Measurement of Static and Dynamic Foams in Characteristic Units

static foam measurements for egg albumin. Foams of egg albumin do not follow a logarithmic subsidence. Beer foams obey a logarithmic rate of subsidenc...
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Measurement of Static and Dynamic Foams in Characteristic Units J

GEORGE L. CLARK AND SYDNEY ROSS University of Illinois, Urbana, 111.

M

ETHODS of measuring the foaminess of liquids have ported in the literature, which are based on the uses of foams been considerably influenced by the ultimate indusfor industrial purposes other than ore flotation. Brewers, trial application to which the foam-producing liquid bakers, and soap, paper, and glue manufacturers, etc.-in is to be put. By far the greatest attention has been paid to the fact, all industries in which the production of a foam is either “dynamic” method of foammeasurement,a term first coined by a desirable or an undesirable occurrence-have reported Hansley (20), in 1928, to describe a system in which the foam methods of foam measurements which do not correspond to is continually produced by a current of gas passing through the dynamic method investigated by Bikerman. Foams can the liquid. The application of foams in ore flotation appears be produced in any one of several possible ways, but these to be the cause for this method of foam measurement. An nondynamic or static methods are characterized by the fact apparatus of this type was used by Aleinikov (1) for methylthat measurements are made after the production of the foam and not during its formation.. aniline and cresol foams, by Foulk and Miller (16) for inorganic salt solutions, and by Ostwald and Siehr (SO) to inStatic methods vary among themselves chiefly in the manvestigate the relation between the height of the foam, gas ner in which the foam is produced. I n the case of egg whites pressure, and blowing time. Finally a unit of foaminess for (4, 6),albumin (IS), and casein (32) a foam was produced by dynamic foams was established by Bikerman (7):based on the whipping; foaming of saponin solutions was promoted by hypothesis that the capacity forcing them through a fine for the formation of foam filter into a vessel maintained is a physical property of a a t low pressure (6); foams liquid. have been produced by pourT h e a p p a r a t u s used b y Dynamic and static foam meters, for Use ing the liquid from a height B i k e r m a n was of the type with any gas which will produce foam in ( w I , 2 2 , 2$,36),by bubbling a recm-~mendedby F o d k and gas through it (8-21, $4, 261, any liquid, are described. The results of measurements of dynamic and static foams by Pouring sand into it (8.4)) Miller (16) and by Lederer (26). A measured volume of or merely by shaking i t (37). air is forced through zt porous of egg albumin sols are tabulated for both All these methods agree in membrane and through a layer air and carbon dioxide i n the gaseous phase, that measurements are not of liquid above it. The volume and comparison is made with correspondmade until after the foam is of foam which is formed is ing beer foams, formed. There is an even measured in a calibrated tube. Evidence for a reaction between carbon greater between It was found that the average the units used to express the dioxide and egg albumin sol is deduced foam volume was proportional results. In fact, it is not from foaminess data. Units of foaminess to the rate of streaming of the possible to find any two methair. Hence, if V is thevolume for both static and dynamic foams are ods employing the same units; discussed, and methods of foam measurein a t least one case where four of air forced through the membrane in time and is the ment classified and correlated. investigators used the same average volume of the foam, method, they each employed totally different units (8, IO, v =fW/Q $2,841. The present work is an attempt to establish a unit for static foams, comparable to the where f is a constant independent of the rate of streaming. Bikerman unit for dynamic foams, and if possible to establish It was found that the value of ut/V observed for large amounts a correlation between the methods. of liquid was independent of the size or shape of the apparatus The attempt to evaluate the average life of a bubble in a or the rate of air flow; hence Bikerman proposed the use of static foam can be made for any system in which the rate of this function as the unit of foaminess. I n the centimetercollapse is known or where sufficient data are available to gram-second system it has the dimensions of time (seconds), enable its calculation. and the symbol Z: was proposed for i t (from the Greek saIt has been found empirically for beer and saponin foams ponos, meaning “lather”). It has been shown that the physical significance of 2 is the lifetime of a unit volume of foam. that The work of Bikerman, however, has left out of account a large number of methods for foam measurement, already rej(t) = = jO(t)e-’lk’ (1) 1594

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where ;fo may be assumed for convenience to be zero, and f (t) is the integral life of a unit of foam volume. Then the average life of a unit volume of foam, 5, up t o complete collapse (or time infinite) may be represented by

wheref’(t) denotes the derivative of Equation 1. I n order to evaluate this equation, i t is assumed that the number of bubbles present a t any given instant is proportional to the weight of the foam. This applies to a static foam when no great height is present and hence presents no large error for variation in the number of bubbles per unit of volume a t the top and a t the bottom of the foam. The validity of this assumption is not affected by the well-known fact that a gradation of bubble sizes exists in the foam. It is supposed that large bubbles are collapsing a t much the same rate as small bubbles are coalescing to form larger ones; hence, a similar gradation of sizes would exist in the foam even after it has been standing for several minutes. It is also recognized that a familiar equation expresses the number of bubbles, N , in unit, mass:

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and E are closed, and stopcocks D and F are opened; at the same time a stop watch is released. Water flows into reservoir R-2 under constant pressure and forces the gas through the membrane in a steady stream, which soon reaches a constant rate of flow. The foam in column G begins to build up and reaches a maximum height which is observed as recommended by Bikerman (7)-i. e., every 5 seconds for slightly foaming substances, every maximum height for strongly foaming substances. (Obviously the ideal heights which would have been attained if no collapse had occurred during formation cannot be measured.) When a sufficient number of observations has been made, stopcock D is closed, and simultaneously the stop watch is arrested. The volume of air passed through the membrane in the recorded time is found by opening stopcock C and siphoning the water out of reservoir R-2 into a measuring cylinder. The volume divided by the time taken for the experiment gives the rate of flow of the gas through the membrane.

1 P2 N = p m r where p = gas pressure within bubbles T = surface tension of bubble substance Ar: = bubble wall thickness p = density

The right-hand member can be constant only if the ratio p / T is independent of pressure and AT is invariable. This is obviously a special case; but the quantity of liquid a t the bubble interfaces may be so large that the material of the actual bubble wall contributes only a small weight fraction to the whole This may be the explanation of the validity of the assumption empirically established by the experimental results to be described. Thus suppose No bubbles are initially present in a weight Wo,and N bubbles are present finally in a weight of foam W after time t. Then, W/Wo = N/No; N

Naf(t)

3

N&-kt = N&-t/k’

(3)

Using Z = l/k = k‘ as the unit of measurement for static beer foams, the authors demonstrated (3s) that the results are independent of the method of producing the foam. It can be formed by bubbling the gas through the liquid, by pouring the liquid from a height (if carbonated), or merely by shaking the liquid in the presence of the gas, and in each casethe value of Z is found to be the same. This establishes the usefulness of the unit for static foams and leaves only the question of the possibility of a correlation between static and dynamic foam units.

Dynamic Foam Measurement

FIGURE 1. DYNAMIC FOAM METER

Egg albumin solutions of 2 per cent concentration were stored in an icebox under a mixture of xylene and paraffin oil. The method of storing and withdrawing the solution was that recommended by Bernhart and Arnow ( 6 ) . Solution was removed from the bottom of the bottle when required and diluted with volumetric accuracy to 0.01 er cent. Table I gives the data on these solutions, using air as &e gas with which to form the foam. Two U-tubes packed with ascarite were employed to remove carbon dioxide from the air. Different volumes of solution were used in separate runs, as recorded, to determine if changing the volume after 100 cc. had any effect on the result. Column G was cleaned with an alkaline cleaning solution ( 2 per cent sodium carbonate and 0.5 per cent sodium hydroxide) between each run, and when not in use, it was kept filled with distilled water. A preliminary rinsing with the solution whose foaminess was t o be determined was made prior to each run. The values of L’ are calculated from Equation 1. TABLE I. DYNAMIC AIR FOAMS OF 0.01 PER CENT ALBUMrN Foam No.

Av. max. ht om.

An apparatus for the measurement of dynamic foams was constructed as shown in Figure 1; it is a modification of the one used by Lederer (26). The chief improvements are the addition of a device for a constant head of water and the extension of the function of the apparatus to allow foams made with gases other than air. The ap aratus is made ready for use by closing stopcocks C, D, E,and %’and opening stopcocks A and B. This allows the first, or reservoir, part of the system to be evacuated. The gas cylinder is connected, stopcock E is opened, and the reservoir fills with the gas t o be used in the foam. When the pressures have been equalized, as indicated by a suitable manometer, stopcock F can also be opened and the second part of the system flushed with the gas. This process is repeated three times at least, to assure only a small concentration of air left in the system. To obtain measurements, the liquid to be investi ated is placed in columnLC, and the level observed. Stopcocks B , C,

3,

v o ~of soln “cc.

1nit’ialht.oflliquid om. Max. foam ht., cm: Vol. of air, ca. Time, sec. Rate of flow, cc./seo. 2, sec.

I 13.3 100 8 2 5.1 500 184 2.7 34.2

2 33.1 100 8.3 4.8 500 185 2.7 32.3

3 13.0 100 8.3 4 7 550 214 2.57 32.8

4 17.8

200 13.6 4.2 675 312 2 16 36.3

8

22.6 300 19 4 3.2

660

320 1.75 33.8

6 12.8 100 8.4 4.4 840 217 2.49 39.2

Measurements in Tables I and IIA were made on the same solution, using an air foam in the first and a carbon dioxide foam in the second case. The results are directly opposite to what might be expected in the case oi a static foam, indicating totally different rates of coagulation for the two methods of measurement. Aleinikov ( 1 ) made the suggestion that in dynamic foam measurements coagulation takes place with the

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of the air foam, the logarithmic law, as evidenced by the constancy of the values for 2 , applies until about 50 per cent of the foam has disappeared, when the values of Z begin to increase. This is precisely what would be expected from the escape of the more soluble gaseous constituents first leaving the less soluble gaseous constituents in the foam. The logarithmic law for carbon dioxide foams holds for at least 90 per cent of the lifetime of the foam. From the data of Helm and Richardt (2222)the ratio of the stabilities of air and carbon dioxide foams can be calculated and compared with the ratio found in the present investigation. The results are 3.8 for the former and 4.5 for the CARBON DIOXIDE FOAMS OF 0.01 PERCENTALBUMIN TABLE 11. DYNAMIC latter method. When it is remembered that Helm Aged Albumin--A. Fresh Albumin--B. 2 3 4 1 2 3 4 1 and Richardt used the Carlsberg method and dif36.8 32.4 29.8 37.0 36.8 Av max.ht om. 33.0 32.6 50.2 ferent samples and units of measurement, this 15.8 15.4 15.2 11.4 17.0 15.8 15.1 17.4 Iniiial. ht. o? liquid, om. is a satisfactory agreement. 16.6 14.4 21.2 17.0 15.2 35.0 25.6 19.8 Max. foam ht., Table IIIB gives data for static air and carbon om. 1160 1570 1255 1860 1665 1635 1550 2027 Vol. of gas. cc. dioxide foams of 0.2 per cent egg albumin. The 973 1014 969 967 1043 598 776 1090 Time 880. 1.29 1.14 1.62 2.40 1.52 Rate'of gas flow, 1.69 1.49 3.38 stop watch was not released until the volume cc./sec. reading was 20 cc. ; consequently 2: is calculated 2 , 8ec. 193 186 189 194 237 238 234 229 from the equation: rupture of newly formed surfaces which are continually being replenished, but that in static foam measurements, coagulation involves the rupture of surface films which have already been standing for some time. Consequently, i t was of interest to observe whether aging the protein solution had any effect on its foam stability. The concentrated protein solution was kept at a low temperature for three weeks, and then further measurements were made. These data (Table IIB) show that a change in the foam stability does take place on aging.

An obvious difference in the appearance of air and carbon dioxide foams produced by this method exists. The air foam rises only to a small volume and is composed of relatively large bubbles; coagulation a t the surface is not observable, and when the foam subsides, i t leaves no solid scum sticking to the walls. The carbon dioxide foam rises to a much greater height, has smaller bubbles, and deposits a solid scum on the wall when it subsides. Occasionally, in such foams the coagulated protein seems to form a rigid film on top of the foam and prevents the proper rise of the froth above it. Results in experiments where this was observed were lower than the others and were consequently discarded. If the rate of flow of the gas through the membrane is very slow, there is more chance for i t to take place, owing to the length of time required for the experiment. On the other hand, if the flow of the gas is very fast, the foam rises rapidly and continues to rise throughout the course of the experiment, apparently not reaching its constant maximum value before the gas in the reservoir is completely used up. Results from experiments of this type are again too low and must be discarded. These two factors fix the possible rates of gas flow within rather narrow limits, as shown in Tables I and 11, and prevent adequate investigation of a possible variation of Z with the rate of flow. Within the experimental range observed such a variation did not occur.

E=

2.3 log

t 80 100 - v

Static Foam Measurement An a paratus for the investi ation of static foams was constructefas shown in Figure 2. Sxactly 100 cc. of the liquid are poured into the graduated bottom of the apparatus, and the foam is produced by means of a Chamberland filter. When the liquid has been dispersed into a foam, a atop watch is released and the filter withdrawn. The time taken for the advance of the liquid level between each 5-cc. graduation mark on the cylinder is noted. When, as sometimes happens, the rise of the liquid is very rapid the time between 10-cc. marks is taken instead.

Results for both air and carbon dioxide foams on the same sample of beer are given in Table IIIA. The calculation of Z is by the following equation, derived from N = Noe--kt and = l/k,

z

-

=

2.3 log

1 -v 100 100

(4)

where V is the volume reading of the graduated tube. The data of Table IIIA obtained by means of the static foam measuring apparatus are in agreement with the logarithmic formula for a carbon dioxide beer foam. In the case

FIGURE 2. STATIC FOAM METBR

It is apparent that egg albumin foams do not obey a logarithmic law. This is also in accordance with the data OF Barmore (6) which do not indicate any logarithmic decline. However, as is the case with static foams of beer, the air foam is considerably more stable than the carbon dioxide foam.

Experimental Results SUMMARY. Measurements of dynamic foams of egg albumin show that the carbon dioxide foam has a higher vaIue for Z than the air foam.

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show that the air Measurements of static foams of egg foam has a higher value for 2 than the carbon dioxide foam. Dynamic foam measurements give higher values for than static foam measurements for egg albumin. Foams of egg albumin do not follow a logarithmic subsidence. Beer foams obey a logarithmic rate of subsidence for 50 per cent of the lifetime of the foam if the foam is produced with air, and for 90 per cent of the lifetime of the foam if the foam is produced with Dure carbon dioxide.

tially (according to Gorter, 17), completely (according to Bull, la), and that the reaction is not immediate (as, 23)* The denatured and, in some cases, coagulated film has a superior tensile strength; consequently, as the foam subsides, those surface films which have been standing for a longer period resist rupture More and more effectively and cause an increase in the average life of the bubble as the foam ages. I n the case of air foams, only denaturation of the surface egg albumin appears to take place. This is in accordance with the x-ray study of FOAMS FOR BEERAND TABLE111. S T a T I C AIR AND CARBON DIOXIDE Astbury ( 3 ) , as well as Bull's study of protein ALBUMIN films ( l a ) , in which he found the amount of deB . 0.2% Albumin A . Beer -Time, set.- y - 2 -Time, set.z naturation to be proportional to the rate of Vol., Air COz Air Con Vol., Air COz Air Cor cc. f o a m foam foam foam formation of new surfaces. However, when carcc. foam foam foam foam 5 10 .. 195 ... 20 0 0 ... ... bon dioxide is used to produce the foam, a solid 10 24 228 .. .. .. 25 3 :$:E scum sticks to the walls of the tube when the 222 30 141 9 15 36 .... 20 51 11 229 49.4 35 21 14 101 67.6 foam has disappeared. This and other evidence 25 64 .. 225 ... 40 35 19 122 72 6 6 ,.; 2 reopens the unsettled question as to whether car30 80 17 225 46.6 45 52 27 139 35 96 .. 223 . . . 50 79 34 168 ; bon dioxide gas reacts with albumin sols. It was 40 115 24 227 47.4 55 116 44 202 45 138 .. 231 ... 60 169 54 244 78.0 so affirmed by Mellanly and Thomas (68), by 50 164 31 237 44.8 65 251 67 304 81.1 84.7 Croak (19), and by Pauli (SI),but denied by 55 195 , . 244 , .. 70 373 83 381 60 228 39 249 42.6 75 566 103 487 i;,: Gregorie (18) on the basis of a lack of increase in .. . 128 ... 65 271 45 258 42.9 80 70 321 53 269 44.5 85 ... 162 ... 96.9 the conductivity measurements. If, however, 75 385 61 278 44.0 90 ... .. . .. 104 116 the reaction is of such a nature as to produce 46.0 93 ... 216 283 74 287 80 462 90 .. . 85 .. . 47.5 94 ... 328 .... . 140 127 coagulation of the protein molecule, the conduc95 .. . 388 .. . 49.6 00 ... 114 tivity need not be affected. Also, Stady and O'Brien (36) proved a carbamate reaction between carbon dioxide and hemoglobin and serum DISCUSSION.The basic assumption in the derivation of Z , albumin, based on analysis for carbamate by the method in Equation 4 for static foams is the postulate that the volume of Ferguson and Roughton (16). Therefore, strong preof the liquid formed from the foam is a measure of bubble desumptive reasons exist to suggest a reaction between egg albumin and carbon dioxide, which forms foams composed of cay. It involves the supposition that this liquid does not come partially from a mere thinning of the bubble walls but is very fine bubbles breaking rapidly and leaving a coagulated residue. The difference of foam measurements of the two derived in toto from the collapse of the bubbles. The observations of carbon dioxide foams can be interpreted to lend some foams, when first a dynamic and then a static foam meter is used, is satisfactorily explained only on the basis of a reaction support to the assumption, based upon an argument originally employed by Barmore ( 5 ) . Barmore found a simple linear between carbon dioxide and egg albumin. If we assume a relation between the specific gravit'y of egg white foams (a carbamate reaction as the most probable one, based on the measure of the bubble size) and the rate of drainage. In analogy of serum albumin, the carbon dioxide reacts with the view of this simple relation, it does not seem probable that amphanion and not with the protein zwitterion. The nature the liquid draining from the foam can be derived from of the reaction is expressed in the following equation: more than one source. If this source is the drainage /NH . COOe"/ from between the two liquid-air interfaces without /"s+ OH-+fC"' R -R H+ any breaking of the bubbles, then on the basis of Poiseuille's equation, \COOH \COO'coo'cooV = Pr4/81v protein zwitter amphanion protein ion carbamate the rate of drainage should be proportional to the fourth power of the cross section of flow. With foams cornThe reaction with carbon dioxide is rapid in the case of amino posed of small bubbles, this cross section, whichis the thickness acids, hemoglobin, and the serum proteins (56). The decrease in the pH of the solution due to the liberation of H + of the liquid layers between the bubbles, must be decreased and hence drainage from the foam would be decreased. The obions on dissociation of the carbamate may well account for. the presence of the observed coagulation. served effect is just the opposite. As shown in Table IIIB, the A reasonable a priori assumption that the value for the avercarbon dioxide foam, which has the smaller bubble size: drains more rapidly than the air foam. Barmore (5) found that age life of the bubble in the foam would remain constant whether the measurements be made by a static or a dynamic foams which were finely whipped (small bubbles) drained faster than loosely whipped foams. These results bear out the foam meter is shown by the present study to be unjustified, a t contention that the liquid volume which is measured origileast for egg albumin foams; further consideration of the mechanism of foam formation and decay will show that i t is nates in the collapse of the bubbles and so considerably strengthens the claim that Z for static foams is a real measure probably invalid in all cases. Foaminess as measured by the of the average life of the bubble. static foam meter is based on the rate of decomposition of the The use of Equation 4 for static foams is, of course, also foam. In the case of the dynamic foam meter the measuredependent on the logarithmic subsidence of the foam. Such a ment depends upon an equilibrium between the rates of formarelation has been found to exist for beer foams (10) and solution and decomposition. For identical results to be obtained tions of different concentramtiomof saponin (a) where no by the two methods, not only must the rates of decomposition chemical reaction takes place between the gas and the liquid. be the same in both cases, but the ultimate value of 2 , in the In the present study of static foams of egg albumin an increase case of dynamic foam measurements, must be independent of of 2 as the foam ages (Table IIIB) may well be explained the rate of formation. The reason for the second condition is by the fact that egg albumin denatures in surface films, parthat the rate of formation is not taken into account in static 7

7 -

--

;g

s,R +

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foam measurements; the results of a previous paper (33) offer proof that the value of Z is independent of the gas pressure, which determines the rate of formation. The first condition, that the rates of foam decomposition be identical, can never be obtained in practice; the degree of deviation between the values depends on the nature of the constituent in the surface layer. As already stated by Aleinikov ( I ) , the coagulation of a dynamic foam involves the rupture of newly formed surfaces, whereas the coagulation of a static foam involves the rupture of surfaces which have had time to age and, moreover, continue to age even while measurements are being made. While this factor may not be significant in many cases, it certainly has an effect in the case of egg albumin foams where, as shown by Bull (18), denaturation, and possibly ultimate coagulation, takes place on the surface layer. It has also been claimed that these changes in the surface layers of proteins are not immediate but take place over a period amounting to several minutes (66, 89). That the aging of solutions affects the foaminess can be observed from the results of Table IIB, where the solution was aged under conditions which tended to minimize any chemical reaction. A much greater effect would be expected when the aging of the solution takes place in the large surface areas which exist in the foam. The result of denaturation appears to be an increase in the foaming property of the protein sol, as manifested by the increase in Z for aged solution (Table IIB) and the continuous increase of B (Table IIIB) as the foam ages. This is in accordance with the results of Loughlin (97) who found an increase in the capillary activity of protein sols on denaturation. The work of Duce (14) on the aging of protein sols showed a series of maximum and minimum changes in the values of surface tension and viscosity as the sol aged. It is well known that these properties also influence the foaminess. I n such circumstances a correlation between the static and dynamic foam measurements cannot be expected. I n his work on the unit of foaminess for dynamic foams, Bikerman (7) demonstrated that the value of Z was independent of the rate of flow of the gas through the membranein other words, independent of the rate of formation of the foam-but his investigation was conducted on air foams of butyl alcohol solutions where no reaction took place between the constituents in the two phases, In the case of carbon dioxide foams of egg albumin, we must postulate a reaction between the gas and the protein in order to account for the fact that dynamic foam measurements show the carbon dioxide foam of egg albumin to be more stable than the air foam, whereas the static foam measurements show just the opposite result. The bubbles in the carbon dioxide foam are much finer than in the air foam; consequently, the volume of foam per unit volume of gas would be slightly greater. At the same time, some of the gas is absorbed by the protein sol and hence plays no part in the tormation of the foam. The result is that even when the same rate of flow of gas through the membrane is used, the rate of formation of the foam in carbon dioxide differs from the valqe it has when the gas is air. This violates the second condition for a parity in results for the two methods. The measurements by the static method indicate the difference in the rates of foam decomposition, and the measurements by the dynamic method indicate that the difference in the rates of formation is more than enough to overcome the difference in the decomposition rates between air and carbon dioxide foams of egg albumin. There can be no doubt that the use of air and carbon dioxide foams of egg albumin exhibits the deviation between the two basic methods of foam measurement to a maximum degree. The results of the present study show that methods of foam measurement can be correlated, even when the foam is produced in different ways, if only static methods of measurement are used. In this connection, a unit of foaminess for static

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foams is provided. The work of Bikerman (7) has provided a comparable unit for dynamic foam measurements. It is now apparent that with these two broad distinctions all the methods of foam measurement can be classified and methods within each group correlated, apparent inconsistencies between the two classes can be explained, and, i t is believed, a sounder bask established for the development of further methods.

Literature Cited (1) Aleinikov, N. A., Kolloid-Beihefte, 36,82 (1932). (2) Arbuzov, K. N., and Grebenshchikov, B. N., J . Phys. Chem. (U. S. S. R.), 10, 32 (1937). (3) Astbury, W. T., Bell, F. O., Gorter, E., and Ormondt, I., Nature, 142, 33 (1938). (4) Bailey, M. I., IND. ENQ.CHEM.,27, 973 (1935). ( 5 ) Barmore, M., Colo. Agr. Expt. Sta., Tech. Bull. 9 (1934). (6) Bernhart, F. W., and Arnow, L. E., J. Phus. Chem., 43, 733 (1939). (7) Bikerman, J. J., Trans. Favaday SOC.,34, 634 (1938). (8) Blom, J., J. Inst. Brewing, 43, 251 (1937). (9) Blom, J., Petit j . brasseur, 42,388 (1934). (10) Blom, J., and Prip, P., Wochschr.Brau., 53, 11 (1936). (11) Blom, J., Prip, P., and Jacobsen, J.,Ibid., 53,25 (1936). (12) Bull, H. B., J. Bid. Chem., 123, 17 (1938). (13) Chang, H.-Y., and Hsieh, M.-S., J. Chinese Chem. Soc., 2, 117 (1934). (14) Duoe, W., Boll. SOC. {tal. bid. sper., 10, 977 (1935). (15) Ferguson, J. K. W., and Roughton, F. J. W., J . Physiol., 83, 68 (1934). (16) Foulk, C. W., and Miller, J. N., IND.ENQ.CEEM.,23, 1283 (1931). 33,1125 (1937). (17) Gorter, E., Trans. Faraday SOC., (18) Gregorie, E., Compt. rend. soc. bid., 99, 1227 (1928). I Groak, B., Biochem. Z., 196, 478 (1928). I Hansley, V. L., Ohio State Univ., doctor’s dissertation, 1928. I Helm, E., Wochschr.Brau.. 50.241 (1933). Helm, E., and Riohardt, O., J: Inst.’Bre&ng, 42, 191 (1936). Laufer, S., and Ziliotto, H., Am. Brewer, 72, No. 8, 25 (1939). Ibid., 72, No. 9, 33 (1939). Lederer, E. L., Seifensieder-Ztg., 63, 331 (1936). Lee, W.-Y., and Hsien, W., Chinese J. Physiol., 6, 307 (1932). Loughlin, W. J., Biochem. J . , 27,97 (1933). Mellanly, J., and Thomas, C. J., J. Physiol., 54, 178 (1920). Neurath, H., J.Phys. Chem., 40, 361 (1936). Ostwald, W., and Siehr, A., Kolloid-Z., 76,33 (1936). Pauli, W., Biochem. Z., 152, 360 (1924). Richards, C. W., and McFail, L. W., Paper Trade J . , 97, No. 1, 29 (1933). Ross, S., and Clark, G. L., Wallerstein Labs. Commun. Sci. Practice Brewing, No. 6,46 (1939). Schuster, K., and Mischke, W., Wochschr.Brau., 54, 177 (1937). Stady, W. C., and O’Brien, H., J. Bid. Chem., 112, 723 (1936); 117, 439 (1937). Westelaken, E”. P. van der, and McCormack, R. H., Am. Brewer, 69, No. 7, 24 (1936). Williams, H. E., IND. ENG.CHEM., 18, 361 (1926).

Correspondence-Evaluation of Nitrocellulose Lacquer Solvents

SIR: A statement in our article [IND. ENG. CHEM.,32, 78 (1940)] regarding the mixed aromatic-aliphatic naphtha used indicates that it is “a commercial material with a distillation range of 95-132’ C., containing approximately 30 per cent aromatics”. Recently we have been informed that considerable improvement in the product has been made during the last few years. The sample in question, used throughout our work, was procured a little over four years ago. Its aromatic content was based upon the dimethyl sulfate solubility of the material, a method generally accepted at that time. V. W. WAREAND W. M. BRUNER