Dielectric Properties of ANIMAL FIBERS
JACQUES ERRERA The Shelton Looms (Sidney Blumenthal 8 Co. Inc.),
Shelton, Conn. A t frequencies between 13 million cycles (mc.) and 120 kilocycles (lrc.) the dielectric constant of wool and mohair fibers and human hair is -1.2. The difference between this value and the square of the refractive index is appreciable. 3Ieasurements at different temperatures (30” to -70” C.) indicate that this difference is due to atomic or ionic polarization. A weak dispersion zone exists in the region below 120 kilocycles. The stretching of the fibers has no effect on the dielectric constant which is interpreted as being in agreement with Astbury’s and Taylor’s new patterns of a- and @keratin C. For comparison. the dielectric constants of silk and nylon fibers were measured. Silk behaves exactly as w-001. Nylon has a lower dielectric constant, and the dispersion zone reaches into the highfrequency region (4 mc.).
I
N 1931 Astbury ( 3 ) interpreted with remarkable intuition the x-ray diagrams he obtained for normal and stretched animal fibers and hairs. For the most important chemical constituent of the cortical layer, the long-chain protein keratin C, he introduced the notion of two mechanical stereoisomers-the normal, unstretched, or folded a-keratin and the extended linear P-keratin. Thus the macroscopic property of the high elasticity of these fibers, which can be stretched about 100 per cent of their length, is reproduced by a n intramolecular movement in the long-chain molecules. These principal polypeptide chains (main chains or backbones) are linked together by side chains, the cystine linkage being the most characteristic. In the normal unstretched form, the main chains are more or less regularly folded like a buckle grid, Indirect physicochemical facts confirmed Astbury’s structural model (14). The question of the complete regularity of the fold in the alpha form is still open: x-ray evidence indicates only a relatively low regularity, but we know from experiments on rubber (10) that x-rays a t ordinary temperatures do not always give a picture of the total regularity because rotational movements may hide to x-ray investigation the fact that longchain nioleculeb are in a parallel orderly state. In any case, the fold is regular in more or less short segments of the main chains. Let us now consider more in detail one isolated, regularly folded, main chain segment xvith its amino acid side chains attached and follow the stretching operation. Figure 1 gives dstbury’s first image of his hypothesis of hpxagonal rings and their opening as a result of the strain. For lactam the distance betmeen the carbon and nitrogen atoms of the keto and imido groups is smaller than would correspond t o their normal radii or volume. To interpret this fact, ilstbury assumed a linkage between these groups as given by a lactamlactim equilibrium; this transformation produces contraction. If the fold occurs on a CHR and C=O group, the necessary contraction of the distance between these two groups is given by a keto-enol equilibrium. 712
HENRI S. SACK Cornell TJniversity, Ithaca, N. Y,
I t is natural to check the hypothesis given in Figure 1 by infrared spectroscopy or dielectric constant determinations. I t is known that the dielectric constant of a substance is a function of the dipole moment of its molecules or atomic groups such as C=O, OH, etc. Astbury’s first hypothesis, considered from the dipolar viewpoint, would result in a variation of the dielectric constant of animal fibers with stretch: in fact, in the stretched beta form we have true C=O and S H groups, whereas in the unstretched folded form the character of approximately one third of these groups is greatly changed (and thus their dipole moments) by the strong interaction resulting from the lactim-lactam (or keto-enol) equilibrium: I t is therefore probable that this change is accompanied by a pronounced variation in the dielectric constant of the fiber.
\ /
NH
/
R /H
HS
I
/CHR\/ 7 i
N
OC
\
NH
NH
>4&
I CHB
/\
co
I
~
CHR
” OC
co NH
CO
I
RCH
/
XH
-
--\
““CHR”CO
\
\
/co
ITCH
\ NH
Lactiin
Lactarn
/
CO
\ i4LPH.&
BETA
Figure 1. Astbury’s First Image of Hexagonal Rings and Openings Resulting from Strain
I n 1941 A4stburyrevised his first hypothesis, not fundamentally but in the details given to interpret this alpha-beta transformation ( g ) , in order to take into account the papers of Pauling (23) and Neurath (12). Neurath built a scale model of the keratin molecules following Figure 1, and proved there n as not enough room to locate the amino acid side chains, especially during the alpha-beta transformation. Pauling’s criticism is based principally on thermochemical evidence against the lactam-lactim interchange in the hexagonal ring. This hexagonal structure had been generalized by Krinrh (18 ) in her three-dimensional “cyclol” hypothesis. Bstbury thus revised his hexagonal model and replaced it by the square model represented in Figure 2 . This model
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1943
covers the requirements necessary for the knowledge of the alpha-beta transformation as he defined it: The experimental and other conditions to be satisfied by the transformation are: (1) the w-form must be about half as long as the p-form, ( 2 ) the density must remain practically constant at akout 1.3, (3) the folds must repeat at a distance of about 5.1 A,, (4) the side chains must protrude alternately in opposite directions from the plane of the fold, (5) the folds must not be so sharp anywhere as to leave insufficient room for the side chains, and (6) adjacent main chains must fold in opposite directions. H
O
/
H
--N-C-C
NH R\ (21)
(d)
NH
I co I
RCH NH
\
co
/
RCH
\ NH
/
CO
\
CHR
/
NH
‘(20
/ \
RCH
PREPARATION O F FIBERS
CO CHR
/
NH “ O H \ NH
(d)
I CO I RCH I
NH
\ /
CO
R&H
\
NH
/ CO \ CHR / NH \ BETA
u = side chain up; d = side chain d o w n
Figure 2.
The liquids used were benzene, carbon tetrachloride, chlorobenzene, and solutions of chlorobenzene in benzene and carbon tetrachloride. The liquids were freshly distilled and completely dried. No influence of the (very low) direct-current, conductivity on the results was observed. No differences were found for measurements made with solutions of identical dielectric constant but different chemical composition, which shows that no interaction takes place between the fibers and the solutions. As these liquids wet the fibers very well, no air bubbles were observed during immersion and no special care was necessary in this respect. All measurements were made a t room temperature (2325” C.), if not otherwise indicated; they were made a t different frequencies in order to detect any dispersion of the dielectric constant. The frequency region extended from 8000 to 13,000,000 cycles per second. Four different methods were used for determining the capacitance of the measuring condenser. At 1.7 mc. and 500 kc. the ordinary “beat” method was employed. At 120, 90, 60, and 21 kc., the beat method was used with the aid of Lissajou’s figures on a cathode ray oscillograph. Measurements in this region are slightly less precise than with the first-method. At 8 kc. the current passing through the condenser a t constant potential was measured with a compensation device; these measurements are the least precise. Finally a t frequencies higher than 1.7 mc. a resonance method was used with good precision.
NH
/
\
713
Astbury’s Revised Square Model
Considered from the dipolar angle, this second pattern, in contrast to the first hexagonal one, would show no noticeable change in dielectric constant upon stretching, because no atomic group changes occur as a result of the unfolding of the main chain. The supposed hydrogen bonds between the true C=O and NH groups of adjacent folds would exist in the stretched and unstretched forms. ELECTRICAL MEASUREMENTS
The dielectric constant of the fibers was measured by the so-called immersion method, in which the solid of unknown dielectric constant (in this case the fibers) is immersed in a liquid between the plates of an electric condenser ( 7 ) . The liquid is so chosen that the introduction and withdrawal of the fibers produces no change in the capacitance of this condenser. Thus the dielectric constant of the liquid is the same as that of the fibers. I n practice several mixtures are prepared, and for each the change in capacitance in introducing the fibers is measured. By interpolation we can then determine accurately the dielectric constant of the fibers.
An investigation was made of 62’s domestic and BA 5’s wools, 24’s mohair, and human hair of several colors. The distribution of diameter sizes of the wools follows: 62’s Domestic (‘/1 Blood), Av. Diameter, 22.6-24.0 Microns Micron diam. Distribution, % 10-20 Not less than 27 10-30 Not less than 88 30.1-40 Not more than 12 4 0 . 1 and over Not more than 0 . 5
BA 5’s Wools, Av. Diameter 38.25-39.75 Microns Micron diam. Distribution, % 10-30 Not less than 1 5 . 5 10-40 Not less than 4 9 . 0 40-50 Not more than 4 0 . 5 50-70 Not more than 1 0 . 5
The general procedure for preparing the fibers was that outlined by Steinhardt and Harris (16). The fibers were washed with cold petroleum ether, extracted for several hours with ethyl alcohol, dried, extracted for several hours in petroleum ether, carded, and washed in several changes of distilled water. Silk samples were used in the form of yarn. They were prepared as follows to isolate the silk fibroin: The skeins were boiled in 1.5 per cent soap solution for 2 hours at 93-97” C. and rinsed in distilled water. After two 10-minute rinses the skeins were allowed to stand in distilled water overnight for approximately 15 hours; then they were dried in a laboratory air dryer. The next step was an ethyl alcohol Soxhlet extraction for 2.5 hours, after which the skein was dried. The third step included a petroleum ether Soxhlet extraction for 2.5 hours, followed by drying. Then the skeins were washed in several changes of distilled water and redried. Two samples were prepared. One sample of nylon fiber mas measured in the undrawn state, and one sample drawn 3.9 times. A second lot of stretched yarns was measured, one part raw as received, and another part purified in the same manner as the silk yarns. The fibers were mounted on a metal frame which held them in a well-defined and reproducible position in the electric condenser and also stretched them (Figure 3). It consisted of two cylinders, 0.5 inch in diameter and 2.5 inches long, which could be rotated around their axes but were otherwise rigidly held in a frame; the distance between the cylinders was 2 inches. The cylinders were cut along the axis into two
714
I N D U S T R I A-L A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 35, No. 6
identical results; therefore the alkaline treatment had no pieces (Figure 3-B), and the two parts could be pressed tight effect on the dielectric constant of the fibers. The frame contopether by screws. The fibers were clamped between these tained a great many fibers, and it vias possible t o stretch most two parts. The stretching was caused by rotating one of the of them 60-70 per cent; in a number of experiments on human cylinders (with a handle introduced into a hole a t the upper hair, the fibers could be stretched only 40 per cent. However, end of one of the cylinders); the cylinders were mevented from *further this should be enough to show a t least a small effect upon moving by stretching, as Astbury's x-ray diagrams showed a change for a stretch of only 25 per cent. screws in the I n all our measurements, the fibers were oriented in such a base of the frame way that the fiber axis was perpendicular to the electric field (not shown). In in the measuring condenser. The condenser (Figure 4) conclamping t h e sisted of two 2.5 X 5 cm. parallel plates, approximately 0.1 fibers, care was cm. apart; the fibers held in the frame (Figure 3) could be taken that they introduced between these parallel plates. be evenly disThe precision of the experiments was limited by the foltributed over the lowing factors: (a) difference between individual fibers, (b) whole cylinder variation in drying process, time during which fibers were exlength. Thus a posed to air after removal from the desiccator and before imrelatively homomersion in the liquid, and (c) the relatively small amount of geneous layer of fiber in the condenser. The experimental error should be befibers was low 3 per cent at the higher frequencies and approximately clamped so tight 5 per cent at the lowest frequency. Within these limits the that they could results have been reproducible. all be stretched The unstretched mohair sample was measured a t 500 kc. a t the same time over a wide temperature range. First, the capacity of the by nearly the condenser filled with pure hexane was determined as a funcsame amount. tion of temperature. The condenser, provided with a tight The whole apcover with an inlet for dried air, was put into an alcohol bath paratus was which could be cooled by adding solid carbon dioxide. The made of chrotemperature inside the condenser was measured with a therm i u m - p 1a t e d mocouple. The measurements were first made by cooling the brass. bath and then letting it come back to room temperature. Before each After a run with pure hexane, the fibers were immersed in the measurement hexane. The temperature range was +35O to -70" C., and the fibers the run was continued long enough to assume that the fibers fixed in t h e Figure 3. Metal Frame for Holdtook on the temperature of the hexane in the condenser. f r a m e w e r e ing Fibers This procedure does not yield an absolute value for the change dried in a n elecA . Stretching frame in dielectric constant of the fibers with temperature] but it was B . Cut through one fiber holder tric oven with sufficiently precise t o shorn that the dielectric constant does automatic temnot change more than 5 per cent in this temperature range. perature control during 1.25 hours a t 95-97' C. Then the fibers were allowed to cool t o room temperature in a desiccator containing calcium chloride. For each measurement the fibers were removed from the desiccator as quickly as possible and immediately introduced into the electric (measuring) condenser. For each measurement the drying process was repeated, which lengthened the procedure; however, it was essential for reproducible and consistent results. Thus in all these measurements] great care was taken to dry the fibers and to avoid any moisture adsorption by them. Even a moisture content of less than one per cent increases the dielectric constant markedly. Therefore we may conclude that even though the adsorbed water molecules are tightly attached chemically, they can still orient themselves in the electric field a t such high frequencies as 1.7 mc. Argue and Maass (1) noted similar behavior for water adsorbed on cellulose. The fibers were stretched after they had been allowed t o soak for 2 hours in a 0.1 per cent sodium hydroxide solution a t ordinary temperature, except for human hair in which case the solution was heated to 40' C. After stretching, Figure 4. Top View of Condenser with Fiber Holder the fibers were washed in tap and then in 1. Container with outlet for liquid 5 . Shield 2. Stretching frame (see Figure 3) 6. Isolated plate supporting one distilled water. Some measurements were also condenser plate 3. Fibers 4. Condenser plates 7. Guides for frame made on fibers stretched in pure water with
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1943
Two such measu r e m e n t s were made. I n addition, t h r e e r u n s were carried out over a smaller temperature range (f30"to -20°C.) in t h e m i x t u r e s of carbon tetrachloride and chlorobenzene ; the results were the same.
715
ring in the zero position over the 8-hour period of the experiment; this change reached a maximum of 0.5 scale division. The difference (column 4 below) due to the presence of the fibers was nearly constant over the whole temperature range. Since, on the other hand, the dielectric constant of hexane changes only slightly with temperature (increase of approximately 5 per cent over the range studied), it can be concluded that the dielectric constant of the fibers also did not change appreciably: Temp.,
C.
Hexane
Hexane
+ Fiber
Difference
PRECISION OF THE METHOD
Figure 5. Change in Capacitance after Immersion of Mohair
Since dielectric constant measureCurve 1. Stretched ments are rather Curve 2. Released standardized, i t is unne cessarv t o give all the readings. However, examples will show h o i the method works and what its precision is. The following table shows the dial readings on the capacitance measuring device for one series, One scale division in this case (beat method, 500 kc.) is approximately equal to 0.3 micromicrofarad, and 0.05 division can be read with a vernier. A stretched sample of mohair was immersed in a series of mixtures whose dielectric constants, e, are indicated below: r
e = 3.8
Liquid alone fibers Liquid Liquid alone
e
49.05 47.70 49.15
+
AC
-
4.0 49.75 49.10 49.80
+0.67
+1.40
-
Capacitance
4.2 48.70 48.30 48.75
10.42
e
-
4.6 50.50 51.30 50.50
-0.80
I n Figure 5 the change in capacitance AC (in scale divisions) when the fibers are immersed is plotted against the dielectric constant of the liquid. The intersection of this curve with the e axis gives the dielectric constant of the fibers. Curve 1 (data from the above table) is for a stretched sample; curve 2 (data not listed in the table) is for the same sample released. The AC values are smaller in the first case, because the volume of the fibers between the condenser plates is smaller than when the fibers are stretched, due to the lateral contraction. The intersection with the axis, however, is nearly the same in both cases, which means that the dielectric constant has not changed appreciably with stretching (or releasing). Single dielectric constant values are given below for two mohair samples as determined by the above method; the stretched sample was first measured, then released, again stretched, and again released:
Mohair 1 Mohair 2
Stretched
Released
Stretched
Releaaed
4 30 4.04
4.26 4.25
4.16 4.28
4.a2 4.08
This table shows how the single values are distributed around the average value, as reported in the next section. Three measurements made on a silk fiber gave dielectric constants of 4.10, 4.25, and 4.30, respectively. Variation with temperature is shown by data from a test made with the condenser filled with hexane and then with the hair immersed in the hexane. The dial readings were corrected for changes occur-
PROPERTIES OF WOOL, SILK, AND NYLON
WOOL. The greatest number of measurements in the stretched and unstretched state were made at 500 kc. The dielectric constant found for approximately fifteen measurements was 4.2 * 2 per cent. The values obtained a t other frequencies are given in the following table; no difference was found for the stretched and unstretched fibers within the limit of experimental error: 8 ma. 4.2 120 kc. 4.2
= 13 mc. = 4.2 Y 240 kc. c = 4.2 Y
e
-
4 mc. 4.2 90 kc. 4.3
1.7 mc. 4.2
60 kc. 4.4
0 . 5 mc. 4.2 20 kc. 4.8
8 kc. 5.4
These values are the same for wool, mohair, and human hair. This table shows that below 120 kc. a dispersion zone sets in. The only value reported in recent literature is that of Holl (11) who found 7.2 * 10 per cent. However, he did not indicate how his fibers were dried, and this high value might have been due to incomplete drying. As indicated above, no pronounced temperature variation is found between 35' and -70" C. Our measurements indicate a small decrease in dielectric constant with decreasing temperature which does not exceed 5 per cent of the original value. Precise refractive index measurements were made by Fox and Finsh (8) at 5890 A. with plane-polarized light. They find for n* a value of 2.4 in comparison with our value of 4.2 for e. SILKFIBROIN.The two samples of silk were measured at 500 kc., 4 mc., and 10 mc.; a t all three frequencies the dielectric constant was 4.2, exactly as for wool. No recently published measurements could be found with which to compare these results'. NYLON.For the undrawn sample, the following values were found : Y
e
-
=
10 mc. 3.15
4 ma. 3.15
0 . 5 mc. 3.26
10 ka. 4.2
The value of 3.15 at the high frequency is considerably lower than the 4.2 value for wool and silk. The nylon long-chain molecule contains many nonpolar CH2 groups. The drawn sample is consistently slightly higher, approximately by one per cent. The du Pont Company indicated that for drawn sheet nylon they found a value of 4.5 a t 1 kc. On the other hand, the Plastics Handbook for 1942 gives values for nylon moldings of 3.6, 3.3, 3.2 a t 1mc., 1 kc., and 60 cycles per second, respectively. The other sample of drawn nylon yarn (210/69/1) was measured in the raw state 1 New measurements at low frequencies show t h a t the dispersion zone of silk begins a t lower frequencies than for wool. Exact data will be published later.
716
INDUSTRIAL AND ENGINEERING CHEMISTRY
and purified. The results n-ere of the same order of magnitude; the dielectric constant of the purified sample was approximately 5 per cent lower than that of the raw sample. STRUCTURE OF STRETCHED AiYD UNSTRETCHED FIBERS
I t is important to find out n-hat mechanism is responsible for the large difference between the square of the refractive index and the dielectric constant as measured a t high frequencies. It could be due to dipoles which would still be able to orient in fields of frequency higher than 13 me., as found in certain solids; or it might be due to atomic or ionic polarization. The constancy of the value over a large frequency region together with the small temperature effect clearly shows that the second hypothesis is the more plausible. . The next question is whether this atomic or ionic polarization is to be ascribed to the main chains or to the side chains in the molecular structure of the fibers. Measurements on silk were made to decide this question. Silk and wool are very similar as far as the main chains are concerned but differ considerably in the length and nature of the side chains; the silk fibroin cont,ains mainly hydrogen and CH, groups as side chains (4). Since the dielectric constants for silk and wool were found to be identical, it is probable that the main chains contribute mainly to atomic and ionic polarization. Furthermore, if we consider the ionic character of the CO and S H groups, it is plausible that such a high atomic polarization is found. Other proteins in t’he solid state also show a highfrequency dielectric constant of the same order of magnitude as those found here for wool and silk ( 6 ) . The high dielectric constant found for cellulose fibers at high frequencies ( 5 ) , however, is due to orientation of the OH dipoles, as shown by the marked temperature and frequency dependence (16). The increase of the dielectric constant below 120 kc. marks the beginning of the orientation of those groups in the molecule which have a dipole moment (NH, CO, etc.). The whole structure of the large protein molecules is not rigid, and therefore a n orientation of these groups under the influence of the field is possible. The same phenomenon is also found in many ordinary plastics-for instance, the polyvinyl chlorides (9)2. However, it has not yet been possible to make measurements.at sufficiently low frequencies to determine the exact relaxation time of these movements. I n t’he case of nylon this orientation of dipolar groups starts a t much higher frequencies and seems to be dist,ributed over a much wider range. This behavior may perhaps be attributed to the fact that we are dealing here with a synthetic copolymer, that this polymer is not so homogeneous as natural polymers and contains simultaneously smaller and bigger molecules, and that in nylon there are no side chains. TRANSFORNATIOS FROM ALPHATO BETAFORN. The experimental results show that the dielectric constant is the same in both forms. It is almost impossible t’o estimate n-hat change could be expected on the basis of Astbury’s first theory; but considering that one third of the active CO and S H groups are strongly altered by their interaction, we should expect some difference in the dielectric behavior. The atomic polarization is intimately related to the vibrations which the lattice of the solid can perform. It is clear that in Betbury’s first picture the alpha form is a much more rigid structure than the beta; therefore thevibrations in the two cases should be quite different. That we do not find any change in atomic polarization (dielectric constant at high frequency) therefore indicates that the structure in the alpha and beta forms are not very different. The same is true if we consider 2 After the manuscript was submitted, a paper appeared by W.0. Baker and W. A. Yager [J. Am. Chem. Soc., 64, 2171 (194211 o n the dieleetrio properties of polyamides. Their results are in many respects similar t o ours but their dispersion zone begins, in general, at higher frequencies. These polyamidcs are, therefore, more similar i n behavior t o nylon than t o wool.
Vol. 35, No. 6
the dipole orientation which shows up a t lower frequencies. Here also t’hemuch more rigid structure of the alpha form in -4stbury’s first hypothesis should hinder this orientation as compared to the ease with which the chain can move in the beta form. We could therefore expect to find the dispersion at a lower frequency for the alpha form than for the beta. Furthermore, in the alpha form the dipoles of the A-H and CO groups which are interacting will contri’but’eless to the dipolar orient’ation and, therefore, to the dielectric constant. Our measurements indicate that there are no profound differences between the alpha and the beta forms, which is consistent with Astbury’s second picture of the structure of keratin C ; it is also consistent with any ot,her picture-for example, that given by Taylor (17)-in which there is no important “chemical” difference (in bonding) between the alpha and beta forms. I t seems that t,he mobility of the chains is the same in both forms, and therefore that the two forms differ only in geometrical arrangement. This behavior has often been compared with the behavior of rubber where the curled up chains also tend towards linear form during sbretching. Only in the case of rubber does this uncurling occur continuously and Tvhen the slightest stress is applied. I n the case of wool it seems that, this transformation from a more bent to a more stra,ight position is not possible continuously, and that the fibers must first swell under the influence of adsorbed water in order for the transformation to take place. These results on rvool are similar to those for nylon where the stretching of the fibers arid therefore the increase of orient’at’ionof the chains does not produce any considerable change in the mobility of these groups and therefore in the dielectric constant. Measurements a t still 1013-er frequencies t o embrace the whole dispersion region will be still more conclusive, and we hope to finish these measurements in the near future. Rut on the basis of t’he data here reported, the conclusion is that the alpha and beta forms are not essentially different and that Astbury’s n e v picture or Taylor’s model represent the actual facts more accurately. ACKYOW~LEDGMENT
The fibers 17-ere prepared by R. Canavan whose help is gratefully acknowledged, as well as that of D. T. Wilber and IT. R. Monroe with the electrical measurements. The nylon samples were made available through the cooperation of the Sylon nirision, E. I. du Pont de Nemours R. Company, Inc. LITERATURE CITED
(1) Argue, G . H., and Maass, O., Can. J . Research, B13, 156 (1935). (2) iistbury, W. T., and Bell, F. O., N a t u r e , 147, 696-9(1941); Astbury. J . Soc. Dyers Colorists, 57, 336 (1941). (3) Astbury, W. T., a n d Street, A., T r a n s . R o y . Soc. (London), A230, 75 (1931); Astbury a n d Woods, H. J., Ihid.,A232, 333 (1933) ; A s t b u r y , “Fundameritals of Fibre Structure”, Ox. ford Univ. Press, 1933. (4) Bergmann, M,,and Kiemann, C., J . B i d . Chem., 122, 677 (1937). (5) DeLuca, H . A , Campbell, IT. B., and Maass, O., Can. J . Reseaich, B16, 273 (1938). (6) Errera, J., J . chim. p h y s . , 29, 577 (1932). (7) Errera, J . , a n d Ketelaar, M . H., J . phys., 3 , 232-47 (1932). (8) Fox, K. R., a n d Finsli, R. B., Teztile Research, 11, 62 (1940). (9) Fuoss, K . >I., and collaborators, J . Am. Chem. SOC.,64, 283 (1942), a n d earlier papers. (10) G e h m a n , 8 . D., and Field, J. E., J . Applied Phys., 10,564 (1939). (11) Holl, Helv. C h i m . Acta, 19, 281 (193F). (12) N e u r a t h , H., J . P h y s . Chem., 44, 296 (1940). (13) Pauling, L . , and Siemanii, C., J . Am. Cheni. Soc., 6 1 , 1860 (1939). (14) Speakman, J. B., Proc. R o y . Soc. (London). A132, 164 (1931). (15) Steinhardt, J . , and Harris, AI., J . Reseawh N u t ! . Bur. S t a n d a r d s , 24, 336 (1940). (16) Stoops, TT7. N.,J ,Am. Chem. Soc.. 56, 1480 (1934). (17) Taylor, H . S., Proc Am. Phil. Soc., 85, 1-12 (1941). (18) Wrinch, D. M., Nature, 137, 411 (1936).
