securing of well-oriented fiber patterns of both crystalline forms in order t o calculate their respective identity periods along the fiber axis. Their results, which are probably the most reliable published to date, give the alpha form of the hydrocarbon an identity period of 8.78 * 0612 A. and the beta form an identity period of 4.87 * 0.07 A. More recently Bruni and S a t t a ( 1 ) have examined gutta-percha by means of electron rays and have given a solution for the alpha' form in which they find the unit cells to be ogthorhombic and to have $he axes: a = 6.53 A., b = 7.68 A , and c = 9.60 or 4.80 A. (fiber period). They find the number of isoprene units (CsHs)per cell to be four. The present work relates to the investigation of vulcanized gutta-percha hydrocarbon by means of x-rays. KO record of such a study has been found in the literature, although it is well known that vulcanization of gutta-percha with sulfur leads under certain conditions of cure to a material possessing distinct rubber-like properties when warmed (14). Preliminary tests on samples of gutta-percha vulcanized for various times showed that after "melting"-i. e., heating until translucentthe samples remained amorphous for a time which increased with the time of cure. Samples previously melted and exposed to x-rays in the stretched condition a t room temperature showed the orientation characteristic of the cold drawn samples but lost their reversible stretch during the time of exposure. However, the fiber patterns of the stretched vulcanized samples showed a considerably higher degree of orientation than was generally obtained by stretching guttapercha alone. Therefore, these fiber diagrams were studied in the light of the work on gutta-percha and balata just summarized with the hope that more information on the cryi;tal structure of these hydrocarbons might be obtained.
GUTTA-PERCHA EFFECT OF VULCANIZATION ON ITS X-RAY DIAGRAM C. S. F'ULLER Bell Telephone Laboratories, New York,N. Y.
ARLT x-ray investigations by Clark (8) and Ott (13) on gutta-percha and balata established the fact that these materials were crystalline in nature even in the unstretched condition. I n 1927 Hauser and his co-workers (5) pointed out the similarity of the diagrams obtained on gutta-percha and balata, but it remained for Hopff and Susich ( 7 ) to establish definitely the identity of the x-ray diagrams of the two substances. In addition they brought out the interesting fact that, on heating above 60" C., a characteristic change took place in the gutta-percha diagram. Hauser and Susich (6) made a detailed study of this change. They showed that gutta-percha (and hence also balata) underwent a change in crystalline structure at about 68" C. The beta niodification, obtained above this temperature, did not transform back into the lowtemperature or alpha form, but remained unchanged unless the cooling took place very slowly through the transition point. The last-named authors gave much attention to the
X-Ray Technic The source of the x-rays was a Philips' Metalix x-ray tube with a copper target (X = 1.539 A,). The tube was operated at approximately 25 kilovolts and passed a current of 15 milliamperes. A nickel filter 0.015 mm. in thickness was employed to absorb the K beta radiation of copper. The lvave length corresponding 1 Bruni and Natta ( I ) list their data as applying to the alpha form of gutta-percha but employ the identity period of 9.60 which corresponds not to the alpha b u t to the beta modification. Since these authors find the alpha spacing 3.3 A , , it is probable that their samples contained both modifications. I n view of the fact t h a t the layer-line reflections cannot be indexed when the identity period given by Hauser and Susich for alpha gutta-percha is employed, i t seems improbable that the unit cell of Bruni and Natta can be correct for alpha gutta-percha. I n addition, in the present work it is shown that this unit cell does not agree with the x-ray data for beta gutta-percha.
A partial transformation of the beta to the alpha form of gutta-percha results by stretching at 80" C., although the exact conditions under which this occurs have not been determined. The identity period in the fiber direction of the beta modification is 4.77 =t 0.03 A., or double this value, and the alpha modification presents an anomaly in that two identity periods are in best agreement with the data. These are 9.00 * 0.05 and 8.70 * 0.13 In the case of the beta modification three possible orthorhombic unit cells which are in agreement with the observed lattice plane spacings are given.
The finding of previous investigators that gutta-percha and balata have identical x-ray patterns is verified. Experiments on the x-ray behavior of vulcanized and unvulcanized gutta-percha show that vulcanization (to the extent carried out here) has no effect in changing the lattice plane spacings of either the alpha or beta crystal modifications. Vulcanization does appear to increase the degree of orientation of the crystallites present in these substances as produced by stretching and to that extent allows a more accurate calculation of the identity periods of the crystalline forms to be made.
A.
90;
FIQUHE2. MIXED FIBERDEAGRAM OF ALPEA AND BETA GUTTA-PERCHA, N O T VULCANIZED Filni distnnoo. 30.6 m u .
to the maximum intensity of the continuous x-ray spectrum was evident on certain photograph but, it is believed, in no case led to an erranems measurement. The cameras employed were similar to those described by Ketz (8). The distance of the sample from t.he photographic film was varied between 3 and 5.5 em. and was checked by two methods: (a) B Debye-Schemer photograph of a bar of finely powdered sodium chloride crystals and (6) accuretely measured stops which could he inserted between the sample holder and the film support. When corrections were made ior the thickncss of the sample of sodium chloride rtnd for the thickness of the black a er covering the film, the two methods checked to 0.02 mm. o ders of brass were provided for the samples sa thst the" could be maintained in a condition of maximulh stretch during the exposure. The time
in the proportions given, but were not subjected to cure. X-ray photographs of these latter samples showed reflections due to the uncombined sulfur crystals superimposed on the gutta pattern, but otherwise were the same as those of the uncompounded gutta-perchas. This effect is sliown in Figure 1. A few photographs n-ere also taken of samples of deresinated balata (containing approximately 1 per cent resin) prepared from tlie latex of .b'irnusops Gamin. These verify
%f
&t in view of theaize'of the reflections obtained. This precision, on separate cheek photographs at different plate distances, w s approximately 1 per cent of the distance being measured and in many cases was less than 0.5 per cent of rhis distance. The percentage error for measurementsof the same photograph was less than 0.5 per cent except in rare cases.
Preparation of Samples Two grades of gutta-percha were eniployed: (a) a pure white Tjipetir hydrocarbon containing 0.05 per cent p-toluidine as antioxidant (this material was obtained in vacuum tins direct from the Dutch Government Tjipetir Plantation), and (b) a fourth-grade gutta-percha with a somewhat higher resin content ( I per cent) and 0.05 per cent hydroquinone. Vulcanized samples were prepared from both grades and had the composition: gutta-percha, 94 per cent; sulfur, 5: zinc butyl xanthate ( a c c e l e r a t o r ) , 1. These samples were cured a t 142' C . for times raiiging from 15 minutes to 3.5 hours. A set of control saniples was also prepared from each of the grades of gutta-percha. They contained t h e h y d r o c a r b o n , s u l f u r , a n d accelerator
TABLEI. CoMPARlSoN OF IATlTCE PLANE SPACLVQB OF SAMPLES OF VGLCANlZED AND UNVULCAhTZED GUTTA-PERCHA, IN ANRSTR6M UNlTS
IleUectioo'
1h
2c
7.92
..
5:s;
..
..
4:7l 3.92
4:73 3.90
3.35 3.00
2:95
2.80 2.38
2.73 2.36
1:is .. .. ..
.. .. .. .. ..
2:95
2:sl
.. ..
.,
.. .. .. .. ..
.. .. .. .. .. ..
... .
3d 7.97 6.90 5.94
5.01 4.71
3.91 3.31 3.00
2.80 2.37 2.12 1.95
4.44 3.90 3.50 3.12
2.92
..
5:08
4.55
4.05 3.32 2.72 3.50
4'
..
.. ,.
4:ia 3.89
2:ss 2.74 2.36
1:i
.. .. ..
2:94
.. .. .. .. .. .. .. ..
5f 7.87
60
6.80 5.92
6175
4:75 3.91 3.32 3.00 2.79 2.38 2.12 1.96 4.45 3.84 3.48 8.14 2.95 2.42 2.25 5.13 4.55 4.04
3.32 2.72 3.48
71
8.
.. .. ..
7.85
4.75
4:76
4.73
3.33
3.30
2.78 2.38
2.78 2.37
1:95 4.46 3.88 3.61
..
5.94 4.93 3.91
2.98
3.14 2.94
2.43 2.25 5.05 4.52 4 05 3.33 3:is
3.92 2.99
,.
..
..
.. .. .. .. .. .. .. .. .. ..
..
5:92 4.88
3.93 3.33 2.94 2.74 2.35 1:s3
4.44
.. ..
2:93
..
5:o; 4.68 4.00 3.32 2.09 3.50
INDUSTRIAL AND ENGINEERING CHEMISTRY
910
VOL. 28, NO. 8
Likewise there is no x-ray evidence which allo\\b a definite TABLE 11. -AVERAGE VllLUES OF LATTICE PLANE SPACINQS choice between the values 4.77 and 9.54 for the fiber period, AND IDENTITY PERIODS OF BETA GUTTA-PERCHA COMPARED since the accepted chain molecule structure for gutta-percha WITH PREVIOUS WORK, I N ANQSTROM UWTS may possess a glide plane of symmetry parallel to the chain Present Hauser and Bruni and Stillwell axis. The larger unit cell of Figure 5 was chosen here as Reflection Work Susicha Natta and Clark d IP d IP d IP d the basis for index assignment because it allows all of the ?dib 9.35 .. .. .. .. .. .. data listed in Table I1 to be included. Assuming a density Bsb 6.75 ,. .. .. .. .. .. (10) of 0.982 for crystalline gutta-percha, we obtain 15.4 5.95 .. . .. .. .. (approximately 16) isoprene groups as the number occupying this unit cell. This unit cell has a volume approximately four times that found by Bruni and Katta since the a and b axes are each about twice those found by these investigators. 2.12 .. .. .. ,. .. Table I11 gives the indices found for each point by means A16 2.02 .. .. of the graphical analysis, the comparison of the observed Ais 1.95 .. .. .. 1:90 :: .. 4.56 4.91 4 45 4 79 and calculated lattice plane spacings, and the relative inI11 .. 3.88 4.94 .. .. 3 87 4 78 112 tensities of the observed reflections for a cell of these dimen.. .. 3 . 5 1 4 80 113 3 : i 7 4:83 3 . 0 8 4 75 114 sions. The following formula has been used for the calcula.. 2:95 2.89 4.79 2 93 4 79 11s 2 42 4 78 tion of the lattice plane spacings: 11~ .. .. ,.
2 : 22 2.42 2.04
2 25 4 78 117 2 36 4 72 2 02 4 74 IVl Average identity period (c axis) : Present work Hauser and Susich Bruni and Xatta
IVo (diatrope)
4.77 4.87 4.80
*
0.03 0.07
a The lattice plane spacings for the layer-line points were calculated from data given by these authors (5,p. 149). b It is uncertain whether these reflections belong t o the alpha or the beta diagram.
TABLE111. TICE
COMPbRISON OF OBSERVED AND CALCULATED LATPLANESPACINGS FOR BETAGUTTA-PERCHA'
Reflection ?Aie A, 0 A4 ?A5
A7
AS l l 0 C
Ai1 d i 2
AirC Ais Ail
I11 I12 118 I14 I16 11s I11
--
IVO IV1
hkl 110 2 10 020 300 3 10 220 400 320 330 040 510 240 430
dcaled. (A.) 9.48 6.55 5.95 5.23 4.78 4.73 3.92 3.93 3.15 2.97 3.03 2.78 2.79
dobavd. (A,) 9.35 6.75 5.95 5.22 4.73
720 540 450 060 640 800 012 2 12 302 032 412 2 42 432 612 342 004 414
2.10 2 16 2 03 1.98 1.96 1.96 4.43 3.86 3.52 3.05 2.93 2.40 2.41 2.25 2.27 2.37 2.01
-
Relative Intensityb
vw
VW VW
VW
vvs
3.91
VS
3.15 2.98
VW
2.78
S
2.12
V \T
2 02
vw
1 95
M
4.45 3.87 3.51 3.08 2.93 2.42
hl
= hZ (0.00407) $. k* (0.00706)
+ l 2 (0,0110)
KO fiber diagrams in which it was certain that the alpha form of gutta-percha was present alone were obtained. For this reason no attempt has been made a t a complete treatment of this modification. The measurements of the reflections, however, indicate certain peculiarities that have not been observed heretofore. Table IV gives the average lattice plane spacings and identity periods along the fiber axis calculated for the reflections not included in the beta diagram, together with those of Hauser and Susich (6) and Bruni and Natta (1) for comparison. Table IV shows that the identity periods calculated for the various layer-line reflections do not agree well with each other or with the data of Hauser and Susich. Since the deviations appear to be greater than the experimental error of measurement, it is difficult to regard all of the reflections as arising from the same crystalline form. Attempts to apply the graphical method to the solution of this fiber pattern employing an average value for the identity period failed to account satisfactorily for all of the observed reflections. It is rather surprising, however, that all of the equatorial reflections except A s are readily indexed by the scheme found to apply to the beta diagram.
Discussion The influence of sulfur in producing better orientation of the crystallites in gutta-percha is interesting in the light of the theories of rubber vulcanization. Apparently no parallel exists in rubber in which case vulcanization appears to require
M
w V \v W
M W
2.25
W
2.36 2.02
W W
TABLEIV. AVERAGEVALUESOF LATTICEPLANESPACINGS AND IDENTITY PERIODS OF ALPHAGFTTA-PERCHA COMPARED WITH PREVIOUS WORK, IN ANGSTR~W UNITS
a 15.7b.; b = 11.9A.i c 9.54A.; 8 3 90'. b V very; W = weak. 6 = strong; M = moderate. 6 It is uncertain whether 'these reflections belong t o the alpha or the beta diagram. 0
1 5
d A. --_
As
As
4. --.
7 91
4.95 3.91
3....33
5.08 4.54 4.05 3.32 IS 2.72 111 3.49 Illl 2.64 -4verage identity Present work I1
B.,
percha by this method resulted aqfollows: a = 15.7 or 7.85 b = 11.9 A,, c = 4.77 or 9.54 A. (fiber period). An orthorhombic cell is indicated. Figure 5 illustrates the relation of these unit cells to one another. It is diflicult t,o choose between these various unit cells on the basis of the observed x-ray spacings alone. However, the larger spacing for the a axis is favored by the presence of the reflection IIaand by the fact that it agrees better with ?ne of the reflections observed by Bruni and Natta (2.48 -4.).
Present Work
Reflection
I2
13 14
IP
.. ..
..
Hauser and SusichO d IP 7 7 .. 4 9 3 9 3 3 4.78 8.76 4.52 8.85 4.02 8.80 3.29 8.76 2.69 8.68 3.48 8.74
9.04 8.94 8.78 9.02 8.55 9.04 8.77 ' . . period (c axis) :
Hauser and Susich
Bruni and Natts d 3186 3.28
..
.. ..
..
..
9.00 8.70 8.78
+ f
0.05
0.13 0.12
a The lattice plane spacings for the layer-line points were calculated from data given by these authors ( 6 , p . 149).
AUGUST. 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
911
the structures proposed for other high-molecular substances such as cellulose and ruobber. If the length, 4.77 A,, is the true identity period in the fiber direction, it is necessary that the methyl groups lie on the same side of the plane which includes the chain carbon atoms. If, on the gther hand, the true identity period in this direction is 9.54 A. as has been assumed above, the methyl groups will lie on opposite sides of this plane. These two possibilities are shown in Figure 6. As stated before, present evidence does not permit a choice between these two fiber periods. Since, however, the presence of one of the types of chains of Figure 6 is well supported by both the chemical and x-ray evidence, it is probable on the basis of the symmetry possessed by these chains that the beta guttapercha crystals belong to the crystal class CpUof the orthorhombic system. This conclusion follows from the fact that the other two crystal classes of this sytesm-namely, Vh and V-both require three mutually perpendicular twofold axes of symmetry, a condition which is difficult to satisfy with structures of the type represented in Figure 6. Just as it is possible for the unit cell of beta gutta-percha to have a c axis one-half that assumed in Table 111, so by FIGERE5. POSSIBLE UNIT CELLSFOR BETA GUTTA-PERCHA neglecting the reflections indicated as questionable or uncertain in Table I1 we can arrive a t a unit cell having an a axis one-half as great. Thus, assuming that the reflections stretching to greater elongations in order to produce the fiber A I , As, AIO,AI^, and IIs (the latter is weak and difficult to measure) do not belong to the beta diagram, the remaining pattein (2, 9). I n the case of sol rubber, however, Clark and co-workers (4) have recently found that vulcanization causes the appearance of a fiber pattern at 400 per cent elongation whereas elongations of 1000 per cent do not cause crystallization without vulcanization. I n the case of gutta-percha the increased orientation produced by vulcanization seems to be connected with the reduction in plastic flow of the vulcanized material a t the temperatures employed to produce the orientation (80" C.). Hence the orientation effect can be interpreted &s being due to the fact that the chain molecules in the vulcanized samples are restrained from flowing from the positions of parallelism which they assume as a consequence of the stretching force. This could well be effected by the hooking together of shorter molecules by sulfur bridges so as to form longer ones which many believe occurs in the vulcanization of rubber. The observation (Figure 4) that both the alpha and the beta forms of gutta-percha may crystallize out together when a sample of the vulcanized hydrocarbon is cooled from 80" C. and a t the same time is subjected to a stretching force, finds an interesting explanation by Hauser and Susich (6). These authors state that, on cooling gutta-percha from temperatures above the transition point (68" C.), the alpha form does not crystallize out a t its freezing point (65" C.) because of its low crystallization velocity, and that from the supercooled FIGURE6. Two TYPESOF THE GUTTAmelt a t temperatures below the freezing point of the beta PERCHA CHAIN modification (56" C.), only the latter form is then capable of ( a ) Repeating distance, 9.54 A. crystallizing. I n the present instance we evidently have an ( b ) Repeating distance, 4.77 A. example in which the rate of crystallization of the alpha modification has been greatly accelerated by the application of the stretching force so that a considerable portion is caused h indices of column 2 , Table 111, become divisible by 2 and to crystallize in the interval 65" to 56" C. Since this effect therefore allow an a axis one-half as Large. The base of this was not observed in the unvulcanized samples, it is possible unit cell will measure 7.85 X 11.9 A. and will have space that the presence of sulfur also plays a part in favoring this for four chain molecules t o pass perpendicularly through crystallization. it. It is interesting toonote the nearness of these dimensions to those (8.3 X 12.3 A.) proposed for the base of the unit The finding of the repeating distance 4.77 8. along the fiber direction in beta gutta-percha agrees well with the calcell of stretched rubber by Susich (12). However, it is not culated length (4.80 8.)of the trans form of the gutta-percha likely that beta gutta-percha will conform t o as high a class chain as has been pointed out by Meyer and Mark (12). of symmetry as does rubber to n-hich Meyer and Mark have This fact makes it probable that the chain molecules of guttaattributed the crystal class V or I'h. Until the dimensions of the unit cell for beta gutta-percha percha are extended with their long axes parallel to the c axis of the unit cell-that is, parallel to the direction of stretch. are established definitely, there is little purpose in attempting Such an arrangement of the molecules is in agreement with to determine its space group and ultimately the atomic posi-
INDUSTRIAL AND ENGINEERIYG CHEMISTRY
912
tions in the crystal. For until these dimensions are known,
it is not possible to distinguish between the presence of simple planes and glide planes of reflection or between simple axes and screw axes of symmetry, and it is thus difficult if not impossible to arrive a t the correct space group. Richer fiber diagrams of this form of gutta-percha or the production of specimens in m hich a higher order of orientation is achieved will aid in this determination. But as has been often pointed out (6, 11), in the absence of suitable macroscopic crystals it nil1 be necessary to employ all available chemical and physical evidence in order to arrive at a convincing solution of this phase of the crystal structure. S o attempt at a complete treatment of the alpha diagram of gutta-percha is possible on the basis of the evidence obtained 50 far. I t is difficult to explain the discrepancies that apparently exist in the identity period calculated for this diagram. Rather than attribute a very long identity period to the gutta-percha molecule [Clark (5) reported finding such spacings in rubber and cellulose], it seems more reasonable to postulate the existence of three forinsoof the hydrocarbon with fiber periods & i i ,9.00, and 8.70 A. or multiples of these figures. The latter tTvo are represented by the points of Table IT’. Speculations of this nature, howel-er, are of little value until further data are secured. One result of this work has been to show that the crystal structure of gutta-percha is considerably more complicated than generally has been realized. Only by continued investigation by means of x-rays and electron rays can the true solution of the crystal structure of this and other high-polymeric substances ultimately be solved. It is hoped that the experiments summarized here will serve a4 a qtep in the direction to this solution.
VOL. 28, SO. 8
Acknowledgment The author wishes to express his thanks to A. R. Keriip for suggesting this work and to F. S. Malm for supplying t,he samples.
Literature Cited (1) Bruni, G., and K a t t a , G., S t t i accad. Lincei, 19, 206 (1934); tr. in Rubber Chem. Tech., 7, 603 (1934). ( 2 ) Clark, G. L., ISD.ENQ.CHEM.,18, 1131 (1926). (3) Clark, G. L., and Corrigan, K. E., Radiology, 15, 117 (1930). (4) Clark, G. L., Warren, W. J., and S m i t h , W. H., Science, 79, 433 (1934).
( 5 ) Hauser, E. A., a n d R o s b a u d , P., Kautschuk, 3, 1; (1927); Z. Elektrochem., 33, 511 (1927); Hauser, E. A , Hunemorder, M., and Rosbaud, P., Kautschuk, 3, 228 (1927). (6) Hauser, E. A , , and Susich, G. von, Ibid., 7, 120, 125, 146 (1931). (7) H o p f f , H . , and Susich, G. von, Ibid., 6 , 234 (1930); Rev. ggn. caoutchoirc, 7, 23 (1930). ( 8 ) K a t a , J. R . , “ D i e Rontgenspektrographie als Untersuchungsmethode,” Berlin, Urban und Schwarzenberg, 1934. (9) K a t a , J. R . , Cummi-Ztg., 41, 2035, 2091 (1927). (10) Kirchhof, F., Kautschuk, 5, l i 5 (1929). (11) M a r k , H., “Physik und Chemie der Cellulose,” p. 136, Berlin, Juliiis Springer, 1932. (121 hleyer, K. H., and Mark, H., “ D e r Aufbaii der hochpoiynieren organischen Naturstoffe,” 1930. ( 1 3 ) Ott, E., ~~\htu,.wl‘ssenschu~~ften, 14, 320 (19%). (14) P a r k , C . R., IND.ESG. CHEX, 17, 162 (1926). (15) Sauter, E . , 2. Krist., 84, 453 (1933). (16) Stillwell, C. IT,, a n d Clark, G. L . . IXD.ENG.C‘HEK, 23, 7‘06 (1931). RECEIVED Alar 15, 1936. Presented before the Dirision of Rubber Chemistry at the 91st Meeting of the American Cheinical Society, Kansas City, Mo.. .ipril 13 t o 17, 1936.
Selenium Compounds in Soils K.T. WILLIAMS A N D H. G.BYERS Soil Chemistry and Physics Research Division, Bureau of Chemistry and Soils,Washington, D. C.
I
S T H E various publications of thi, di-
vision, in which the presence of selenium in soils and vegetation is discussed and which have recently been summarized (1, Z), only incidental attention has been directed to the particular chemical forms in which the element is found. This question is important, since there appears to be no definite relation between the quantities of selenium present in a soil and that absorbed by a given species of vegetation ( 2 ) . Considerable work has been directed by Franke and other. (6, 7 ) and by Horn, Selson, and Jones (S) to the selenium compounds present in grain, but no definite compound has yet been identified.
Pyritic Selenium Work previously reported (8) showed that selenium is apparently always present in varying quantities in iron pyrites. I t always appears that selenium is always present in the shales of the upper Cretaceous period and particularly in the lower portion of the Pierre and the upper portion of the Siobrara formations ( 2 ) . In these formations pyritic nodules are often found. Two of them haye been analyzed. One is from the Pierre shale in Boyd County, Sebr., and contains 205 part.s per million of selenium. The other is from Lane County, Kans., and contains 80 p. p. in.
Since the soils of the semi-arid areas have undergone relatively little weathering during the soil-forming processes and are very iminat.ure, it may be inferred that one of the forms in which selenium may be present in the soils is as pyrit’es carrying selenium as a part’ial substitute for sulfur. Such occurrence is relatively unimportant, except for its bearing upon the problem of the sources of selenium, since in pyrited selenium is certainly not available as a plant food or poison. This statement, while true in general, may not apply to cases where the sulfur compound is marcasite which weathers rapidly and may release selenium in soluble form (8).
Selenium as Selenite In the course of examination of hundreds of samples of shale, of svhich only a small portion has been reported, analyses of separate limonitic concretions or limonitic pieudomorphs frequently showed a selenium content higher than the ininiediately adjacent strata, For example, an iron oxide concentration in the Siobrara shale had a selenium content of 48 p. p. in.lvhereas the adjacent shale had only 3 p. p. m. It was therefore a,..sunied that the high results represented residual selenium from m-eathered pyrites. This assumption, however, does not establish the chemical form in which it remains either in the concretions or in the soil itself.
