T H E COLORS O F COLLOIDS. IX BY WILDER D. BANCROFT
Colloidal Metals I n 1857, Faraday1 made colloidal solutions of gold which were red, violet or blue by transmitted light. Nearly forty years later, Zsigmondy2 worked out methods for preparing stable colloidal solutions of gold. I n the red solutions the particles are green by reflected light, they are yellow to reddish brown in the blue solutions, while the violet solutions contain a mixture of the two. By precipitating metallic silver under suitable conditions, Carey Lea3 obtained colloidal silver which was yellow, red, or blue by transmitted light. Carey Lea of course assumed that these were allotropic forms of silver and this view was shared by Blaken4 No evidence has been brought forward to show the existence of allotropic modifications and it is, therefore, necessary to account for the color phenomena solely on the basis of the size and the structure of the suspended particles. Garnettj has given a theory of the phenomenon which has been summed up by Wood6 as follows: “We will now consider the effects of these small gold spheres upon the color of the transmitted light. The subject has been very fully discussed by Garnett, who has investigated it from the standpoint of the electromagnetic theory. His treatment is much too long to give in full, but we can examine to advantage the general method of attack and some of the conclusions. Let the light of wave-length fall on a metal sphere of Phil. Trans., 147, 145 (1857). Liebig’s Ann., 301,30 (1898); Zeit. anal. Chem., 40, 711 (1901). 3Am. Jour. Sci., (3) 37, 476; 38, 47 (1889); Phil. Mag., ( 5 ) 31, 238, 320, 497; 32, 237 (1891). Zeit. anorg. Chem., 37, 243 (1903). 6 Phil. Trans., 203A, 385 (1904); 205A, 241 (1906). 6 “Physical Optics,” 643 (1911). 2
The Colors of Colloids. I X radius a, refractive index Further let
PZ,
555
and absorption coefficient k .
Nfn(~-Zik)
=
42,
being the complex dielectric constant. This case has been considered by Lord Rayleighl who showed that the sphere excited by a periodic electric force E , emits the waves which would be emitted by a Hertzian doublet, which at time t was of moment equal to
E’
N2N2
+
I 2
u3Eo.
If there are a large number of spheres in close proximity, the electric force exciting each one will be E’, i. e., the force E,, together with forces due to the neighboring doublets. This force E’ causes the polarization N2- I f ( t ) = a3 N 2 2 E’*
+
If the average moment of a doublet be f ( t ) , and there are doublets per unit volume, the polarization of the medium will be f ’ ( t ) = ~ j ’ ( t ) . By means of analyses by Lorentz and by Larmor it can be proved that %
provided the doublets are distributed through a space large in comparison to the wave-length. This gives us
E’
Eo
=
4~ N’--I’ I - - na3 3 N2+z
so that N2-
I
Eo sqz
.i=I -4” N’-I nu33
N2+2
By substitution of these units iii Maxwell’s equation, the com1
Phil. Mag., 44, a8 (18973,
556
Wilder D. Baficroft
plex dielectric constant of the medium containing the spheres is found to be N2-I 3D
€ ’ = I +
I-D-
N2N2
~
_-
+
I 2
in which D is written for cuta3, denoting the volume of the 3
metal per unit volume of the medium. This is for spheres iut vacuo: in glass of refractive index p the equation becomes
The optical constants of the medium ut’ and k’ thus depend only on D, the relative volume of the metal, and not on the size of the spheres, restricting them, however, to sizes small in comparison t o A. By reducing the above equation, and substituting in it the values for N and p, the absorption can be found for any given value of D. “Now D varies with the nature of the glass. The gold glass as first prepared is colorless, becoming red on reheating, the process causing the metal spheres to form within the body of the glass-(excretion of the metal’ Garnett calls it. Colorless gold glass turned red on exposure to the emanation of radium, and it is probable that the blue color of X-ray tubes, and tubes which have contained radium, is due to the excretion of metallic potassium or sodium by radiation. Sir William Ramsay ekposed glass -containing silver to radium rays and found that it turned yellow. Quartz glass is not colored, as no metal is present. “Elster and Geitell found that salts of alkaline metals, colored by the action of cathode rays, exhibited photo-electric properties which suggested the presence of free metal; this supports the view held regarding the coloration of glasses by X-rays and radium rays. Wied. Ann., 59, 487 (1896).
The Colors ~f Colloids. IX
55 7
“Garnett took the values of N calculated from Drude’s tables of the optical constants of the metals, and the value of D calculated from the total gold content of the glass, and the observations of Siedentopf and Zsigmondy, and showed that the medium should be much more transparent for red than for yellow light. Values +z and y for green and blue light not being available, the best that could be done was to infer that, since yellow is less freely transmitted than red, the medium is still more opaque to green and blue. “Garnett next develops an expression for the scattered light, and finds that the amplitude a t any point of the light emitted from a sphere is proportional to jy2-pz ~
~
7s
+
I a_3 2p2
I
X2’
The X 2 in the denominator indicates that the scattered intensity increases with the inverse fourth power of the wavelength, but that it is also dependent on Tu’, i. e . , on the optical constants of the metal. Calculations showed that yellow light would be scattered more powerfully than red, from which it was inferred that green would be still more powerfully scattered, which is in agreement with the observations of Siedentopf and Zsigmondy. Certain types of gold glass scattered a muddy, red light however. I n this case the particles are probably so large that they reflect light in the ordinary sense, and, as we know, gold reflects red light in greater excess than any of the other colors. “In an appendix to the paper, the transmission of gold and silver glass has been calculated for red, yellow, green, and blue light, from values of .tz and k given by Rubens. The colors in the order of the degree in which they were transmitted were found to be : for gold glass-red, yellow, blue-green; for silver glass-yellow, red, green, blue. Certain gold glasses appear blue by transmitted light, and it appears probable that large particles (diameter > O . C G O I ) , by reflecting out the red and orange, give the glass a blue color. “In the second part of the paper above referred to, Garnett examines the conditions which hold in cases where the
558
Wilder D. Bimcroft
metallic granules are deposited in thin films. The equations already given were developed on the assumption of a uniform polarization in the medium, which is only the case when the spheres are distributed in three dimensions. For a twodimensional distribution, in the xy plane, it is shown that the complex dielectric constant in the direction of the xy axes is the same color as for the medium in bulk, whereas the constant in the direction of the z axis may be quite different. If this were the case the film would behave like a doubly refracting substance, the ‘optic axis’ being perpendicular to the film. It is found that for films of thickness greater than z / 3 of X the absorption is governed by n k , while in t h e case of films less than X it is governed by n2k. Curves are given showing how the absorption depends on D, the volume of metal per unit volume of the medium. The values of xk, or n2k,are plotted as ordinates, and the values of D as abscissae. In the case of a nongranular film of solid metal, it is evident that D = I . “Garnett was able to explain all of the effects observed by Wood in the case of the sodium and potassium films deposited in exhausted bulbs, at least all of the effects which fell within the scope of his equations. The curves for a potassium sodium amalgam show how the absorption depends on the value of D. For D = I , i. e., solid metal, the absorption is strongest for red and weakest for blue. For D = 0.5, that is, for a film with equal volumes of metal and empty spaces, the absorption is strongest for yellow, while for D = 0.3 the blue is practically the only color absorbed. For thin films we find that for D = 0.5 yellow is very powerfully absorbed, which agrees with Wood’s observation that when the conditions were such as to cause an absorption band in the yellow, the band was much blacker and narrower than when it occurred in some other part of the spectrum. “The graphs for gold indicated that for D = I the color in the case of very thin films of gold leaf should be blue. This was the color observed by Mr. Beilby in the case of the thinnest leaf which could be procured. For thick films the graphs
The Colors
OJ
Colloids. I X
559
showed that the color of the transmitted light should be green. Many very curious and interesting changes, observed by Mr. Beilbyl in the colors of thin, gold films, produced by heating and annealing, are discussed and explained by Garnett, whose paper is by far the best on the optical properties of metallic films which has appeared up to the present time. “It appears that the theory of optical resonance can be regarded as in a measure confirmed by these investigations, though the phenomenon is very much more complicated than in the case of large resonators and electro-magnetic waves. The optical constants of the metal enter as a factor, and for very small particles a t least the absorption depends not on their size, but on the total bulk of metal in unit volume. It is probable that very definite and more easily interpreted results can be obtained by experimenting with very long heatwaves, either with cross-ruled films of metal or fine metal powders. ” While Garnett’s papers are very interesting, I do not find them as helpful as I should like. They are very mathematical in form, and yet it is difficult to find any clear-cut general statements. I t has seemed to me that it might be of assistance to others if I were to put the matter as I see it, laying more stress on simplicity of treatment than on accuracy. N o discussion will be attempted in regard to polarization. It does not seem to me that the theory to be outlined is the same as Garnett’s; but it could perhaps be deduced from his equations. I n the case of selective reflection, some or all of the light which is absorbed very strongly by a given substance is reflected strongly from a polished surface of that substance. The surface color of magenta is green, while the body color or the color due to the transmitted light is red. With indigo the surface color is red and the body color blue. The surface color is believed to be due to resonance, the substance emitting the rays which i t absorbs very strongly. If this is the case, it is easy to see that a very small granular particle might emit surface color by resonance on the opposite side from the source 1
Proc. Roy. SOC.,72, 2 2 6 (1903).
Wilder
560
D.Bawcroft
of light, in which case we should have the surprising phenomenon of a substance apparently transmitting the light which it absorbs most strongly. On the other hand, the amount of reflection from the back surface of a particle of the light ordinarily transmitted will be greater, the less the thickness of the particle. In the case of a colloidal solution of a substance showing selective reflection, we should expect that with decreasing size of particles, the apparently transmitted light will contain more and more of the light which is usually reflected, while the reflected light will contain more and more of the light which is usually transmitted. In other words, we should expect a more or less complete reversal of the usual colors as the suspended particles became smaller and smaller. While this is theoretically sound, it might easily happen that one did not get apparent transmission of the strongly absorbed light at any size of particle which could be realized satisfactorily. Fortunately, this seems not to be true. “Wien focussed sunlight upon the highly polished edges of thin plates of various metals, and observed that light was diffracted far into the region of the shadow, the edge of the plate appearing luminous. The color of the light varied in a remarkable manner with the nature of the metal, appearing red with copper and gold screens, orange with silver, yellow and yellow-green with platinum and tin-foil. The color only appeared when the edge was clean and quite free from dust; it was complementary2 to the color most strongly absorbed by the metal, and polarized with the vibration perpendicular to the diffracting edge. If the incident light was polarized to start with, the color was only seen when the vibration was perpendicular to the edge. The phenomenon is evidently related in some way to resonance, vibrations being set up in the metal along the edge which emit energy into the region behind the screen. I n addition to the colored light, Wien found that white light was also present, and that it could also be cut off by a Nicol See Wood. “Physical Optics,” 634 (1911). [This is wrong. It does not agree with the statement in regard t o resonance two sentences further on.-W. D. B.] 2
The Colors of Colloids. I X
561
prism, though its plane of polarization appeared to depend upon the azimuth in which the incident light was polarized, and also upon the angle of diffraction.” The thickness of the film at the edge of the plate is finite, and it, therefore, seemed probable that the desired effect could be realized experimentally with colloidal solutions. Since indigo in mass transmits blue and reflects red, a colloidal solution of indigo should be red by transmitted light. Professor H. N. Holmes of Oberlin was good enough to try this experiment for me and found that the prediction was verified. Apparently, the reason that this has not been formulated clearly before has been that people have worked chiefly with colloidal metals which are so very opaque that the reversal of colors has not been noticed. One should really study the problem using aniline dyes having a metallic surface color. We can now apply this general principle to the case of gold. Under ordinary conditions gold reflects yellow; but the color is said to change to red with multiple reflecti0ns.l Pulverulent gold is black, and, consequently, gold will change from yellow through brown to black, as it becomes more and more porous. Brown gold is obtained in assaying.2 This turns yellow again when subjected to pressure, or when heated so that it sinters. Very thin gold leaf is green.3 Consequently, we should expect gold particles to reflect yellow, brown, or green light, depending on the conditions. Massive particles would reflect yellow and very fine particles green, while porous particles would reflect brown light, more or less irrespective of size. Zsigmondy‘ points out that colloidal gold solutions contain ultramicrons which reflect yellow, brown, or green. “Both green and brown ultramicrons may have all possible dimensions from the amicroscopic to 1 2 0 p p and over. As a general thing, however, the large particles are yellow or brown while the very fine subdivisions are green. At present, there 1
3
Wood: “Physical Optics,” 456 (1911). Hanriot: Jour. SOC Chem. Ind., 30, 89, 216 (1911). Faraday: Phil. Trans., 147, 145 (1857). “The Chemistry of Colloids,” 94 (1917).
.
562
Wilder D. Bancroft
is no explanation for the fact that very small particles are sometimes brown. Nevertheless, the following may be the key to the situation: According to Mie’s theory, particles of gold having a diameter of 40 ,up and under must be green. The assumption is thereby made that the shape is spherical and the particle a compact mass of metallic gold. Any divergence from the theory may mean that the conditions are not fulfilled. I n other words, when the very small particles are brown, either the shape is not spherical or the entire space occupied by ultramicrons is not filled with metallic gold. The first assumption does not seem to be entirely necessary. It may also be contended that the divergence from the theory is due to allotropic modifications of gold. The assumption is quite unnecessary for the explanation of the color and leads to contradictions in certain cases. “With regard to the brown color of very small particles, a large number of experimental facts point to the assumption that the ultramicrons are not composed of massive gold. For instance, whenever the green particles become flocculent or approach very close to one another, the color changes to brown, even when the aggregate is still amicroscopic. It would seem, therefore, that small, brown particles are in reality conglomerates of the green. Green ultramicrons, on the other hand, are composed of compact gold, and are the result of the normal growth of amicroscopic particles, or, better, perhaps, they are tiny crystals.” This agrees admirably with the theory as outlined. Compact gold reflects yellow when it does not resonate, and green when it does. Porous gold is brown. Apparently the green particles cannot resonate after agglomeration. While the smallest particles must be green, there is no reason that some brown particles should not be smaller than the coarser green ones and this is the case. When green ultramicrons are pressed between the cover glass and the platform of the cardioid ultramicroscope, they become brown. The color of colloidal gold solutions by transmitted light is red, violet or blue. The solutions containing green particles
The Colors of Colloids. I X
563
transmit blue light, those containing yellow to reddish brown particles transmit blue light, while solutions containing a mixture of green and yellow to reddish brown particles transmit violet light. Since in general, the green particles are the largest, it follows that coagulation of a colloidal gold solution should cause a change in the color by transmitted light from red to violet to blue, which is what actually occurs.1 “A characteristic property of all pure red gold solutions is the change to blue during coagulation. This change is occasioned by the union of two or more particles that diffract green. The complex thus formed diffracts only brown light waves. It is impossible to explain the color change on the grounds of an increase in the size because i t occurs regardless of whether amicrons or submicrons unite. I n the first of these cases the complex may still remain amicroscopic and have a mass several hundred times smaller than that of alarge, red particle. Nevertheless these tiny complexes diffract brown and the liquid appears blue. “It seems necessary to assume that the particles unite to form a somewhat loose, flocculent mass, and do not melt into one another as drops of liquids do. For if the latter were the case the color would be the same for all particles of the same substance having like dimensions. However, as already stated, there is no relation between size and color unless the growth has been normal; that is not caused by union of several ultramicrons larger than molecules. “There is another important conclusion to be drawn from the considerations discussed in the foregoing, viz., that a decrease in the surface is not a very prominent factor in the coagulation. Even if one assumes that the liquid films between the particles are broken, the decrease of surface must be confined to the edges or faces that touch. “A reversible change of color may be brought about by evaporating a colloidal gold solution with a very small amount 1
Zsigmondy: “The Chemistry of Colloids,”
IOI
(1917).
5 64
Wilder D.Baxcroft
of gelatin.’
The dried residue is blue while the color changes to red if moisture is added. The change of color has been explained by Kirchner2 on the basis of Planck’s3 dispersion theory for isotropic dielectrics. He regards the particles as resonators that, on coming into close proximity with one another, displace the absorption maximum toward the red end of the spectrum, at the same time causing a widening and increased intensity of the maximum. This is borne out very well in practice. Mie4 has raised objections to Kirchner’s theory so that a satisfactory elucidation is not yet a t hand. A complete optical theory of metal colloids must unquestionably explain the change of color that is so characteristic of gold and other metal colloids. “Siedentopf has observed an unmistakable dichroism of gold gelatin films when viewed a t an oblique angle. He assumes that the change of color on drying is due to a change of form of the particles and not to the distance between them. It is difficult to conceive of a reversible change of form, however, and it seems much better to assume an orientation of the particles parallel to the distention surface of the film. But this cannot be the only factor involved in the change of color on dry desiccation, because the color of the residue seen through a Nicol’s prism suitably placed is a turbid violet-red and differs greatly from the deep red obtained by the addition of moisture. The distance between the particles must play a part here just as it does in the coagulation of gold solutions. “A word may be added with regard to blue-gold hydrosols. The blue obtained on the reduction of gold chloride solutions may be attributed to three causes: First, the reduction I F . Kirchner and R. Zsigmondy: Drude’s Ann., 15, 573 (1904);R. Zsigmondy: Zur Erkenntnis der Kolloide, I 14 (1905). 2 F.Kirchner: Ber. Kgl. Sachs. Ges. Wiss. Leipzig, 54, Math. phys. Kl., 261 (1902). Planck: Drude’s Ann., I, 69 (1900);Sitzungsber. Akad. Wiss. Berlin, 1902, 470. Drude’s Ann., 25, 429 (1908). 6 Verh. deutsch. phys. Ges., 12, 36 (1910).
The Colors
OJ
Colloids. I X
565
may be incomplete and colloidal gold oxide be formed instead of golds1 Further reduction, perhaps a t higher temperatures, might cause the blue to change to red. This condition has not been taken cognizance of up to the present time. Secondly, the reduction may be complete and the blue color be attributed to the flocculent union of particles already spoken of; or perhaps to the irregular growth so that, instead of flakes or needles, husk-shaped bodies are called into being. These, of course, may be submicroscopic. Finally the liquid may contain large, massive, gold particles that, according t o the theory of Mie, would account for the blue color.” The red color which is said to be obtained by multiple reflection from gold is undoubtedly the same as the red color transmitted by certain colloidal solutions, while the purple color which one gets with less effective multiple reflection, probably corresponds to the so-called violet solutions. Wood2 has summarized the data obtained by Hagen and Rubens on the reflecting and absorbing power of gold with varying thickness of film. “We see from the data that the reflecting power increases with the thickness, reaching its maximum value a t about 80 p y or 0.00008 mm., which is about oneeighth of the wave-length of red light, after which it remains constant. For red light this maximum value is 90 percent; for green light it is less than 50 percent. This explains the yellow color of gold and the green color of gold leaf by transmitted light.” From this it is clear that red light is reflected the most by gold, and, consequently, it should be the color transmitted most readily by the finest particles of colloidal gold, which is actually what happens. It may be of interest to include here the table given by Helmholtz showing which spectrum colors are complementary, even though the surface color and the body color are not necessarily strictly complementary.
3
R. Zsigmondy: Zur Erkenntnis der Kolloide, 114, 133 (1905). “Physical Optics,” 467 (1911). Wood: “Physical Optics,” 440 (1911).
Wilder D. Bancroft
566
Name
Red Orange Yellow Yellow
Yellow Yellow
Greenish yellow
Wave-length? in w
Name
Wave-lengths
656.2 607.7 585.3 573.9 567.1 564.4 563 6
Blue-green Clear blue Clear blue Clear blue Indigo Indigo
492.1 489.7 485.4 482. I 464.5 461.8 433 .o
Violet
m
Pfi
It may be asked why it should not be possible to make gold leaf thin enough so that it would transmit red light. The answer seems to he that a coherent film does not resonate so readily as a granular one. Beilbyl found that a coherent green film becomes granular when heated and then transmits either purple or blue light, even though the diameters of the particles must be greater than the thickness of the film from which they were formed. We get a similar thing in the case of the dried gelatin films which were blue by transmitted light instead of red, showing that the contraction due to drying brings the particles of gold sufficiently near together to be equivalent to reversible agglomeration, thereby destroying the resonance. There is also some evidence that Beilby did actually get some continuous films which were thin enough to transmit purple light. Garnett2 says: “A very thin leaf of gold should not show the green color distinctive of gold leaf, but the red color should predominate over the yellow. The arbitrary graph for n2kfor blue would, if correct, show that blue should predominate over either yeliow or red. The color of a thin film of gold leaf would, therefore, be chiefly blue, less red, and least yellow, i. e., blue-purple, and this is the color observed by Mr. Beilby in the thinnest piece of gold leaf he possessed. It should be noticed that it has not been proved that a very thin film will let through more red than yellow light and that it, therefore, will not exhibit the green color Proc. Roy. SOC.,72, 226 (1903). Phil. Trans., 203A, 385 (1904).
.
The Colors of Colloids. IX
567
of gold leaf. It has only been stated that it seems probable that it will let through more blue than either.” The case of silver is not so simple because the polished silver has no marked color of its own. The color obtained by repeated reflections from silver is said to be golden yellow, while the color of a thin film of silver is blue. We should, therefore, expect the colloidal solutions having the finest particles to transmit yellow and that actually happens. By reflected light, these particles should be blue, and they are. Muller1 reports that “the very coarse particles of a colloidal silver solution are red in the ultramicroscope; but the color is not saturated because shorter wave-lengths are also present. The smaller the particles, the more the color changes to the blue end and the more saturated and intense it becomes. At 80 p p diameter they are an intense blue.” If no other factor came in, the color reflected from massive silver would be yellow and this is said to be true in the case of multiple reflections. The apparent reflection is gray. To account for the facts, one must assume that on repeated reflections from silver, the light becomes first red and then yellow; but I do not know whether this has actually been observed or not. With gold the apparent reflection changes from yellow to purple to red and silver might easily change from gray to red to yellow. The particles of silver which reflect red, transmit blue, while a mixture of the particles reflecting blue and those reflecting red give a reflection of blue-green and a transmission of red. Colloidal solutions of silver transmit yellow, red, and blue with increasing size of particles. The transmission of yellow follows from the application of the theory to known facts. The transmission of blue cannot be predicted from any qualitative facts now known t o me. Starting with the experimental fact that the coarser silver particles reflect red light, it is possible to predict that one should get red light reflected from massive silver under suitable conditions and this will have to be tested experimentally a t some time. Since Drude’s Ann., 35, 500 (1911).
568
Wilder D. Bancroft
a thin film of silver transmits blue to green light it is probable that it can easily be made to reflect red light. It is to be noted that the blue which is transmitted cannot be a t all pure because Gallagherl has shown that when a blue film is precipitated on glass, and then a second superposed on the first and so on, the color changes from blue to yellow to red, which could not happen if the coefficients of absorption were not very different for different wave-lengths. The very small particles of gold reflect green and transmit red, while those of silver reflect blue and transmit yellow. This shows that the phenomenon is specific, which is not in accordance with the views of Wolfgang Ostwald.2 “The order in which the colors change from one to the other as the degree of dispersion changes seems also to be definite. As a rule, the most highly dispersed colloidal metals are yellow or orange; in other words, they absorb violet and blue light. As the degree of dispersion decreases, the color passes from yellow through orange to red, violet, blue and finally green. The absorption maximum gradually moves toward the side of the greater wave-length as the degree of dispersion decreases. The same order is frequently observed in organic dyestuffs, when the colors of any homologous series are studied. Yellow is usually the color of the chemically simpler members, while the dyes of greater molecular complexity in the same series are often blue and violet.” Since copper has a red surface color, it is natural that colloidal copper solutions should b e red by transmitted light when the particles are very fine, and blue when the particles are coarser. The conditions for the brown and the olive-green solutions are probably very simple; but I have not had time to look up the literature on this point carefully, so I shall omit a discussion of them. With metallic fogs, Wood3 has obtained results similar Jour. Phys. Chem., IO, 701 (1906). “Introduction to Theoretical and Applied Colloid Chemistry,” 62 (1917). “Physical Optics,” 639 (1911).
The Colors of Colloids. I X
5 69
in principle to those of colloidal solutions. A condensing cloud of sodium vapor scatters violet light and transmits yellow, which, of course, it apparently ought not to do, because sodium vapor absorbs the D-lines and lets through blue. What happens is that we get resonance and the sodium particles transmit the yellow light which they absorb and reflect the blue or violet light which they do not absorb. Wood says that “it was difficult to understand this a t first, since the vapor is perfectly transparent to blue light, and somewhat less so to yellow-green light. The cause was finally found to be a scattering of the violet and blue rays by the fog of condensing vapor, which was so powerful that none of these rays was transmitted. ,’ A potassium fog behaved similarly but scattered longer waves than the sodium fog. By regulating the conditions, it was possible to get a potassium fog to scatter red, yellow, and green rays, the blue being transmitted. This is not in accord with the fact that metallic sodium in sodium chloride transmits blue and that a colloidal solution of sodium in ether is blue by transmitted 1ight.l There may be a confusion between transmitted and reflected light. The scattering of light which is ordinarily transmitted has been shown well by Wood2 in the case of an iodine fog. “In the course of some experiments on the fluorescence of iodine, the precipitation of what appeared to be an iodine fog in one of the glass bulbs was observed. This fog scattered powerfully light of a deep red color, and on examining it with a Nicol prism it was found to be plane-polarized in a direction a t right angles to that which is usually observed in the case of light scattered by small particles. When a powerful beam of light was sent through the bulb in a horizontal direction, the scattered light came off a t right angles, with its direction of vibration (electric vector) horizontal instead of vertical. If the light was polarized before it entered the bulb, the light was scattered laterally in the directions of the vibration in the incident light.. . . . . . The best method of producing the Svedberg: Ber. deutsch. chem. Ges., 38, 3616 (1905). “Physical Optics,” 628 (1911).
.
Wilder D.BancroJt
570
colored fog is to precipitate the iodine upon a smoke-cloud already existing in the bulb.. . . . . . The red fog persists for some time, once it is formed. When at its best, its scattering power is so great that a reddish color is seen within the bulb at a distance of a meter from the arc without any concentration. Examined by transmitted light in a well-lighted room, no trace of color is to be seen, which proves that the red light is selectively scattered and not produced by the absorption of iodine vapor. With the concentrated beam from the arc, the scattered light is blood red and of great intensity. A Nicol placed with its long diagonal horizontal nearly, but not quite, extinguishes it.” Wood1 has obtained some very interesting results while working with heat waves of very great wave-length ( I I O p ) , though it seems to me that his interpretation is not entirely satisfactory. Polished marble reflects over 40 percent of the radiation in question. When reduced to an impalpable powder, and pressed into a flat cake with a smooth surface, it was found to reflect practically nothing, though the irregularities of the surface were much too small to account for the absence of specular reflection. “The particles were apparently so small that the resonance necessary for selective reflection could no longer take place. Similar results had previously been obtained with films of very finely divided aniline dyes which showed colors quite different from those exhibited by continuous films, Metal powders were found to behave in the same way. A film was obtained by shaking reduced copper in a jar and then allowing the finely divided dust to settle from the air upon a quartz plate. This film was absolutely opaque for visible light, yet it transmitted go percent of the longwave radiation, and reflected practically nothing. A continuous film of copper of much less thickness would reflect nearly IOO percent and be absolutely opaque to the radiation.” Wood considers that the particles are not resonating ; but it seems to me that this is another case in which a granular material transmits by resonance the wave-lengths which it 1
“Physical Optics,” 632 (1911).
The Colors of Colloids. I X
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ordinarily reflects selectively. There is no necessary contradiction in the two points of view because Wood is assuming that the particles are too small to reflect selectively by resonance and he is not considering the possibility of transmission by resonance. There are a great many points in regard to the colors of colloidal metals on which I have not touched; but it seems to me t h a t this qualitative, non-mathematical theory is more likely to be helpful as a guide to research than the more elaborate mathematical theories on which this is based. The general results of this paper are: I . With very fine particles the light which is ordinarily reflected selectively is transmitted by resonance, while t h e light which is ordinarily transmitted is scattered. 2 . Massive gold is red by multiple reflection and thin films are green by transmitted light. Very small particles reflect green and transmit red. 3 . Massive gold reflects yellow when compact and brown to black when porous. Particles which do not resonate are yellow or brown by reflected light and transmit blue light. 4. Silver is yellow by multiple reflection and thin films are blue to green by transmitted light. Very small particles reflect blue and transmit yellow. Particles which do not resonate transmit blue (probably blue-green) light and reflect red. It should be determined experimentally whether compact silver reflects red light under suitable conditions. 5 . The effect of porosity has been discussed only for gold. The matter should be studied experimentally with silver, copper, etc. 6. Colloidal indigo solutions transmit red light and t h e surface color of indigo is red. 7 . Sodium fog scatters blue light and transmits the yellow which the vapor absorbs. There is an apparent contradiction with Svedberg’s experiments on colloidal solutions of sodium in ether. 8. Iodine fog scatters red light. Cornell University
