HYDROUS OXIDES AND HYDROXIDES
497
RECEXT X-RAY DIFFRACTION STUDIES ON T H E HYDROUS OXIDES AND HYDROXIDES’ W. 0. MILLIGAN Department of Chemistry, The Rice Institute, Houston, Texas Received January 24, 1950
The structures and properties of hydrous and hydrated oxides and hydroxides are of continued interest because of their importance in both pure and applied chemistry. This paper is concerned with recent work on the hydrous oxides and hydroxides at The Rice Institute and elsewhere, which has been carried out since an earlier survey (36) of the literature was made in 1942. The discussion will be limited primarily to x-ray and electron diffraction studies on single oxides such as cupric oxide, alumina, chromia, and the trivalent oxides of indium, scandium, and the rare earths, and dual or multiple oxide systems such as SiO-XI203, BeO-InpOa, Cr203-Fe203, Cr203-Zr02, SO-Cr203, and SiO-Cr*O3-Zr02. CUPRIC OXIDE AND HYDROXIDE
Blue gelatinous cupric oxide exhibits an x-ray diffraction pattern distinct from that of broivn or black hydrous cupric oxide. Phase rule dehydration isotherms and isobars (37) indicate that the pure blue gel is hydrous cupric hydroxide, lvhich is \-ery instable and decomposes to form brown to black hydrous cupric oxide, especially in the presence of traces of alkali. It is well known that a slight excess of cupric sulfate stabilizes the blue color (33). The blue gel stabilized with excess cupric sulfate exhibits an x-radiogram distinct from that of cupric hydroxide or any of the known basic cupric sulfates (25, 26). X-ray diffraction and electrometric titration methods demonstrate (37) that the stabilized blue gel is actually a new basic cupric sulfate, 5CuO.SO3.zH20. This conclusion has recently been confirmed by Feitknecht (3), who kindly sent to the author the electron photomicrograph shown in figure 1. “AMORPHOUS”
ALUMINA
It has been previously shown (cf. 36) that alumina precipitated from nitrate or chloride solutions consists of minute crystals of hydrous r-AlOOH, whereas alumina precipitated from sulfate solution at a pH value below 5.5 is amorphous to x-rays. Alumina precipitated from sulfate solution a t pH values greater than 5.5 consists of r-AlOOH. Under alkaline conditions some a-.41(OH)a may be formed. Electron diffraction examination of alumina from sulfate solution a t pH values of 5.5 to 6.5 gives relatively sharp diffraction lines characteristic of r-hlOOH (36), in contrast to the broad and diffuse bands detectable by x-ray diffraction methods. The sharper lines observed by electron diffraction are at-
’
Presented at the Symposium on thestructure of Hydrous Oxides, which was held under the auspices of the Division of Physical and Inorganic Chemistry a t the 116th Meeting of the American Chemical Society, Atlantic City, S e w Jersey, September 19-23, 1949.
498
W. 0. MILLWAN
tributed primarily to the greater efficiency of diffraction of the electrons (32) and t,o a lesser extent to the shorter wave length. Recently Cole and Jackson (1) ohrained relatively sharp electron diffrnction patterns for gels of basic ferric and aluminum phosphates, which mere essentially amorphous to x-rays. “Amorphous” alumina, precipitated a t room temperature from the sulfate a t pH values of approximately 4 and 6, ages in the boiling mother liquor to form crystals (30) of a basic aluminum sulfate (AlSO3.8O3.I-ZHO) and 7-AIOOH, respectively. The format,ion of distinctly different products by aging suggests that the original gels are likewise distinct materials. The original amorphous gel formed at low pH values may age to form relatively large crystals according
to one of the following mechanisms (“7): (e) The t.ransit.ion during the aging period of an amorphous material into a crystalline one of the same chemical composition. ( b ) The growth of extremely fine crystals or crystallit,es, initially too small OT too few in number to be detected by x-ray examinetion, the initial and end produets of the process k i n g of the same chemical composit,ion. (e) The reaction of “amorphous” alumina with sulfate ion8 in the mother liquor to form the basic sulfate. ( d ) The formation of B cryst,alline hasic sulfatehy hydrolysis of aluminum ions already prrrent in the mother liquor because of incomplete precipitation a t such low pH values, or because of desorption from the gel as a result of the aging process. Current work on this problem is concerned &h obtaining monochromatic x-ray diffraction patterns of the unaged amorphous gels precipitated a t pH
HYDROUS OXIDES
AND
nYnnoxInEs
499
values of 4 and 8. Bath quartz and sodium chloride monochromators have been employed, with exposure times of 100 hr. for “Xo-Screen” and 2W hr. for “Type A” x-ray film. Although the amorphous bands (figure 2) for the two ssmples appear to he somewhat different, and the results suggest that mechnnism (a) may be the mast probable, further studies are in progress.
F I G2. Microphotometer traeingaoi monoohromstio x-ray diffreotion patterns of unnged niurnioil precipitated from aulfate solution. ALUM1NT:M TRIlfYDBOXIDES
Two definite aluminum trihydronidesare recognized (36): y - A l ( 0 H ) ~orgibbsite and a-AI(OH)a or bayerite. The crystal structure of gibbsite has been carefully determined (lo), but the structure of bayerite in doubtful. Numerous invcstigators hare examined bayerite, but it has been difficult to be certain that the samPIPS are not contaminated wit,h gibbsite. Attempts to prepare pure single crystals of bayerite have heen unsuccessful. More recently, Montoro (22) reiixaminecl the x-ray diffraction data for bayerite, and concluded that certain of the diffraction lines xere characteristic of gibbsite. Montoro rejected the diffraction lines which Ire at,tributed to gibbsite, and attempted to deduce the structure of bayerite from the remaining lines. He concluded that bayerite is hexagonal, with two molecules of AI(OH)z in a unit cell of dimensions aa = 5.01 A,, co = 4.76 A,, e = 0.95. This unit cell implies an x-ray density of 2.49, comparing favorably with an experimental (6) vdue of 2.529. Montoro’s proposed strurture for bayerite may be described as H layer structure similar to that. of gibbsite, except that the pseudohexagonal arrangement of the anions in gibbsite is replaced by a. regular hexagonal arrangement in bayerite. Montoro (22) rejected an intense line a t about 4.36 A. This diffraction line cannot be accounted for on the basis of Montoro’s propused st,ructure. Recent experiments have been carried out. at, The Rice Institute with the object of
sw
W. 0. MILLIQAN
determining whether or not the 4.36 A. line is attributable to bayerite. Samples of hayerite have heen prepared by slow aging of “amorphous” alumina under water a t room temperature and by other methods. In dl instances, the 4.36 A. line appears concurrently with other diffraction lines, especially the 4.76 A. line, which definitely is part of the bayerite diffraction pattern. The ratio of intensities of the 4.36 1.and 4.76 1.lines is always constant within experimental error. Dr. Allen S. Russell (28) of the Aluminum Company of America has informed us that he alsa considers the 4.36 A. line to he a part of the bayerite pattern. The patterns (figure 3) of our purest bayerite agree well with that of samples considered by Russell to be pure bayerite. It is therefore concluded that Mont,oro’s struct,ure is not correct. The structure of hayerite will be referred to below in connection with the structure of chromic hydroxide, Cr(OII)3. CHROMIUM
TnlnYDROXlDE
Procipitated chromia gels have been usually considered to be amorphous. Recently (14, 10) it was found that air-dried chromiu gels precipit,ated from nitrate solutions are crystalline. The x-radiogram is distinct from that of an-
R e . 3. X-ray dinraetiori patterns of e-AI(OH), (bsyovite)
hydrous chromic oxide and the monohydrate (CrOOH) of Simon, Fiucher, and Schmidt (31)prepared in a bomb a t hi& temperature and pressure. The diffraction pattern closely resemhles that of hayerite (figure ?) (14, 16). A dehydration isobar ( I G ) demonstrates that the airdried material is actually hydrous chromic hydroxide, rvhich decomposes readily to form hydrous ebmmic oxide. More recently, J h . W. ?;.Lipscomb (8) has prepared single Crystals of chromic hydroxide, but x-ray structural determinations have not bcen made as yet. The chromic hydroxide gel in the form of extremely small cryst,als made by precipitation and air-drying is extremely unutahlc, decomposing a t room temperature, itti shown in figure 5. The larger single crystals of Lipscomb (8) are stable in air a t room temperature. The two strong ditrraetion lines a t the highest d/n values compare so closely with the similar linea of the bayeritc pattern that it is assumed that the stmetures of the Cr(OH)a and n-.kl(OH)~are closely related, and that the 4.36 A. line is definitely part of the n-A1(OH)r pettern (figure 4). It is hoped that the structure of a-AI(OH), e m be deduced as 8oon as the complete structure of chromic hydroxide is determined from measurements on single erystala.
501
HYDROUS OXIDES AND HYDROXIDES INDIUM AND SCANDIUM TRIHYDROXIDES
Several years ago (11, 20) it was observed that precipitated indium oxide consisted of hydrous h ~ ( o H )It ~ .was found that the crystals were cubic with a. = 7.95 A., and eight molecules of In(0H)S in the unit cell. The x-ray density of 4.35 agreed well with the experimental value of 4.1. More recently Moeller and Schnielein (21) and Palm (23) have made x-ray diffraction studies on indium trihydroxide. Mise Palm obtained a value of a0 = 7.90 k,and found eigkt molecules of In(OH)3in the unit cell. Fricke and Seite (4) found a0 = 7.92 A. with an x-ray density of 4.45. In our laboratory microscopically visible crystals of indium trihydroxide were found to have deposited from an indium hydroxide sol that was 10 yr. old. In a more recent publication Schubert and Seite (30) have examined indium trihydroxide by single-crystal x-ray methods. Their pre-
WOW3 AIR-DRY
uhc,
IO"
20"
30"
I
A -
40"
I -
50°
28
Cr203.xH EAYS 0
AIR-DRIED30 60°
70"
8
FIG.4 . X-ray diffraction patterns of cu-AI(OH)3 and Cr(OH)3
cise value of a. = 7.923 0.005 i. and a pycnometric density of 4.41 require eight molecules in the unit cell. Fricke and Seite (4) have been able t o prepare crystalline scandium trihydroxide by digestion of the precipitated gel in 12 M sodium hydroxide solution in an autoclave a t 160°C. Their dehydration isobar clearly establishes the existence of Sc(0H)3 and likewise indicates the formation of ScOOH, previously found in this laboratory (34). It appears likely that the freshly precipitated gel is hydrous ScOOH, which is transformed in the presence of hot alkali solution into Sc(OH)r, in a manner analogous to the transformation of hydrous y A 1 0 0 H to cy- or yAl(0H)a. Fricke and Seite (4) found scandium trihydroxide to be isomorphous with indium trihydroxide, with a0 = 7.88 -1.and eight molecules of Sc(0H)r in the unit cell, agreeing with a pycnometric density of 2.68. Schubert and Seite (30) have recently examined scandium trihydroxide by singlecrystal x-ray methods. These investigators concluded that the space group is
502
U’. 0. MILLIGAN TIME
DRIED OVER Pz05 0.4 Day
1.5 Days
1.1 0.ys
5.5 Days
6.5 Days
15.0 Davs
11.0 Dayr
0
5
10
15
20
l i n e , Day8
R o . 5. llato of decomposition of chromium tiihydroxide ever water and over phosphorue pentoxide.
503
HYDROUS OXIDES AND HYDROXIDES
T: with scandium atoms in the eight (c) positions and the oxygen atoms in the twenty-four ( 9 ) positions, with the parameters y = 0.307 f 0.005 and z = 0.182 f 0.005. A value of = 7.882 f 0.005 b. and a pycnometric density of 2.65 require eight molecules in the unit cell. Schubert and Seitz consider indium trihydroxide and scandium trihydroxide to be isomorphous and point out that their structure is closely related to that of Reo3. Quantitative x-ray studies have not been made on ScOOH, but it has been observed that the powder photographs of ScOOH and y A 1 0 0 H are closely similar (35). RARE EARTH HYDROXIDES
Precipitated lanthanum oxide was found to have the composition La(0H)3 by Hiittig and Kantor (7) in 1931, using isobaric dehydration methods. Some indication of a monohydrate was observed. Later Weiser and Milligan (35) found from dehydration isobars that precipitated neodymium and praseodymium oxides were hydrous N d ( 0 H )3 and Pr(0H)a. The dehydration isobars indicated also the existence of NdOOH, and possibly PrOOH. More recently Fricke and TABLE 1 Data on trihudroxides of the rare earths ATOMIC NUMBER
u
57 59 60 62 64 (39) 66 68
3.55 3.55 3.52 3.54 3.54 3.55 3.53 3.53
C
0.565 0.566 0.561 0.564 0.566 0.565 0.563
1
X-PAYDERSIIY
4.78 4.74 4.93 5.37 5.75 3.81 5.83
Seitz (5) have established by dehydration isobars the existence of La(OH)3, Pr(OH)3, Er(OH)3, Y(OH),, Sm(OH)a, Gd(OH)3, and Dy(0H)a. These investigators obtained evidence for the existence of LaOOH, NdOOH, YOOH, SmOOH, GdOOH, and DyOOH. S o indication of PrOOH was found. In earlier work (35) the evidence for PrOOH was inconclusive, in contrast to the evidence for NdOOH. Marsh (9) believes that a partial oxidation of Prz03 to Pro, may account for the slight inflection found in the dehydration isobar. The crystalline hydroxides prepared by Fricke and Seitz were made by the aging of precipitated oxides in a sealed tube for 25 hr. a t 20O0C., in contrast to the author’s investigations which were carried out on the freshly precipitated gel dried in air. In this earlier work the oxides of praseodymium and neodymium were definitely crystalline, whereas the oxides of samarium and yttrium were amorphous. More detailed work must be carried out before one can answer the question as to whether the unaged amorphous oxides of samarium and yttrium are amorphous hydrous oxides or amorphous hydrous trihydroxides. Fricke and Seitz (4) found that all of the rare earth trihydroxides studied are hexagonal, with two molecules of M(OH)3 in the unit cell. These data are summarized in table 1.
504
W. 0. MILLIGAN
Single crystals of yttrium trihydroxide were studied in more detail by Schubert and Seitz (30), using single-crystal x-ray diffraction meth:ds. The dimensions of the hexagonal cell were found to be a0 = 6.24 f 0.01 A. and co = 3.53 0.02 b. The space group was found to be &. Quantitative x-ray diffraction studies have not been made on the monohydrates of the oxides of lanthanum, neodymium, yttrium, samarium, gadolinium, and dysprosium. MULTIPLE OXIDE SYSTEMS
Multiple oxide systems have been extensively investigated by numerous techniques during the past decade. When mixtures of two or more oxides are formed simultaneously by coprecipitation or otherwise, and then are subjected to heattreatment a t elevated temperatures, several phenomena may result: (a) the product may consist of a simple mechanical mixture of the separate oxides possessing the additive properties of each constituent; ( b ) the presence of one or more oxides may retard or prevent the crystallization of the other, resulting in an enhancement of surface properties; (c) a solid solution may occur; ( d ) one or more compounds may be formed; or ( e ) some combination of the above processes may take place. In this laboratory several systems of mixed or multiple oxides have been studied and examples of many of the above possibilities have been observed. MUTUAL PROTECTION
In the system CuO-FepOs it was observed (12) that 10-30 mole per cent of ferric oxide prevented or retarded the crystallization of cupric oxide in mixed gels precipitated and dried a t room temperature. At lower concentrations of ferric oxide, there was observed the x-radiogram of cupric oxide alone, the results indicating that the cupric oxide can also retard the crystallization of the ferric oxide. This phenomenon, designated as mutual protective action, mas observed to persist a t temperatures as high as 1000°C. In the system CuO-FesOs, the existence of the compound C u 0 . F e z 0 3was confirmed a t higher temperatures. The system Ni0-AlZo3 (figure 6) yields similar results, the mutual protective action being still pronounced at 5O(r1O0OoC.However, equimolar mixtures of N O and Also3reacted a t 1000°C. to form NiO.Al2O3,which formed a solid solution with each of the pure oxides. The system F e ~ O d k ~ also 0 3 exhibits the phenomenon of mutual protection (14). Because of the similarity in the structures of a-Fe203and chromic oxide a continuous series of solid solutions were found a t temperatures aboye 500"C., in confirmation of the results of Passerini (24). Mutual protective action has also been observed (38) in the following systems: A120aBe0, A1203-Zr02, A1203-Bi203,and A1203-Sn02. In these systems it was found that 10-30 mole per cent of either constituent tended to retard or prevent the crystallization of the other constituent. In addition to the mutual protective action the results suggest the existence of two zones of composition wherein maximum protection occurs. In the zones of protection the gels are amorphous
505
1 l Y U I K ~ l SO l l U E b .\XU WYDKOXIDES
t,o x-rays or consist of extremely tinely divided crystals which may correspond to regions of maximum surface a n d \vhich, therefore, may exhibit enhanced sdsorptirr and cattdytic propcr1,ies. tern it h:m I m n found (17) that samples containing as of hcryllium oside aftcr heat-treatment at temperatures as high as lO0O"C. esliilit. only an s-wtliogrum correspondir.g to indium oxide. The.= rciiults haie lreen recently eonfirmed by Ensslin and Valentiner ( 2 ) . It is assumed that the beryllium oxide forms :in interstitial solid sohition with the indium oxide. I n our earlier insestigations (17) and in those of Ensslin and Valen-
100%
11
0%
2:O%
70%
Pic. li. Zlo,,ur.hiom:i!ic I~.~ii!iricn! fin 2
lu
y . 1 ' ' ~dif7r:tction ~
p:i!!erm i n tbr, ~ p l r r nSiO-AlnOJafter tieat-
a! 5!13'C. (1&1110 mole pcr cent iiirteluua
.ride).
liner. (2) no indic:rtioo of eompound formation nas ohserved. In current work it has been found that at ahout 5 W C . a new crystalline phase occum at an equimalar ratio of h Ilium wide and indium oxide. At lower temperatures the samples m e amorphoiis, and at higher tempratures the ncw phase decomposes l a givr the interstitial solid solution of BeO-In& The x-radiogram of RrO.InzO, or BrInA), is not similar to known spinels or to chrysoberyl (BeO,.AI&). Homerev, the pattern resembles somewhat that of CaO.InlOl, whirl1 Ensalin and Valentiner consider to he tetragonal (Hausmanite type). Further studies on BeO.InzOaare in progress. I n the system CrdkZrO. an indication of solid solution has been observed.
506
FV. 0. MILLIGAN
In the system ;C'iO-CrzOa at temperatures of heat-treatment of 500" and 600°C., 30 mole per cent of chromic oxide retards the crystallization of nickelous oxide, whereas smaller or larger amounts are not as effective. THE SYSTEM
Ni0-CrZO3-ZrOz
Sixty-six gels in the system X0-CrzO3-ZrO2 have been systematically examined (18) by x-ray methods a t a temperature of heat-treatment of 500°C. In the ternary diagram four principal regions mere observed. In the high chromic oxide region the pattern of chromic oxide alone was obtained. In the high nickelous oxide and high zirconium oxide region the patterns of nickelous oxide alone and zirconium oxide alone, respectively, were observed. In the NiO-ZrOz portion of the system, samples near the equimolar ratio yielded diffraction patterns corresponding to a mixture of crystals of nickelous oxide and zirconium oxide (tetragonal). h large area of the ternary diagram yielded amorphous x-radiograms. It is assumed that the amorphous region will decrease in area as the temperature of heat-treatment is increased. Other mixed oxide gels being studied a t present (15, 19) include the system M203-VZ06,where M is scandium, lanthanum, thallium, indium, gallium, iron, chromium, aluminum, rhodium, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, erbium, terbium, thulium, yttrium, or lutecium. The freshly precipitated gels are essentially amorphous to x-rays, but heattreatment to temperatures around GOO-1000°C. results in the formation of heavy metal orthovanadates. The orthovanadates of the rare earth metals form a tetragonal isomorphous series. The author is grateful to Mr. James L. McAtee, Jr., and Mr. Olin B. Cecil for the results of their recent studies on alumina and the nickelous oxide-alumina system, respectively. REFEREXCES (1) COLE A N D JACKSON: Abstracts of the Twenty-Third Sational Colloid Symposium, Minneapolis, Minnesota, June 6-8, 1949, p . 6. (2) ENSSLINA N D VALENTINER: Z. Naturforsch. ab, 5 (1947). (3) FEITKNECHT: Private communication, December 16, 1948. (4) FRICKE A N D SEITZ:Z.anorg. Chem. 266, 13 (1947). (5) FRICKE A N D S E I T ZZ. : anorg. Chem. 264, 107 (1947). ASD S E V E R I N Z :. anorg. Chem. 206, 287 (1832). (6) FRICKE (7) HUTTIGA N D KANTOR: 2.anorg. Chem. 202, 421 (1931). (8) LIPSCOMB: Private communication. (9) MARSH:J. Chem. SOC.1946, 15, 17, 21. (10) MEGAW: Z.Krist. 87, 185 (1934). (11) MILLIGAN: Abstracts of the 96th Meeting of the American Chemical Society, .Milwaukee, Wisconsin, September 5-9, 1938, p. P-20. (12) MILLIGAN A N D HOLMES: J. Am. Chem. SOC.69, 149 (1941). ASD MERTES:J. Phys. Chem. 60, 465 (1946). (13) MILLIGAN (14) MILLIGAN A N D MERTEN: J. Phys. R: Colloid Chem. 61, 522 (1947). (15) MILLIGAN, RACHFORD, A N D WATT:J. 41x1. Chem. Soc. 70, 3953 (1948). (16) MILLIGASA N D WATT:Abstracts of the 110th Meeting of the American Chemical Society, Chicago, Illinois, September 9-13, 1046, p. 14E.
LIGHT ABSORPTION DUE TO ELECTRON EXCHASGE
507
(17) MILLIGAN A N D WATT:Abstracts of the 2nd Texas Regional Meeting of the American Chemical Society, Dallas, Texas, December, 1946. (18) MILLIGAN ASD WATT:J. Phys. & Colloid Chem. 62, 230 (1948). J. Phys. & Colloid Chem. 63, 227 (1949). (19) MILLIGAN,WATT,ASD RACHFORD: (20) MILLIGAN.AND WEISER:J. Am. Chem. SOC.69, 1670 (1937). A N D SCHNIZLEIN: J. Phys. & Colloid Chem. 61, 721 (1947). (21) MOELLER (22) MOSTORO:Ricerca sci. 13, 565 (1942). J. Phys. & Colloid Chem. 62, 959 (1948). (23) PALM: (24) PASSERINI:Gam. chim. ital. 60, 544 (1930). A N D TUNELL: Am. J. Sci. 16, 1 (1929). (25) POSNJAK A N D TCNELL: J . Phys. Chem. 36, 929 (1931). (26) POSNJAK MILLIGAS, MCATEE,A N D DRAPER: Abstracts of the 3rd Southwest Regional (27) PURCELL, Meeting of the American Chemical Society, Houston, Texas, December 12-13, 1947, p . 12. (28) RUSSELL:Private communication. June 9 , 1949. ASD SEITZ:Z. anorg. Chem. 264, 116 (1947). (29) SCHUBERT A N D SEITZ:Z. anorg. Chem. 266, 226 (1948). (30) SCHUBERT ASD SCHMIDT: Z. anorg. Chern. 166. 107 (1930). (31) SIMOS,FISCHER, .AND COCHRANE : Theory and Practice of Electron Diflractz'on, p. 97. .lfacmillan (32) THOMSON and Co., Ltd., London (1939). (33) WEISER:J. Phys. Chem. 27, 501 (1923). : Phys. Chem. 42, 669 (1938). (34) WEISER A N D ~ ~ I L L I G A S J. (35) WEISERA N D MILLIGAS:J. Phys. Chem. 42, 673 (1938). Advances i n Colloid Science, Vol. 1, p. 227. Interscience Pub(36) WEISERA N D MILLIGAN: lishers, Inc., Xew York (1942). (37) WEISER,MILLIGAX,A N D COOK:J. Am. Chem. SOC.64, 503 (1942). ASD S , MILLS:J. Phys. & Colloid Chem. 62, 942 (1948). (38) WEISER,~ ~ I L L I G A
LIGHT ABSORPTIOX AS A RESULT OF AN INTERACTION OF TWO STATES OF F'ALESCY OF T H E SAME ELEMENT W. A. WEYL Department of Mzneral Technology, School of Mzneral Industrzes, The Pennsyluanaa Slate College, Slate College, Pennsylvania Receaved Aprzl 14, 1960 I. ISTRODUCTION
.J. F. Gmelin (1779) observed that blue colors result in ceramic glazes under reducing conditions. The blue color is the result of iron, but it seemed unacceptable to most scientists that it resulted from ferrous ions alone. Ferrous ion in aqueous solution and in certain glasses is a strong infrared ahsorher, but it has little absorption in the visible region. J. J. Berzelius pointed out that the deep color of the ink made from galls might be due to the formation of a double salt containing both the lower and the higher oxide of iron. He drew the analogy between it and the blue color obtained from the complex cyanides of iron. When A. Werner made his study of the complex platinum compounds, he noticed that the color was considerably deeper whenever the salt contained platinum in dif-
