Symposium on Lecture Demonstrations1 SEQUESTRATION, DISPERSION, AND DILATANCY -LECTURE DEMONSTRATIONS THOMAS Xi. DAUGHERTY Calgon, Inc., Pittsburgh, Pennsylvania
I
I Three simple lecture experiments are described to demonstrate the phenmnaa of sequestration, dispersion, and dilatany, which employ a complex sodium phosphate glass represented by the cmnmercial product, Calgon. The terns, sequestration, dispersion, and dilatany, are defined and industrial applications of thefirst two have been cited. N o industrial application of d i l a f a n y i s known to the author.
J u s T ABOUT 100 years ago, in 1849, the German chemist Rose (1) made the following statement i "No substance offers the chemist greater difficulties than phosphoric acid; the more the behavior of the acid is studied, the more the difficulties increase. Every new investigation presents the chemist with new anomalies, whilst the older and already known difficulties are by no means cleared up." This short summary of the field of phosphate chemistry is remarkably apt today. On the one hand, even larger quantities of phosphates are being used, so much so that demands now exceed the ability to produce particularly for such types as fertilizers, detergents and water conditioning agents; each year reveals new uses for phosphates; and new forms of phosphates are being discovered with astonishing frequency. On the other hand, chemists in general, whatever their specialties, are becoming more concerned about the differentiation between the various forms of the phosphates, the properties and structures of these forms, analytical procedures for determining their compositions and concentration, and their interferences in analytical procedures for other constituents. Industrial chemists, in addition to exploring new fields of application for both old and new phosphates, are busy in applying known properties of available forms for solving age-old problems in all the fields that comprise our modern civilization. The chemistry of the phosphates is undoubtedly still in its infancy with no end in sight. In 1833, just 16 years before Rose made his classic statement, Thomas Graham (2), British chemist and
founder of the field of colloid chemistry, discovered a new form of phosphate which he called sodium metaphosphate and which became known as Graham's Salt. It is a glassy or amorphous form and has the empirical formula NaPOa with a molecular ratio of 1N h 0 : 1Pz06. It can be made, as Graham made it, from monosodium dihydrogen orthophosphate, NaH2P04, by heating to drive off a molecule of water, then fusing, and finally chilling suddenly, as by pouring the melt onto a cool metal surface. The clear and colorless glass which forms is soluble in water in all proportions to form an unbuffered, nearly neutral solution. It is distinguished from the pyrophosphate and the orthophosphate forms by its ability to precipitate with egg albumin. In 1849, Fleitmann (3) concluded, on what must be termed insufficient evidence, that this material was a "hexametaphosphate." During the next 80 years, this chemical was confined to the laboratory with its principal application being $he well-known mktaphosphate bead test developed by Emerson (4) in 1866. Hall and Jackson (6) first produced and used the chemical commercially in 1929 for boiler water conditioning and Hall (6) announced his discovery in 1932 of its property of softening water without precipitate formation and without the removal of the hardness constituents. This remarkable property has led to a large-scale manufacture and a widespread use of Graham's Salt. Also it led to an intensive study of the composition and properties of this "Cinderella of the Phosphates." While much has been learned about its properties, comparatively little is known about its structure. It is still believed to be complex, in that it contains more than one NaP03-unit per molecule, but it is known not to he the hexametaphosphate claimed by Fleitmann (3). Probably its structure is that of chains of molecules, the chains being of various lengths and intertwined, in which respect it is similar to the organic polymers. Lamm (7) reports molecular weight values for this compound ranging from 8400 to 14,900, determined from sedimentation studies in the ultracentrifuge. In the literature, Graham's Salt is referred to as a glassy or vitreous phosphate, a complex phosphate, a molecularly dehydrated phosphate and a polyphosphate, these terms serving to distinguish it from the parent orthophosphate. It is the purpose of this paper to present three of the
Presented before the Division of Chemical Education a t the 113th meeting of the American Chemical Society in Chicago, April 19 to 23, 1948. 482
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properties of glassy sodium metaphosphate in a manner suitable for lecture demonstration. One property, sequestration, has found wide commercial application; the second, dispersion, is used commercially to a lesser but still important extent; and the third, dilatancy, is still a laboratory curiosity with no commercial application a t present. The commercial sodium phosphate glass, C a l g ~ nemployed ,~ in these experiments is a uiodified Graham's Silt characterized by a molar ratio of about 1.1 NazO: 1 PzOs. The slight increase in NazO content makes its solution more nearly neutral and improves its rate of dissolving. It is obvious that these experiments can be employed either from the standpoint of illustrating the properties of this form of phosphate or of illustrating the phenomena of sequestration, dispersion and dilatancy. SEQUESTRATION
The term sequestration as applied to the softening of water is of comparatively recent origin (6). The word itself means the act of retiring, secluding or withdrawing into obscurity or the act of taking possession of, by confiscating or appropriating. In the chemical sense, sequestration is the reduction of the concentration of a multivalent positive ion in solution, by combination with a negative ion to form a complex negative ion, to the extent that the remaining concentration of the multivalent positive ion is insufficient to be precipitated bv a eiven negative ion with which it has a low solubility product constant. The sequestration value is a stoichiometric ratio or weight relationship expressed as the quantity of a sequestering agent required to capture a unit quantity of a multivalent positive ion and form with it a complex negative ion which is stable against precipitation by a given precipitant for the multivalent positive ion. The sequestration value for calcium differs from the calcium value of Andress and Wiist (8, 9),which is expressed as the quantity of a sequestering agent required just to dissolve its own precipitate formed with a unit quantity of calcium ion. Thus, in sequestration, the multivalent positive ion has practically disappeared from the solution witahoutbeing evolved as a gas, removed as a precipitate or deposited as an element. The atoms are still in solution hut the chemical characteristics normal to their simple ions are gone. This phenomenon can best be explained by an example, the best example being the one first discovered by Hall (6), namely, the sequestration of calcium ion by Graham's Salt. Hall speculated that this glassy sodium rnetaphosphate might a primary ionization as - - produce . follows: (NaPO& + 2 N a +
+ [Na,2(P03),I--
(1)
and possibly a secondary ionization as follows: [Na,r(POs).]--
i3 2Na+
f [Na.-,(P08),,1--
(2)
The subscript n represents the number of NaPOa units 1
A product of Calgon, Inc., Pittsburgh, Pennsylvania.
comprising the polymeric molecule in solution. With calcium ions, the following metatheses or changes of partners would be possible:
By combining equations (I) and (3), the following.e.&aar ., t,ion results : . . Cat+
+ (NaPO,),
-,4Ya+
+ [Na.,-Ca(POi)nl--
(53
By conibining equations (1) and (4), the following equation results: Equation (5) shows one calcium ion, and equation (6) two calcium ions, sequestered by one polymeric sodium metaphosphate molecule. How many calcium ions can he sequestered by one polymeric sodium metaphosphate molecule is dependent upon the value of n and upon the ionization constants of the complex negative ions formed. In any event, by an ion exchange, similar to the base-exchange reaction of zeolites but occurring entirely in solution, each positive calcium ion replaces two sodium ions in a complex negative ion. The calcium ions remaining as s ~ ~ cwould h be the equilibrium concentration represent:ert by the reverse of reactions (3) and (4).
Figvre 1.
Sequent ation of Calcium Ian with Calgon
Beaker A contains rater of 10 grnina per gallon hardness and s o w . Beaker B oontsins the snme water and noall but i n treated with Calanll
That this residual calcium ion concentration is extremely low can be shown bv addine soaw . which normally forms a very insoluble salt with calcium ions, hut which will not precipitate with calcium when sufficient .>
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Graham's Salt is present. Conversely, the glassy sodium metaphosphate will dissolve precipitated calcium soap. Thus the concentration of calcium ions as such in a solution sequestered with Graham's Salt is less than that corresponding to the saturation value of the very insoluble calcium soap. No calcium has been removed from the solution by volatilization, plating out or precipitation and a true solution remains; however, calcinm ion is no longer present in more than a minute amount, of the order of a few parts per million. This is sequestration. At the turn of the century, Wilhelm Ostwald in his, "Principles of Inorganic Chemistry" (10) stated: "No 'complex' ions are known in which calcium forms a part; whenever, therefore, calcium is present in aqueous solution, calcion is also present." Today such complex ions containing calcium are known and a high concentration of calcium can be present in aqueous solution with only a vanishingly small concentration of calcium ion present. This sequestration of calcium ion is affected by the pH of the solution but it is not primarily a pH effect. Thus, calcium carbonate dissolves in acid and not in alkali b u t i t can be dissolved in alkaline solutions by the addition of neutral Graham's Salt. Other bivalent metal ions than calcium ion can likewise be sequestered to a greater or lesser extent such as Ba++, Sr++, Mg++, Fe++, Co++, and Ni++. Trivalent metal ions, such as Fe+++ and Al+++, are sequestered to a lesser extent than are the bivalent metal ions and do not appear to k s o tightly held in the complex negative ion. Certain other materials than the glassy polyphosphates also are capable of sequestration, in general, howew,-to a lesser extent. Of the phosphates, crystalline sodium pyrophosphate, Na,PzO, or N%HzP2Or, crystalline sodium tripolyphosphate NasPsOlo and sodium phytate, CsHs(NazP04),(also called inositol hexaphosphate) from corn steep liquor, are the more common examples. Likewise certain organic compounds which contain no phosphorus have bten found to exhibit sequestration, among which may be mentioned the Trilons (If), which have the followingformulas:
Trilon A
Trilon B
vented and a lather formed with considerably leas soap. Hence, sequestration can be employed in laundering, in dishwashing, and in every cleaning operation where soap and water are used. In fact, sequestration was employed in removing the radioactive residues from pigs exposed a t the Bikini Atoll atomic bomb test (19). DEMONSTRATION OF SEQUESTRATION
To each of two one-liter beakers, A and B, add 500 ml. of distilled water and 9.5 ml. of a solution containing 10 g. anhydrous CaClpper liter. Each beakkr will then contain water with a calcium hardness equivalent to 171 ppm. CaCOa or 10 grains per gallon. To Beaker B, add 15.5 ml. of a 5 per cent Calgon solution, which provides 9 parts of Calgon per part of equivalent CaCOs. To each beaker, add a few drops of phenolphthalein indicator solution and then add sufficient dilute NaOH solution just to produce the pink color indicative of a pH value above 8.2. Now add 10 ml. of standard soap solution to each beaker with vigorous stirring. Observe the persistent precipitate of calcium (lime) soap and lack of soap suds in untreated Beaker A and the absence of precipitate but presence of copious suds formation in the Calgon treated Beaker B (Figure 1). The Calgon in Beaker B has sequestered the calcium ion, thus preventing it from reacting with the soap to form insoluble calcium soap and leaving the sodium soap free to form its characteristic suds3 DISPERSION
Dispersion signifies the state of suspension of finely divided particles in some other substance. A short review of elementary principles will serve to introduce this experiment. The particles are called the dispersed or comminuted phase and may be solids, liquids, or gases. They are composed of more than one molecule except in the case of extremely large molecuks (such as hemoglobin) and are usually in the colloidal range of particle size from 10" to mm. The other substance is termed the dispersion medium and also may be a solid, liquid, or gas. Since gases are completely miscible with one another, a gas-in-gas disperse system would be a misnomer. However, the attenuation or thinning out of the earth's atmosphere with increasing height from the surface is very A slight turbidity will form in the Calgon-treated water upon st,andinc.whioh is ehmacteristic of sodium soao in distilled water. This tu%idity disappears when the water is h&ed to 12OSF.,the minimum temperature of most washing operations. The adjustment of the alkalinity is necessary to prevent formation of fatty acid from the soap by hydrolysis which occurs in any water at pH values below about 8.2. If 15.5 ml-of a 5 per cent Calgon solution is subsequently added to Beaker A with stirring, the calcium soap precipitate will redissolve and suds will be produced inst as in Beaker B. Instead of a cslcium chloride solution. ordinary trtp water containing'hoth calcium and magnesium hardness may he employed for this demonstration. The soap hardness of the water is first determined by titration with Stsndard Soap Solution and then Calgon is added in the ratio of 9 parts Calgon per part of equivalent CaCOo hardness or of calgon per grain of hardness per 100 gallons of water. This is the ratio normally employed in commercial laundering. ~
The sequestration phenomenon is not just a lahoratory curiosity but instead has found wide commercial application. It is principally employed in softening water which is to be used with soap. All natural water, with the possible exception of win water, contains some calcium and magnesium ions and these form quite insoluble precipitates or Scums with soap. Before a Soap can form the lather, which is an attribute of the alkali metal soaps, it mustfirst be added in sufficient amount to precipitate the calcium and maw=ium ions, these being the so-called hardness constituents. By the use of a sequestering agent, this precipitation can he pre-
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~
~
~
~~~
~
SEPTEMBER. 1948
similar to the concentration gradient in a suspension ( I S ) . The following dispersions, with typical examples, are possible:
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rates into an oil and a water layer which is disatlvantageous for spraying. A ceramic slip can be made with a certain total solids content that will have a satisfactory viscosity. By the addition of Graham's System Example Salt, this viscosity can he reduced or else the total Solid in Solid Antimony in Lead Solid in Liquid Colloids1 Graphite in Watcr or Oil solids content can be increased materially while mainSmoke in Air Solid in Gas taining the viscosity at the satisfactory level. The Liquid in Solid Iran Amalgam fluid properties of oil well drilling muds, composed of Oil in Water Emulsion Liquid in Liquid Iiauid in Gas Cloud-Water in Air clays and weighting agents dispersed in water, are con~ a ' ins Solid Hydrogen in Steel trolled by the application of polyphosphates. The Gas in Liquid \?hipped Cream dispersion of finely dividcd metal oxide and salt pigIn colloidal dispersions, the particles do not settle ments, such as kaolin, clay, titanium dioxide, calrium appreciably under the influence of gravity but the carbonate, lithopone, zinc oxide, talc, and barium sulparticles of a coarse colloidal dispersion or of a coarse fate, can be improved and the settling rate markedly suspension do settle slowly. Under ordinary condi- retarded by the use of very small quantities of Graham's tions the particles are prevented from coalescing by Salt, the amount required being dependent upon the possessing an electric charge, the origin of which is type of pigment, the pH of the slurry and the solids still a matter of controversy. Ina solid-liquiddispersion, content of the slurry. This action is probably due to the charged particles will travel to the anode or to the the strong adsorption of the phosphate on the surface cathode (depending npon the sign of the charge) when of the particles which increases their affinity for water. subjected to a unidirectional electric current. They do not conform to Faraday's Law in that 96,500 coulombs DEMONSTRATION OF DISPERSION of electricity liberate more than one gram-equivalentTo each of two one-liter beakers, A and B, add 200 up to 6 or 8-of the material. When the charge is ml. of distilled water. To Beaker B, add 1.5 g. of removed, as by the addition of an electrolyte, the par- Calgon and stir until dissolved. To each beaker, ticles coagulate or flocculate and the sol (i. e . the add 200 g. of titanium dioxide pigment4, which makes colloidal solution) forms a gel (i. e . the product of coagulation). A flocculated system (where the d i d particles are clustered together) settles fast, leaves a clear liquid layer with a sharp dividing line between the liquid and the solid phase and the solid phase ultimately occupies a relatively large volume. A deflocculated system (solid particles geometrically independent and unassociated with adjacent particles) settles slowly, leaves a cloudy or turbid liquid layer usually with no sharp dividing line between the liquid and the solid phase and the solid phase ultimately occupies a relatively small volume. Solid-liquid and liquid-liquid dispersions are of great industrial importance and particularly those employing water as the dispersion medium. 'fhe following familiar examples of widely used dispersions may he cited: paints, printing inks, pigments suspensions, paper and textile coatings, liquid polishes and dentifrices, emulsion cleaners, milk of magnesia, laundry starch, glues, mayonnaise, ceramic slips, ore flotation Figvre 2. Dispersion of Titanium Dioxide with Calgon media, oil well drilling muds, and insecticidal sprays. In water dispersions, Graham's Salt serves a m;ltiple Beaker A oonthins s 50 per cent slurry by weight of cornmerein1 TiO.. purpose: that of sequestering multivalent ions which Beaker B contains the same alurry but is treated with Calpon. would otherwise flocculate the dispersion or break an emulsion, that of reducing the viscosity of dis- a 50 per cent slurry by weight, and stir until all the persions containing a high total solids content and that particles are wetted, noting that the pigment in the Calgon-treated Beaker B is more easily wetted than is of retarding settling of dispersions. The addition of a calcium salt to many dispersions the pigment in the untreated Beaker A. Turn the or emulsions produces flocculation or coagulation untreated Beaker A upside down and note that the whereby the utility is impaired or lost. For example, slurry is too thick to pour out (Fig. 2). Pour the slurry certain agricultural sprays consist of an oil-in-water from the Calgon-treated Beaker B into an empty emulsion with soap as the emulsifying agent. When liter beaker and note that the slurrv is thin and fluid this soap is precipitated, as by the calcium and magne'Tit,anox A, produced by Titanium Piqment Corporation, New sium saltsin hard water, the emulsion "breaks" or sepa- York City, is satisfactory.
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(Figure 2). The titanium dioxide particles have been dispersed by the Calgon. If 1.5 g. of Calgon (0.75 per cent of the pigment weight) is now added to the untreated Beaker A with stirring, the thick slurry will also become thin and fluid. DILATANCY
Dilatancy is a property of a system of close packed particles in a liquid medium characterized by an increased resistance to deformation upon rapid application of an external force; the packing can only hecomc looser and consequently liquid is sucked in which leaves the mms of solid apparently dry and hard. When the pressure is released, the whole mass hecomes moist and fluid again (14). Wet sea sand, when stepped upon suddenly, becomes firm, appears dry on the surface and resists indentation; when the foot is removed, it again appears moist and loses its rigidity. The particles in most naturally dilatant systems are more or less spherical and hence interfere less with each other than would anisometric (unsymmetrical) particles (15). Thixotropy is explained as the formation of a mechanical structure or house of cards by the orientation of the particles, which structure is readily altered liy an external force, that is, the viscosity changes with increased velocity of motion. Rheopexy is a form of
Figuro 3b.
Dilatmnt Dispersion
Showing stiffening of Liquid atream during pouring. The top beaker is held steady. Photographed at '/io.moeeoondlight Bash.
~i~~~~ 30.
nilatant~
u
~
~
~
i
.
S h o r i n g stiffening under tension. This slurry (80 per cent solids by veigbt) contains precipitated CaCOa. Z n ( N 0 a ) ~m . d Cdgon, Stirring rod is being rapidly withdrawn. Photographed at Vlo.mo second light Bash.
thixotropy in which certain sols solidify as the result of a regular gentle motion instead of vigorous shaking. A given solid-liquid dispersion can he most simply tested for exhibiting these phenomena by being placed in a beaker, allowed t o stand for about 10 minutes, and stirred with a stirring rod. If gentle stirring produces a marked increase in fluidity (reduction in plastic viscosity) and, after being allowed t o stand a t rest again, the fluidity decreases (increase in viscosity or gelation), the system is thixotropic. If upon gentle stirring the system sets up or gels and tends to become more fluid again when allowed to stand at rest, it is rheopectic. If upon rapid stirring the system sets up and resists the motion and if the stirring rod penetrates easily when inserted slowly but with difficulty when inserted rapidly, the system is dilatant. In a so~ called Newtonian fluid, no change in viscosity would occur upon standing or upon being stirred either rapidly Or slowly.
SEPTEMBER, 1948
487
Quite a number of systems exhibiting these phenomena have been described in the literature, an excellent summary of which is contained in Alexander's Colloid Chemistry, Vol. 6. A few typical examples will be cited. The mud employed in the rotary drilling of oil wells is composed of swelling-type clays and weighting agents in water suspension and it, is thixotropic. This mud is pumped down inside the drill stem, picks up the cuttings from the drill bit and returns to the surface around the drill stem. When the mud flow stops for any reason, the mud gels and holds the cuttings which ot,herwise would settle and hind the drill stem. When mud flow is resumed, this gel becomes fluid again. Thus thixotropy has an important industrial application. A kaolin with anisometric particles mostly below one micron produced a suspension, when mixed with 1.6 ml. 2 N NaCl per gram kaolin, which had a spontaneous (thixotropic) solidification time of 17 minutes. Unon t a n ~ i n irheowectic) -,e , the solidification time w k 30 seconds which could be reduced to i d s e c o h h i irradiation in an ultrasonic field of moderate energy (16). Gypsum water paste normally in 10 minutes but sets in a few seconds when tapped softly or ge~tlvrolledhetween the palms of the hands (rheopexy). Venad~um pentoxide behaves similarly (17). While rheopexy is encountered in industrial dispersions, no practical utilization of the phenomenon is known to the author. Dilatancy is also encountered frequently, such as in paints, printing inks, and paper coating dispersions, hut it is disadvantageous in that it hinclers desired operations d h these systenls (18). No useful a~plication of dilatancy is known to the author. An interesting example of int,ense dilatancy was encountered in the author's laboratory by Messrs. G. W. Smith and E. T. Laurin which can be easily arranged for a lecture demonstration. .A
DEMONSTRATION OF DILATANCY
To an 800-m1' beaker' addO' ml. water' and 20 ml' Of lo mL of per cent Zn(NOa)z 5 per cent Calgon solution. Then slowly add approxi-
Figure 3:.
Dilatant Dispersion
Show'ng brittle fracture of a ball of slurry thrown onto table top at moment of impact. Photogmphed at v > o , a a a ~ e e o n dlight Bash.
mately g, of precipitated CaC03 while stirring with a stainless steel stirring rod or spatula, stopping the addition when the suspension hecomes dilatant as characterized by the following behavior5. Stir very slowly and note fluidity. Then attempt to stir rapidly and not,e strong resistance to motion. Insert the stirring rod slowly and note ease of penetration to the bottom of the beaker. Then remove it rapidly and note and slicking noise- (Figure 3a,, Pour liquid from a height of about 2 .feet into another 800-ml. beaker and note wavy or jagged stream indicative of stiffening (Figure 3h), Finally, roll a small of the liquid into a ball between the palms of the hands during which a stiffening action can be f4$,and hurl the ball hard upon the table top, noting that i t shatters like a solid and the fragments then .kollapseinto liquid (Figure 3c). This stiffening action upon the sudden application of force and relaxation upon removing this, force is,dilatancy. LITERATURE CITED (1) ROSE,H., Poyg.Ann., 76,l-2811849). 2 G M T Trans. Roy. Soc. (London), 123, 253-84. (1R??i ,-u-",. (3) FLEITXANN, T.,Pogg.Ann., 78,233-60,338-66 (1549). (4) EMERSON, G . H.,Proe.Am. Aend. Arts n Scienms, 6, 47694 (1866). (5) HALL,R. E.. AND H. A. JACKSON. U. S. Pbtent 1;903;041' 1 ,; 119331. t,,,.
Figure 3d.
Dilstent Dispsroion
After slurry in Figure 30 has relsmdintoliquid.
The exact amount of precipitated CaCO, required is,dcpend-, ent upon the physical charaeteristicmf tht..partkular biand cmployed. Dilatant slurrirs from 67 per cent hy weight (142.g. of precipitated CaCOl from U'yandotte Chemicals Odrporation, Wyandott,e, Michigan) to 80 per cent by weight (260 g:af CaC'Oa from the 0. Hommel Company, Pittsburgh, Pennsylvania) have been prepared. Quite a number of metallic salts can he suhstituted for the zinc nitrate to produce these dilatant dispersions; such as: sodium aluminate. ~otassiumtitanium oxalate. lead acetate, and barium chloride:
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HAL&,R. E., U. S. Patent 1,956,515 (1934); Reissue 19,719 (1935). LAMM,O., Arkiv. Kemi. Mineral. Geol., Bd. 17A (25), 1-27 (1944). ANDRESS,K. R., AND K. W ~ ~ S ZT., Anorg. Allgem. C h a . , 237, 113-31 (1938). HAFFORD, B. C., F. LEONARD, AND R. W. C W M ~ NInd. S, Eng. Chem., Anal. Ed., 18,4115 (1946). OSTWALD, W.. "Grundlinien dm Anorgankchen Chemie," ' Second Edition, 1904, p. 527. ZUSSMAN, H. W., Soap and Saniimy Chemieab, 24 (2). 57 (1948).
United Press Release. Lo8 Awelea Euntinu Hwald Express. . . July 4, 1946. S ~ o ~ u c n o wM.v., s ~ ~Ann. , Physik., 21 (4) 756 (1906). REYNOLDS, O., Phil. Mag., 20 (5) 469 (1885); Nature, 33, 429 11RR6i. DANIELIF.~ . , ~ nRubber d i ~ World, 101 (4), 33-7 (1940). BURGER,F. J., AND K. SOLLNER,T 7 a ~ sFarad. . Soe., 32, 159&1603 (1936). FREUNDLICH, H., AND F. JULIUSBURGER, ibid.. 31, 9m-I (1935). SHEETS, G. H., Technical Association Papers, TAPPI, XXV, 528-36 (1942).
