Nature and Constitution of Shellac VII. Determination of Acid Number HAROLD WEINBERGER AND WM. HOWLETT GARDNER, The Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
T
HE chemical c o n s t a n t s of a s u b s t a n c e , in the
An accurate determination of acid number of shellac is a necessary prerequisite to the study of its behavior with respect to several of its applications. This is particularly true when studying shellac varnishes. Some of the methods which have been developed for acid number cannot be accurately applied to shellac. These include the Albert method which has recently found wide use in this country for the general determination of acid numbers of resins. Results given in this paper show that for shellac it does not give true values. Partial hydrolysis of the alkali salts formed during the titration leads to a false end point with phenolphthalein, which is the only indicator that can be employed with this method.
case of many n a t u r a l m a t e r i a l s , are one of the few means available for identification and in some instances the only method f o r e s t i m a t i n g p u r i t y . Strictly speaking, in most cases the numbers obtained for unadulterated materials are not true constants, but represent ranges in p r o p e r t i e s by which the material may be characterized (34). It is unnecessary to strews the fact that the narrower the ranges the greater is their utility. The numbers representing these ranges might more correctly be referred t o as “identification n u m b e r s , ” or “kennzahlen.” A great deal of studv has been given t o the determination of t h i various values i n d numerous methods have been developed for each of these numbers. It has been the authors’ experience (32), however, that in some instances methods which have been accurately developed for a specific substance have been generally applied to other materials without a complete study of the new applicability. The literature is filled with conflicting data (6, 35), which may in part be accounted for by the use of different methods.
METHODS FOR ACIDNUMBERS I n the determination of acid numbers a wide variety of methods and various modifications have been employed. In some cases a method has been developed to meet the peculiar characteristics of a particular substance, but even here there has been a tendency t o make the method general. Since many of the methods used for resins were first developed for fats, oils, and waxes, it is desirable when considering methods for acid numbers of resins t o include these latter substances. The high coloration of the extracts of some of the substances, notably orange shellac, and the peculiar solubilities of other materials account t o a large extent for the multiplicity of methods which have been developed (6). Even when employing the original method, which consists in titrating a n alcoholic extract with alkali direct, numerous sets of directions are given. Dieterich, who has made an extensive study of resins, recommends for storax ( 7 ) that a 1-gram sample be dissolved in 100 cc. of cold 96 per cent alcohol and titrated with 0.5 N alcoholic potassium hydroxide, using henolphthalein as indicator. This method is very similar to t i e standard method adopted by the American Society for Testing Materials for linseed oil ( 2 ) . Richmond, on the other hand, recommends the use of a solution of 1 gram in 50 cc. of absolute alcohol for manila copal ( Z S ) , whereas Slack (28) refers a 5-gram sample for fats, oils, and waxes and titrates wit{ 0.1 N alcoholic alkali. Richter (24) uses 30 to 40 cc. of an equal mixture of absolute alcohol and ether with 1.0 N alkali for this purpose. Niegemann ($0) recommends 0.2 N aqueous alkali and states that a t least 2 cc. of a 1 per cent solution of phenolphthalein should be used. Gardner and Coleman (10, cf. 18) point out that colloidal substances do not give true acid
numbers when determined in the usual manner. For substances incompletely soluble in alcohol, they recommend a mixture of equal parts of alcohol and benzene, dissolving 1 to 2 grams in 40 cc. of the mixture, refluxing for 30 minutes, and allowing to cool before titrating with 0.1 N sodium hydroxide. Steel and Sward (g9) have used this alcohol-benzene mixture for vegetable oils. K e t t l e (la), on the other hand, has em loyed a mixture of 90 per cent o?benzene with 10 per c e n t of alcohol for oils. Singh (27) dissolves 1 gram of shellac in 100cc. of boiling alcohol, refluxing for 5 minutes. He filters and cools before t i t r a t i n g with standard aqueous potash, using phenol hthalein as i n d i c a t o r . Parry $29) claims that the presence of lac dye in some samples obscures t h e end p o i n t w i t h phenolDhthalein. Kranz and Mairich 114) find that a-naphtholphtha”1eiq alkali blue, and phenolphthalein are the most practical indicators for resins when titrated with 0.1 N potassium hydroxide. For shellac Nagel and Kornchen (19) recommend alkali blue 6R, using alcohol solutions. Whitmore, Weinberger, and Gardner (32) use thymol blue. To overcome the inaccuracies caused by high coloration of solutions, Dieterich (6) has developed a method where the resin is dissolved in a standard amount of alkali and back-titrates with standard acid. The alkali serves both to neutralize the resin and as a solvent. The method cannot be used with substances which contain esters which are readily saponified. For dammar and sandarac (8) he dissolves 1 gram in 20 cc. of 0.5 N alcoholic potassium hydroxide, adds 50 cc. of benzene, and after standing for 24 hours titrates the excess with 0.5 N sulfuric acid. For ammoniacum a preliminary refluxing with water and alcohol for 15 minutes is recommended (9) and part of the filtrate is treated with 0.5 N alcoholic alkali. In this case only 5 minutes’ contact is required before titrating with acid. For similar reasons potentiometric methods have been employed. Kremann and Muss (15, 16) titrated fatty acids dissolved in alcohol with 0.5 N alcoholic sodium hydroxide in this manner; Seltz and McKinney (26) fatty acids and lubricating oils. Gardner and Whitmore (11) studied the applicability of the method to resins, including shellac; and Caldwell and Mattiello (3) linseed oil and its fatty acids. The latter authors found they could use successfully not only 95 per cent alcohol, but also butyl alcohol, and mixtures of equal volumes of alcohol and benzene as solvents. These methods are unquestionably the most accurate that can be employed, since they are totally free from inaccuracy in judging the end point of the indicators or due to color of the solution. They are not as convenient as the other methods, however, since the electrodes have to be carefully prepared (a, 11). For investigation urposes they are unexcelled, since the titration curves give at t i e same time considerable information concerning the nature of the material being titrated. By comparing results obtained by other methods with those obtained by the potentiometer, considerable improvement in technic and means of judging end points can be effected. A fourth type of method for acid number is typified by the Albert method ( 1 ) . Here the substance is dissolved in a 2-to-1 mixture of benzene and alcohol, and a two-layer system formed by the addition of a saturated aqueous salt solution. Aqueous alkali and acid are employed in the titration, using phenolphthalein as indicator. These methods have the advantage of giving a clear end point, since the phenolphthalein is retained in the aqueous layer while the resin and its salts remain in the upper organic solvents. The same end point may be obtained several times by adding a slight excess of alkali and back-titrating with
25 7
268
Vol. 5, No. 4
ANALYTICAL EDITION
acid. Stock, who modified the original method t o a slight extent, determined values for a variety of resins (80). It is clear from his value for shellac that he was not dealing with a usual sample. Coburn (4) with similar modification used this method for darkcolored samples of rosin. His values agree better than Stock’s with these obtained by the usual method of titrating spirit solutions directly with alcoholic alkali.
It can be seen from this review that many of the methods which have been used differ widely both in technic and in principle. I n only a few cases have attempts been made to compare results obtained by more than one or two methods, so that we are left in serious doubt when attempting to establish limits of variation for many substances, especially by consulting the literature for possible climatic variations over long periods of time. The adoption of standard methods and systematic recording of results in the literature are highly desirable. They have been accomplished to some degree in the case of some oils (a),but should be rapidly extended to include many other substances. ACIDNUMBERS FOR SHELLAC The values given for shellac do not vary as widely as do many of those given for other substances, but in some instances the influence of method is obvious. Parker (21) obtained values of 60 and 66 for button and flake shellac; Williams (33) 47.6 to 64 for the same varieties and garnet. His widest variation occurs with the flaked variety. Kremel for a yellow lac; Schmidt and Erban (6) 5; and Dieterich 59 to 63. Singh (27) used aqueous alkali for titration and obtained values of 60 to 65. His values are the only example of shellac derived from a recorded source. On drying or melting, he found that the values were reduced to 52 to 60. Umney (31) obtained values of 53 to 78 for all grades of shellac, and a value of 44 for sticklac. Coffignier (6),using the conventional method, found 60 for shellac and 35 for stick-lac, Rudling (26) obtained 53 to 59 for seed-lac and button-lac, Parry (22) 55 to 65, and Ulzer and Defris (6) 65.4. Nagel and Kornchen (19) stated that the acid numbers of all grades range from 60 to 70 and for rosin-free samples 60 to 65. Gardner and Whitmore (11) showed values of 67.5 to 73; and Whitmore, Weinberger, and Gardner (32) 70 to 75. For bleached shellac Kremel (17‘) obtained 73.7, Coffignier (6) 81, Gardner and Whitmore (11) 85.7, and Whitmore, Weinberger, and Gardner (SI) 93.6. For refined wax-free bleached shellac the latter authors obtained 107.5 to 117.6. The general uniformity of results ranging between 60 and 65 may be due to the common use of alcohol solutions and phenolphthalein as indicator, The values which are lower than 60 almost invariably occur for the poorer grades of shellac and may in part be accounted for by difficulty in judging the end point, which is masked by the deep wine color of the alkali salt of the yellow dye erythrolaccoin present in the orange grades. It is also possible that in these grades the acid value has been reduced, as Singh (87) has shown, by excessive heat or drying. Since the resin content is given in only a few cases, it is difficult to judge what the true value should be (88). The higher range of values of Gardner and Whitmore is unquestionably due to the more accurate method employed in judging the end point. DIRECT METHODFOR DETERMININOACID NUMBERSOF SHELLAC It had been the previous experience in this laboratory that by following the potentiometric titration with outside indicators, for an accurate end point with phenolphthalein a fairly distinct pink must be taken. Because of the difficulty in judging such an end point, especially in the presence of the aforementioned dark purple color, the authors have substituted thymol blue as the indicator. Although the potentio-
metric titration curves show a sharp inflection for all grades of shellac, nevertheless even with thymol blue there is a, slight range in color change from yellow through green to blue a t the end point. From a comparison with fatty acids of similar acid strength it might be expected that both these indicators would show a rapid rate of color change. What is sometimes overlooked in change of solvents in a determination is that the indicator range may also be changed and not in the same proportion. It would appear that the use of 95 per cent alcohol depresses the range of alkalinity of the two indicators. By taking as the end point the first indication of a permanent blue with thymol blue when using two to three drops of a 0.04 per cent alcohol solution of the indicator on a spot plate with one to two drops of the solution, the authors have been able to obtain values which closely approach those obtained with the potentiometric method. This would account for the relatively higher results of Whitmore, Weinberger, and Gardner (32). The exact method employed consists of the following: Dissolve 5.000 grams of shellac, which has been carefully rolled, in 50 cc. of neutral 95 per cent ethyl alcohol. Titrate with standard 0.5 N alcoholic potassium hydroxide, using thymol blue (thymosulfonphthalein 0.04 per cent in 95 per cent alcohol) as outside indicator. A porcelain spot plate is convenient for this titration. The end point is taken as that point where one to two drops of the solution on a glass rod gives the first tern orary blue colorationt o the indicator. The alcoholic alkali shourd be standardized against aqueous acid each time before using t o correct for any error in change in density due t o fluctuations in temperature. Identical results are obtained by this method when 50 per cent of benzene is added to the solution before titration. Since all the acid constituents of shellac are spirit-soluble, it is unnecessary to use any other solvent. At least 5 grams should be taken as a sample in order to obtain uniformity of material for duplicate determinations. Proper rolling of sample is very important. As can be seen from Table I, excellent checks are obtained with duplicate samples.
TABLEI. INFLUENCE OF AMOUNT OF BENZENE USEDIN ALBERT M~~THOD T. N. oranee shellac ACIDNUMBER Weinberger-Gardner Albert method direct method
Samule:
RATIO
CSHR: CgHsOH
, 1:l
..
..
66.5 66.4
1:l
..
0:l
76:O
INDIRECT METHOD Because of the somewhat greater ease in judging the end point of thymol blue when titrating with acid, an attempt was made to determine the acid number by the indirect method: Dissolve 5 rams of shellac in 25 cc. of neutral alcohol, add 26 cc. of 1.0 N t-8coholic potassium hydroxide, and titrate the solution immediately with 0.5 N alcoholic sulfuric acid.
As can be seen from Table 11, this method gives high results. For confirmation that saponification takes place in such a rapid manner one sampIe was aIlowed 10 minutes’ contact with the alkali before titrating. This raised the acid number by 20, and leaves no doubt as to the inapplicability of these methods to shellac. TABLE11. PRESENC~ OF SAPONIFICATION IN INDIRECT METHOD Sample: Superfine (oomposite) orange shellac DIRECT METHOD I N D I R ~ METHOD CT 70.2
..
70.2 a
Titrated after standing for 10 minutes.
77.2 75.6 90.7O
July 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY ALBERTMETHOD
For reasons cited above, it is advisable to use a t least a 5gram sample of shellac. This procedure was followed when employing the Albert method, and in order to compensate for the larger sample the amount of solvents was increased. The determination was conducted in the following manner: Dissolve 5.0000 grams of shellac in 100 cc. of a mixture of two parts of benzene and one part of alcohol. When solution has taken place, add 100 cc. of neutral saturated sodium chloride solution, with several grams of the solid salt and a few drops of phenolphthalein solution. Titrate the solution with 0.5 N a ueous sodium hydroxide with careful but thorough agitation. (goo vigorous agitation will cause the shellac to become wetted with water and adhere t o the sides of the flask. A swirling motion was found to be the best.) Add hydroxide until the aqueous layer has a definite red color and then titrate with 0.5 N sulfuric acid until the pink just disappears. Repeat the addition of alkali and back-titration with acid until two or more consecutive readings give the same value for acid numbers.
269
tions, however, are obtained only if sufficient alcoholic alkali is added to correspond to the acid numbers obtained by the direct method. Since any free shellac is highly insoluble in water, these experiments would favor the contention that the higher acid numbers of Gardner and eo-workers represent the truer values for shellac. CONCLUSIONS The results obtained by the Albert method might be explained in the following manner: Since it has been shown that shellac behaves as a weak organic acid (11, l a ) , its potassium salts would have a tendency to hydrolyze in water. Because of the marked insolubility of shellac (acids) in water, this hydrolysis proceeds only to the normal degree in aqueous solution; however, when there is a two-liquid layer system in which shellac is very soluble in the organic layer, any free shellac which is formed as a result of hydrolysis will be removed from the aqueous layer and hydrolysis can proceed to a much greater extent:
Varying the amount of liquids did not affect the results. With seed-lac, the red lac dye present prevents the obtaining Single-layer systems: of an end point with phenolphthalein. Thymol blue and Potassium shellacate + K + shellac ions other indicators of this range were either affected by the HOH e OHH+ saturated salt solution or showed too great a solubility in the organic layer to be effective. Attempts to remove the &ellac (solid) lac dye by the formation of an insoluble lake with soluble Two-layer system: Potassium shellacate +K f shellac ions lead or barium salts were not satisfactory. The effect of HOH +OHH+ changing the ratio of benzene to alcohol for orange shellac is shown in Table I. As can be seen, an increase in alcohol dkellac (solid) depresses the acid number. 4 A comparison of the values obtained by this method and Shellac solution (alcohol-benzene) those determined by the direct method is given in Table 111. When selecting a method for study as a standard for the The numbers obtained by the Albert method are lower than those by the direct. They do not, however, show any con- acid number of shellac, extreme care should be taken to consider some of the peculiarities of this material. Because of stancy in difference. its difference in source and nature, methods which may sucTABLE111. COMPARISON OF DIRECTAND ALBERT METHODS cessfully apply to other resins will not necessarily give ac-ACID NUMBER? curate results with shellac. It would appear from this study Direct Albert SAMPLE method method DIFFERENCE that only the group of methods known as the direct methods T. N. orange 76.0 67.2 8.8 and the potentiometric methods are open to selection. In Heart orange 70.8 56.0 14.8 A. S. 0 . ( 8 ecial) 73.9 64.6 9.3 developing such a method, consideration should be given to Bleached siellac 93.7 87.7 6.0 the amount of sample taken for analysis, care of rolling, the Seed-laca 69.4 59.5 9.9 Palmitic acid 216.6 218.8 -3.2 neutrality of the alcohol, constancy of titer of alkali, proper a This sample contained very little lac dye. selection of indicator, and standardization of method in I n seeking an explanation for the results obtained with the judging end point. Since this work was undertaken, the importance of acid Albert method, 5 grams of shellac were dissolved in alcohol for shellac has become recognized in this country. numbers and just neutralized with alkali by following the procedure in the direct titration method. Twice the amount of benzene Further study is in progress upon this subject. was added to this solution, and then the saturated salt soluLITERATURE CITED tion. Upon addition of phenolphthalein the aqueous solution Albert, K., Albertschrift No. 15, p. 55. was found to be alkaline. Back-titrating with aqueous acid Am. SOC.Testing Materials, Rept. Subcomm. IX of Comm. D., gave a value identical with that originally obtained by the 26 (1926). Albert method. From this it was clear that the difference Caldwell, B. P., and Mattiello, J., IND.ENQ.CREM.,Anal. Ed., in values was an inherent function of the two methods. 4, 52 (1932). Coburn, H. H., Ibid., 2, 181 (1930). At first it was thought that the difference might be caused Coffignier, C., Bull. soe. chim.,8, 1049 (1910). by a difference in range of end point of the indicators in the Dieterich, K., “Analyse der Harre, Balmme und Gummi-Harre,” two different solvents, alcohol and saturated brine solution. Springer, Berlin, 1930; “Analysis of Resins, Balsama and However, the authors were able to show that by adding water Gum Resins,” 2d ed., tr. by Stock, Scott, Greenwood, London, 1920. to a neutralized alcoholic solution of shellac and boiling off Dieterich, K., Pharm. Zentralhalle, 40, 423, 439 (1899). the spirits, an aqueous solution neutral to both thymol blue Ibid., 40, 453 (1899). and phenolphthalein was obtained. Addition of a drop of alIbid., 40, 467 (1899). kali to this solution rendered it alkaline to either of these indiGardner, H. A., and Coleman, R. E., Paint Mfg. Assoc. U. S., Tech. Circ. 87 (1920). cators. By adding amyl alcohol to the neutral aqueous soluGardner, W. H., and Whitmore, W. F., IND.ENQ. CEIEM., tion derived in this manner by the direct titration method, a Anal. Ed., 1, 205 (1929). two-liquid layer system is formed and the aqueous layer Gardner, W. H., Whitmore, W. F., and Harris, H. J., IND. ENQ. readily becomes alkaline. CHBM.,25, 696 (1933). Kettle, S., ChemistRnaZyst, 18, 7 (1929). By using filtered alcoholic solutions of shellac it can be Kranz, C., and Majrich, A., C h m . Obzor, 1, 81 (1926); Chem demonstrated that clear aqueous solutions of the potassium Zentr., 98, 1402 (1927). salt of shellac can be obtained in this manner similar to those Kremann, R., and Muss, F., &$e, 7, 161 (1921). obtained with wax-free bleached shellac (12). Clear soluKremann, R., and Schopfer, F., Ibid., 7, 612 (1922).
++
++
ANALYTICAL EDITION
270
(17) Kremel, A., Notizen I. Prufe. d. Arzneimittel, p. 33, m. Berucksich der Herausgabe e. neuen osterreichischen Pharmacopoe, Frick. Vienna. 1889. (18) Marcusson, J., and Winterfeld, G., Chem. Umschau Fette, Ole, Wachse Harze, 16, 104 (1909). (19) Naael, W., and Komchen, M., Wiss. Yerdfentlich. SiemensK&zern, 6, 235 (1927). (20) Niegemann, C. N., Farben-Ztg., 21, 207 (1915). (21) Parker, W. B., J. Oil Colour Chem. Assoc., 5 , 197 (1922). (22) Parry, E. J., Chemist Druggist, 59, 689 (1901); 62, 175 (1903). (23) Richmond, G. F., Philippine J. Sci., (A)5, 177 (1911). (24) Richter, R., Pharm. Ztg., 59, 238 (1914). (25) Rudling, A., Chem. Rev. Fett- Harz-Ind., 10, 51 (1903). (26) Seltz, H., and McKinney, D. S., IND.EXQ.CKEM.,20, 542 (1928). (27) Sineh, P., Chem. Rev. Fetf- Harz-Ind., 18, 85 (1911); 19, 234 (1912); J . SOC.Chem. Ind., 29, 1435 (1910).
Vol. 5 , No. 4
Slack, H. F., Chemist Druggist, 87, 673 (1915). Steel, L. L., and Sward, G. G., J. IND. ENO.C H ~ M14,57 ., (1922). Stock, E., Farben-Ztg., 34, 1727 (1929). Umney, J. C., Phnrm. J.,(4) 21, 653 (1905). Whitmore, W. F., Weinberger, H., with Gardner, W. H., IND.ENQ.CHEM.,Anal. Ed., 4, 48 (1932). (33) Williams, R., Pharm. Zentralhalle, 30, 152 (1889). (34) Wolff, H., Chem...Rev. Fett- Hnrz-Ind., 21, 142 (1914); Chem. Umschau Fette, Ole, Wachse Harze, 28, 99 (1921); Farben- Ztg., 26, 1573 (1921); 27, 3130 (1922); Chem.-Ztg., 46, 265 (1922); Farbe u. Lark., 31, 245, 258, 269, 282 (1926). (35) Wolff, H., “Die Naturlichen Harze,” Wissenschaftliche Verlagsgesellschaft, m. b. H., Stuttgart, 1928.
(28) (29) (30) (31) (32)
RECFOIVED March 11, 1933. Contribution 7 from the Shellac Research Bureau of the U. S. Shellac Importers’ Association.
Glass Spheres for Viscosity Determination of Cuprammonium Solutions of Cellulose L. S. GRANT, JR.,AND W. M. BILLING, Hercules Powder Company, Inc., Hopewell, Va.
T
Large quantities of glass spheres, all of which that recommended by the CelluH E accuracy of the falltirne of fall through a standard lose Division of the AMERICAN have the ing-sphere m e t h o d for CHEMICAL SOCIETY and then to viscosity determination liquid, can be obtained by the method described make all other beads have the depends in a large m e a s u r e herein. Since all these spheres have fhe same on the degree of care exercised s a m e t i m e of f a l l t h r o u g h in the selection of the spheres characteristics of fall in a given uiscometer, standard castor oil. The selecthey are particularly suited for those viscosity tion of such a standard sphere used. which require glass spheres, as, f o r and the method employed for When metal spheres are used, matching this s t a n d a r d with t h i s selection p r e s e n t s no example, cuprammonium solutions of cellulose. other spheres are d e s c r i b e d difficulties because of the unibelow. form aravitv of metal and the precisyon to” which such beads may be ground. I n certain SELECTION OF STANDARD SPHERE solutions, however, the use of practically all metals is proThe AMERICANCHEMICAL SOCIETY standard method calls hibited, because of the corrosive nature of the solution, the high specific gravity of the metal, or the opacity of the bead. for a glass sphere 0.125 inch (3.175 mm.) in diameter and Since this is especially true of cuprammonium solutions of having a specific gravity ranging between 2.4 and 2.6. The beads on hand, when difficulties were encountered cellulose, glass beads should be used for such viscosity work. I n the standard method for the determination of the in obtaining additional supplies, had a diameter of 3.17 mm. viscosity of cellulose as described by the Division of Cellulose and a specific gravity of 2.45. These spheres fell through Chemistry of the AMERICANCHEMICAL SOCIETY (I), a bead standard castor oil of 686 centipoises at 25’ C. in a standard of certain specifications is required. Glass beads when viscometer tube (A. C. S.) a t a rate equivalent to 22.3 centimolded or blown are not uniform in shape, size, or specific poises per second of fall. Since these spheres are well within gravity. The effect of these irregularities is overcome in the A. C. S. limit they were set up as the standard for comthe above method by calibrating each bead separately and parison of all future lots of beads. reporting the viscosky in poise;. This tediois procedure PROCEDURE FOR CALIBRATION leads to the use of a large number of spheres, each of which The essential steps in obtaining spheres which will give may have a different centipoise rating. The use of such spheres is obviously impractical where a large number is in the same time of fall as the standard sphere are as follows: constant use. selection from laboratory supply houses of glass beads In seeking t,o overcome such difficulties, it was found that approximately 4 mm. in diameter; segregation of these certain types‘ of glass beads can be ground to an exact di- beads by means of dense liquids into batches having narrow l i m i t s of specific g r a v i t y ; ameter with a great deal of and grinding these beads on precision. However, differ32 the basis of t h e i r a v e r a g e ent lots of glass beads vary 31 gravity to a diameter which w i d e l y in specific g r a v i t y has been d e t e r m i n e d from and quite often beads in the 90 the Ladenburg modification same lot will vary, in part b e c a u s e of different com29 of Stokes’ law (2, 3). positions of the individual SELECTION OF BEADS. The selection of the unbeads, and also because of 26 ground beads is im p o r t a n t the o c c l u s i o n of small air bubbles. These conditions 27 because the H a r t f o r d Steel p r o m p t e d the a u t h o r s to 2 0 Ball Company, H a r t f o r d , 1. CORRECTION OF VISCOSITY SPHERE DIAMETER FOR Corm. , h a s f o u n d t h a t select a “standard” sphere of FIGURE SPECIFIC GRAVITYTO GIVE CONSTANTTIMEOF FALL not all b e a d s will s t a n d closer s p e c i f i c a t i o n s than
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