonging the action of the vapor of bromine, but the color of the mercury suffered no further change.

Chlorine with Mercury and Copper.

Mercury—a slight film. Copper no alteration of color. Mercury—deep gold color. Copper deep red on some parts, blue on others. Mercury—red tinged with blue. Copper blue, white. Mercury—blue. Copper same as before.

"With respect to the bromide and chloride of mercury, it is necessary to view them obliquely in order to perceive all the changes of color they undergo; for if looked at perpendicularly, there is seen on both a dull uneven film of white which reflects none of the above colors: consequently, to avoid any error, the copper must be inspected under the same angle.

Bromine with Bismuth and Silver.

Silver—pale gold. Bismuth—not apparently changed. Silver—deep gold, blue. Bismuth—yellow, blue. Silver—blue, yellow. Bismuth—dull colorless film.

Chlorine with Bismuth and Silver.

Bismuth is slowly attacked with chlorine gas, much in the same way as with iodine and bromine in vapor.

Bromine with Lead.

At the common temperature neither bromine nor chlorine forms colored films upon this metal, which it is very difficult besides to bring to any high state of polish on account of its softness. But when lead is heated, as over the flame of a spirit-lamp, the vapors of bromine then form very fine colored films, which are in succession gold, deep blue, &c.

Iodine with Iron.

These two may be made to form colored films when combined rapidly together, but generally a dull coating without any spectral color is obtained, on account of the deliquescence of that salt.

Until we know the index of refraction of the different films enumerated, it would be impossible to give a correct table of the combining powers in the experiments that have been detailed; nor is the table of the relative thickness of transparent plates as it has been transmitted to us by Newton, sufficient in the present instance, if any great degree of precision be required. Besides these objections, it is necessary before leaving this subject to pass in review several others inseparable from the mode of performing the experiments themselves. The principal circumstances complicating these experiments and liable to vary in different observations, are,—

First, the hardness of the metal acted upon; 2ndly, the obstacle opposed to the continuation of chemical action by the inert film formed upon the metal; 3rdly, the force of the vapors that attack the metal. The influence of the texture of the metallic surface on chemical action is most evident when bismuth is the metal employed. Here the chemical action may be seen to commence on small isolated portions of the surface, which have already assumed a deep gold color, before other parts are in the least changed, from the natural appearance of the metal. To determine how far this might influence the formation of the iodide of silver, a silver coin was exposed to iodine with a piece of pure silver; as the former was so much the harder of the two, it was naturally supposed that the chemical action would be slower in exerting itself on it than on the latter. This, however, was not the case, as may be seen by the following statement of the result of the experiment:

Silver coin—pale gold color. Pure silver—pale gold. Silver coin—deep gold. Pure silver—deep gold. Silver coin—light blue. Pure silver—light blue. Silver chin—yellow. Pure silver—blue, white, yellow not visible. Silver coin—yellow, red at edges. Pure silver—yellow, no red edges. Silver coin—red, blue at edges. Pure silver—yellow, no red apparent.

The intensity of the resistance offered by the different films of iodide of silver to a continuation of the chemical combination, may be determined by noting the moment at which the various spectral tints make their appearance.

Color of the film of iodide of silver.

' "

0 50—beginning to darken.

2 0—pale gold.

4 40—deep gold.

6 40—orange blue.

7 30—blue.

9 30—light blue.

11 30—commencement of yellow.

18 30—orange red.

20 15—blue.

22 55—deep blue.

24 40—green.

28 0—yellowish green.

30 25—ruddy brown.

40 10—green.

46 30—green.

50 10—red.

53 15—green.

By comparing the thickness of the colors with the space of time required for their production, it will be found, however imperfect the table given by Newton may be when applied to this subject, that towards the end of the experiment above given, the chemical combination is retarded by the presence of the inert film, and that to obtain the same thickness of film as at the commencement, about double the time is required.

The third cause of error may be avoided by operating with vapors of about the same force. In those described, the average time employed in passing to the maximum was generally about half an hour; if that were not taken into consideration, different results might be obtained.

In regard to chlorine, there exists another cause of complication—the affinity which it possesses for water; for when disengaged in the ordinary manner, chlorine carries with it a certain quantity of water which may very much alter the results of the experiment.—Philosophical Magazine, 1842.