HEAT O F WETTING O F CHARCOAL AS A MEASURE
OF I T S ACTIVITY’ BY RUDOLPH MACY?
The use of charcoal as a gas adsorbent in the industry and in warfare has led to a voluminous literature on the methods of determining the quality of the charcoal. Probably the most complete bibliography on adsorption by charcoal is that included in the monograph on the subject by Bluh and Stark.3 A typical discussion of the activity of charcoal is the recent article by L0wr3i4 in which it is stated that most investigators use for their measure of activity of charcoal the “adsorptive capacity” of the charcoal under certain arbitrary conditions. In this category he includes the “retentivity” as defined by Chaneyjv6 and later explained more fully by Allmand’. The above mentioned paper by Low@ contains a good resume and bibliography of recent work on the relationship between activation of charcoal, the capacity of the charcoal, and its surface area. It is noteworthy, however, that practically nothing is to be found in any of the English or American journals on the usefulness of the heat of wetting of charcoal as a measure of activity. A fairly complete historical account of investigations concerning heat of wetting is to be found in the text book by Freundlich.* The important names in this early work described by Freundlich are Pouillet ( 1 8 2 3 ) , Melsens ( 1 8 7 4 ) ~ Gore (1894), Chappuis (1883) and Gaudechon (1913). More recently, Berl and AndressQmade a study of a precise method of determining adsorption isotherms of vapors on charcoal and followed this by determination of the heat of wetting in benzene. Their assembled data and curves show that the ether isotherms for various charcoals are practically quantitatively related to the heat of wetting in benzene. Berl and Andress consider the heat of wetting to be a function of both the “saturation capacity” and “intensity factor” of the adsorbent. Herbstlo obtained the heat of wetting in water and in benzene and found that these data were proportional to the activity of the charcoal, when the activity is based on the adsorptive ability as compared with * Contribution from the Chemical Warfare Service, Edgewood Arsenal. This problem was suggested in June, I929 by Captain M. E. Barker, Chief of Research Division, Chemical Warfare Service. Research Division, Edgewood Arsenal, Maryland. Bliih and Stark: “Die Adsorption” (1929). Lowry: J. Phys. Chem., 34, 63 (1930). Chaney et al: Trans. Am. Inst. Chem. Eng., 15, 283 (1923). OCChaney: Trans. Am. Electrocbem. Soc., 36, 91 (1919). ’Allmand: J. Soc. Chem. Ind., 47, 370 (1928). Freundlich: “Colloid and Capillary Chemistry.” Berl and Andress: Z. angew. Chem., 34, 369 (1921); 35, 722 (1922). l o Herbst: Kolloid-Z., 38, 320 (1926).
‘398
RUDOLPH MACY
a standard high grade charcoal. Honig,” also, has shown that there is a distinct relation between heat of wetting and adsorptive capacity. Alekseyevskiil* applied the methods of Berl and Andress (loc. cit.) to discover whether the heat effects could be used to grade the value of absorbent materials for a variety of vapors. Finally, Burstin and Winkler,13state that there is a direct proportionality between maximum adsorption of benzene vapor and heat of wetting in benzene. It is the purpose of this article to show that the heat of wetting in benzene is at once the most rapid and one of the best methods of determining the quality of a charcoal for gas adsorption. The heat of wetting is proportional not so much to the total adsorption capacity as stated by these earlier authors, as it is to the capacity for firmly held vapor, or retentivity. The distinction between these two capacities has been explained by Chaney (loc. cit.) as follows. Adsorption of vapor is due to two forcesa) Capillary, or physical, forces. The vapor held by capillary adsorption practically has the properties of the liquid state, such as relatively high vapor pressure, and is easily eliminated either by application of vacuum or passage of dry air over the sample. b) Adsorptive, or chemical, forces. Part of the vapor is held by secondary forces to the surface of the carbon. Its vapor pressure is extremely low and it can be completely eliminated only by heating. The amount of vapor held by true adsorption forces is the “retentivity.”
Experimental Part The charcoals used were of various types. A large number were obtained from commercial sources, and some were prepared a t Edgewood Arsenal. The nature of the samples is described in Table 11. The charcoal samples were dry-screened 8-14 mesh on Tyler Standard Screens, and in most cases they consisted of equal amounts of 8-10 and 10-14mesh. The charcoal was dried for two hours a t I jo”C. before use. The adsorption capacity, or saturation value, of the charcoal samples was determined in several ways. I n all cases the capacity for benzene vapor was found by placing a weighed sample of two to five grams in a desiccator over the liquid. The temperature was that of the room and did not vary more than two degrees from an average of 25°C. The error introduced by this temperature variation was negligible considering the nature of the material being studied. Constancy in weight of the samples exposed to vapor was usually reached in about two days. I n one series of experiments the capacity of charcoal for chlorpicrin vapor was determined by the method just described for benzene, that is, by exposing weighed samples to the saturated vapor of chlorpicrin a t 25OC. In another series of determinations the adsorption capacity of the charcoal was obtained Honig: Kolloidchem. Beihefte, 22, 345-420 (1926). Alekseyevskii: Zhurnal prikladnor Khimi (Moscow), 1, 182 (1928). 13 Burstin and Winkler: Preemyel Chem., 13, 114 (1929). 11 1’
HEAT O F WETTING OF CHARCOAL AND ACTIVITY
I399
a t a chlorpicrin concentration in air of 47 mg./liter. This was done by passing the chlorpicrin-air mixture through a I O cm. layer of the sample contained in a tube of I .41 cm. diameter according to the usual procedure for measuring the service time of the charcoal. This method of determining service time has been discussed by Lamb, Wilson and Chancy." After the service time had been obtained the passage of chlorpicrin through the charcoal sample a t z 5 T . was continued until the sample no longer gained weight. The total time required was usually less than two hours. The retentivity values were determined by placing the saturated charcoal samples in a U-tube of 1.4cm. diameter and passing dry air through the sample a t the rate of 1000 cm. per minute for two hours a t z 5 T . The amount of vapor retained a t the end of this treatment was arbitrarily called the retentivity. Before using this method the retentivities had been measured by a method developed by Dr. Leo Finkelstein in this laboratory, by suspending the saturated samples in a bucket from a quartz fibre spring balance according to the procedure of McBainl6and evacuating the system to about ,002 mm. Hg a t z5'C. The loss of weight was followed by a telescope and cathetometer arrangement and the system kept evacuated until no noticeable loss in weight occurred for about three hours. A single experiment occupied about 2 5 to 40 hours, but practically the same results were obtained by passing dry air over the sample for two hours as already described, and the data were also more consistent and more easily duplicated by the latter rapid method. As already stated, the retentivities obtained in this work are arbitrary values in that they are measured by weighing the retained vapor after air has been passed over the sample for exactly two hours. In Fig. 3 a set of curves is given which show the rate of loss of vapor under these arbitrary conditions. In two hours all the charcoals give up the loosely held vapor, and then begin to give up the remainder at a slow rate which is nearly the same for all the samples. The two hours desorption period therefore should furnish good relative retentivity values, even though the results obtained are arbitrary ones. The apparatus required to determine the heat of wetting was found to be very simple. The usual practice in measuring the heat effect when a solid is immersed in a liquid is to use a well insulated reaction vessel, the most common apparatus for such a purpose being a Dewar flask. The preliminary experiments on heat of wetting of charcoal were accordingly made in a zoo cc. Dewar flask, fitted with a Beckmann thermometer and a glass stirrer both of which passed through a rubber stopper to prevent evaporation of the liquid. Measurements made in this way were very accurate, but tedious, and would not be appropriate for the use of routine analysis in plant control work. In order to keep all such heat measurements comparable it is necessary to start. each experiment at some common temperature such as zj"C., since the specific heats vary with temperature. Temperature changes in a vacuum flask are so slow that each time fresh materials are placed in the flask a considerable period must elapse before the actual determination can be made. Lamb, Wilson, and Chaney: J. Ind. Eng. Chem., 11, 420 (1919). McBain and Bakr: J. Am. Chem. SOC.,48,6yo (1926).
I 400
RUDOLPH MACY
Although the heat of wetting measurements should begin a t some common temperature, this fixed point can vary several degrees without having an appreciable effect on the results obtained. Furthermore, the heat of wetting determination with charcoal is very rapid, the maximum temperature rise when it is wetted being reached often in 3 0 seconds. Heat transfer to surroundings are therefore very slight. All that is necessary then is to use a test tube for the reaction vessel with an outer test tube to serve as an air-jacket, assembled as in an ordinary freezing-point determination. When a fresh supply of liquid at room temperature is placed in such an apparatus only a few moments elapse before it reaches a fairly constant temperature which will not be far from that of the room and can be taken as the initial temperature of the experiment. The room in which the present work was conducted seldom varied more than z o from an average of z s 0 C . The Beckmann thermometer had to be dispensed with, since it would necessitate starting each measurement at a predetermined fixed temperature, whereas it is more convenient to begin the heat measurement at whatever the temperature in the test tube might be, within a limit of z to 3 degrees. I n its place a thermometer graduated to o.oz"C. was used, reading from I 7 O to 3 I O C ' . Benzene was chosen as the wetting liquid after a study of the adsorption curves for several vapors on charcoal showed that it is one of thb most highly adsorbed substances. It would therefore be reasonable to suppose that the heat effects would be comparable with other determinations on charcoal in which high adsorptive capacity is a predominant feature. Such tests are the accelerated chlorpicrin tube test, and retentivity. The volumes of charcoal and of benzene used were simply those found by trial to be most convenient in the apparatus employed. The apparahs used can be found in any laboratory. Tne following method of carrying out the test has been found to be satisfactory. After pouring 20 cc. of benzene into the tube, the thermonietkr and stirrer were inserted and the contents stirred a few Inonients until the temperature was constant, and the temperature recorded to o.or"C. About 5 cc. of charcoal WMS weighed out to 0.001g. in a slender weighing tube about I , / Z inch in diameter. The thermometer and stopper were partially lifted from the tube, leaving the stirrer loop resting a t the bottom, and the charcoal was poured in rapidly. The stopper and thermometer were replaced and then on the first upstroke of the stirrer the charcoal and benzene were well mixed. Stirring was continued vigorously, the mercury thread climbing rapidly to a maximum position. A little practice is necessary before duplicate results can be obtained. An active charcoal gives a very rapid rise (thc maximum reached in less than 30 seconds); less active charcoals are characterized by a slower rise and slower fall in temperaturc. The maximurn rise in temperature is recorded to O . O I ~ < : . After a determination i8hetest t,ube is rapidly dried by a stream of compressed air. From the weight of sample and rise in temperature the heat of wetting is calculated in "C. 'g. The apparent density is determined separat,ely, and the heat of w t t i n g is then calculated in "C. ;cc.
1401
HEAT O F WETTING O F CHARCOAL A N D ACTIVITY
The heat capacity of the apparatus and its contents was calculated from the standard physical constants of its components, assuming an average specific heat for charcoal of 0.2 and an average weight for the sample of 2.0 grams. The water equivalent was found to be almost exactly 10.0, so that it was only necessary to multiply the heat of wetting data by the factor I O in order t o convert it to calories per g. or per cc. of charcoal. This factor is not strictly accurate because of variations in the weight of sample and specific heat of different charcoals.
Summary and Discussion of Results Heat of Wetting and Particle Size. The effect of the size of the charcoal granules was studied, using a high grade commercial coconut charcoal. This wivas screened to give fractions with the mesh sizes listed in the following PII mmary : Mesh 6-8
8-10 10-14 14-20
Apparent Density
Sample g.
"C./g.
Heat of Wetting "C./cc. cal./cc.
,487
2.318
1.88
0.915
.47 ,465
2.257
2.13
1.00
10.0
21.3
2.322
2.18
1.01
,455
1.827
2.15
0.98
IO. I 9.8
21.8 21.5
cal./g.
9.15
Within the range of particle sizes usually employed] that is, 8 to the variation in size has practically no effect on heat of wetting.
18.8
20
mesh,
Relation between Heat of Wetting in Benzene, and the Maximum Adsorption and Retentivity of Benzene The relations between these quantities are shown graphically in Figs. I and 2 , and the data are presented in Table I. Burstin and Winkler (loc. cit.) claim that the relation between total adsorption of benzene vapor and heat of wetting in benzene is expressed by a straight-line function A = K Q, where A is maximum adsorption in percent by weight Q is heat of wetting in calories per gram K is a constant, equal to 1.9 percent calories (A being expressed in parts adsorbed per I O O parts of charcoal). 4 careful study of their paper indicates that they did not use more than two different commercial samples of charcoal (one made by Bayer and the other by Merck) and these were both of similar characteristics. The work reported in this art,icle, however, is based on a wide variety of types, as shown in Table 11. I n Fig. I the data have been plotted to agree with the system of units employed by Burstin and Winkler, the broken line representing the constant K as evaluated by t,heee authors. In Fig. z the data are given on the more usual volume hapis. In both diagrams it is clear that the data are more
I 402
RUDOLPH MACY
accurately represented by smooth curves rather than by straight lines, and this is also evident for the corresponding chlorpicrin results shown in Figs. 4 and 5 . Inspection of Figs. I and z proves beyond doubt that the heat of wetting is a function of the retentivity of the charcoal rather than of its adsorption capacity. This to be expected from the very nature of the experiment. The heat evolved when chatcoal is wetted by liquid benzene is due to some sur-
FIG.I Adsorption data-ircles;
retentivity data-wosaes.
face phenomenon-whether it is considered to be an initial formation of a monomolecular layer or condensation in capillaries of molecular size, or both, is immaterial. It is hardly likely that penetration of the larger capillaries by the liquid would give rise to heat effects of measurable magnitude, since it is simply a sponge effect and not true adsorption. Thus, the Urbain charcoal and the sample of German charcoal in Table I have very high saturation capacities (this is more readily appreciated from Fig. z ) but these saturation values are out of proportion with their corresponding heat effects. At saturation they probably store up considerable benzene vapor in large capillaries in practically the liquid state. This excess vapor is easily lost, as shown in Fig. 3, when dry air is passed over the charcoal. The amount of vapor firmly held by Urbain and the German charcoal is, however, accurately predicted by the heat of wetting. It is a logical conclusion, therefore, that the heat of wetting is a measure of the amount of vapor held by true adsorption forces, or retentivity.
HEAT OF W'ETTING OF CHARCOAL AND ACTIVITY
FIQ.2 Adsorption data-circles;
retentivity data-row.
FIG.3 Rate of loss of adsorbed benzene by charcoal in a current of dry air.
I404
RUDOLPH MACY
Tryhorn and Wyatt'6 found that a discontinuity occurs in the rate of adsorption of benzene vapor by a coconut charcoal when it has adsorbed 1 2 0 x 1 0 5 moles per gram, and that its total saturation is 204 X 105moles. The break in the curve was ascribed to the sudden formation of a liquid layer. The ratio 1zo/zo4 is nearly identical with the ratio of the retentivity and saturation data given in Table I for the coconut charcoal. This confirms the assumption that the retentivity value is a measure of the adsorbed vapor which is held firmly by secondary valence bonds, while the part that is loosely held has properties corresponding to the liquid state.
TABLE I Relation between Heat of Wetting in Benzene, Maximum Adsorption of Saturated Benzene Vapor, and Retentivity of Benzene a.t 2 5 O C . Sample and Number
Apparent Density
124 Tec-Char .29 1 2 5 Norit ' 19 108 Batchite .78 61 Pine '24 j 6 Oak .44 24A Ironwood .40 120 German .29 107 CoalBriquet's.42 IOI Urbain '25 23 Ironwood .15 39 hfaple ,26 126B Oak .44 I 5X Ironwood .4; 38 Oak 35 5033 Oak 32 ~ I Oak A 42 102 Coconut 48
Heat of Wetting Maximum Adsorption cal./g. cal./cc.
2.4 15.8
0.7
5.1
4.0 5.0
20.8 11.4 13.5 19.6 14.3 25.2
14.5
0
26 367 129 496 206
5.4
206
83
5.7 6.0
655 354 780
190 110 203
227
102
3.1
5
6.3 6.5
17'5
6.6 7.7
2;
4
I4,9
7.0
234
8
2 7 5
8 8 99
236 23 o
mg./g. mg./cc.
11
2
o
T.4BLE
Retentivity mg./g.
mg./cc.
14
4
137
26
101
48
120
142
91
128
37 34 j6
7 70
560
146
30t 244 437 623 486 371
135 I i j
153 zoo 204
178
233 310 263 231
82 99 IIO 111
11
Kature of Charcoals listed in Table I anti Figs. 4 and ; Name
Tec-Char liorit Batchite Pine Oak
Description
wood charcoal made by the Tennessee Eastman Corporation. Serial S o . K-200. --i commercial decolorizing charcoal made from wood. -The generic name for steam activated anthracite coal. -Made by carbon dioxide activation of yellow pine. --Made by carbon dioxide activation of oak. --A
Tryhorn and Wyatt. Trans. Faraday SOC.,22, 134 (1926).
HEAT O F WETTING O F CHARCOAL AND ACTIVITY
1405
TABLE I1 (Continued) Nature of Charcoals listed in Table I and Figs. 4 and 5 Name
Description
-Made by carbon dioxide activation of maple. --Made by carbon dioxide activation of ironwood. -Made by steam activation of coconut shells. Commercial samples were obtained from: Barneby-Cheney Engineering Co., Columbus, Ohio. National Carbon Company, Cleveland, Ohio. Carbide and Carbon Chem. Corporation, New York German -From a German world-war canister; probably made by zinc chloride impregnation and subsequent activation of wood -Made from a mixture of hard and soft coals, steam Coal Briquets activated. Urbain -Briquetted lignite coke, made by the Urbain Corporation of Niagara Falls, N. Y. Lamp Black -Made by steam activation of lamp black briquetted with a sugar binder. Aqua Dag Graphite-Made by neutralizing Acheson's Aqua Dag, filtering, washing, drying, and crushing to 8-14mesh.
Maple Ironwood Coconut
Relation between Heat of Wetting in Benzene, and the Maximum Adsorption and Retentivity of Chlorpicrin Vapor The discussion of these three factors leads to exactly the same conclusions which have been expressed in the preceding section, that is, the heat of wetting of charcoal is proportional more to its retentive capacity than its adsorptive capacity. The data obtained on the adsorption and retentivity of chlorpicrin are so numerous that it was not thought advisable to use up space by a tabulation of the results when they could be adequately represented diagrammatically. Most of the data obtained are shown in Fig. 4 and Fig. 5 , all the results being plotted on a volume basis. The adsorption capacities in Fig. 4 were all obtained by exposing the samples to saturated chlorpicrin vapor at 25OC. I n many cases the capacity was measured in a stream of chlorpicrin vapor, as described in the experimental part; the results obtained showed that condensation of liquid chlorpicrin on the charcoal in saturated vapor probably did not take place since the relative capacities obtained were proportional to the concentration. In Figs. 4 and 5 the average curves drawn were influenced principally by the data for coconut charcoal since that is the standard type by which the quality of other charcoals is judged. The most striking proof that heat of wetting is a function of retentivity and not of capacity is given by the data on a sample of Acheson Aqua Dag, which is really not a charcoal but a high grade of graphite. The material had been treated as described in Table 11.
1406
RUDOLPH MACY
The heat of wetting of this graphite was very small, yet it had an adsorption capacity equal to that of a fairly good coconut charcoal. Its retentivity, on the other hand, was practically zero, in agreement with the heat of wetting.
e COAL BRIQUETS L8
2
4
6
8
LAMP BLACK
/O
/2
FIQ.4 Relation between heat of wetting in benzene and maximum adsorption of eaturated chlorpicrin a t 25OC.
Relation between Heat of Wetting and Surface Area of Charcoal The data presented in this paper show that there is a proportionality between the heat of wetting of charcoal and its retentive capacity. In Chaney'sV definition of retentivity there is the implied assumption that the retentivity of charcoal is determined by the extent of its surface. This idea appears to have a fairly wide acceptance. If this is true then the heat of wetting is a measure of the available surface of the charcoal as well as the retentive capacity. While this article was being written for publication, two other papers have appeared in which the authors also express the opinion that heat of wetting is a function of surface area.
HEAT O F WETTING OF CHARCOAL AND ACTIVITY
1407
Berl and Burkhardt” measured the heat of wetting of a series of activated charcoals, and also measured the adsorption of methylene blue from solution according to the procedure of Paneth and Radu.’* According to the latter authors methylene blue probably forms a monomolecular film on charcoal and can be used to estimate the surface area of the adsorbent. The heat of wetting and methylene blue adsorption were found to be roughly equivalent; a heat
0 COCONUT
360
B
BATCHITE
G GERMAN
320
u
GRAPHITE
0 COAL BRIQUETS
280
L E LAMPBLACK
240
200
/GO
120
80
40
0 0
2
4
6 FIG.
5
Relation between heat of wetting in benzene and retentivity of chlorpicrin a t 25°C.
of wetting of 1°C. produced by 1.5 g. of charcoal and I O cc. of benzene is proportional to a surface area of 60 square meters per gram. Bartell and Fulg have made a study of the various methods of measuring the surface area of charcoal and silica gel, and showed that the surface area Berl and Burkhardt: 2. angew. Chem., 43, 330 (1930). Paneth and Radu: Ber.. 57, 1221 (1924). 19 Bartell and Ying Fu: Colloid Symposium Annual, 7, 135.
li
1s
I 408
RUDOLPH MACY
can be calculated from heat of wetting and adhesion tension. “This method of determining surface area does not depend on the measurement of adsorp tion from solution; it is, therefore, free from those uncertainties which arise from the use of doubtful assumptions concerning the shape and dimensions of the adsorbed molecules, and the unknown thickness of the adsorbed layers.” They obtained a value of 635 square meters for the specific surface of charcoal, which is higher than most values recorded in the literature. When the above values published by Berl and Burkhardt are recalculated to the same basis on which the heat of wetting was determined in the present work, it is found that a heat of wetting of I O C . per gram of charcoal is equivalent to a specific surface of 180 square meters. This is in good agreement with the value of 2 0 0 square meters calculated from the following data for a high grade commercial coconut charcoal. The calculation is based on the assumption that chlorpicrin retentivity is a measure of the amount covering the charcoal in a monomolecular layer: Heat of wetting = 2.64OC.;g. in 2 0 cc. of benzene. Chlorpicrin Retentivity = 330 mg.jcc. = 718 mg.jg. Cross-Section Area of Chlorpicrin Molecule = 20 square Angstrom units (an average figure for molecules in thin films given by Adam, International Critical Tables 4, 476). Area covered by chlorpicrin = 528 sq. m./g. of charcoal. Heat of wetting of I0C.jg.is equivalent to zoo sq. m./g. The above sample of coconut charcoal is one of the best obtainable commercially. Its specific surface area of 528 sq. m. is in fairly good agreement with that found by Bartell and Fu for a sugar charcoal, and the proportionality between heat of wetting and specific surface area is in good agreement with the above-mentioned finding of Berl and Burkhardt. In Table I about six charcoal samples will be found which have a heat of wetting on a weight basis close to that of the above sample of coconut charcoal; these sampli include oak, Urbain, and maple charcoals. The heat of wetting in cal./g. in Table I should be divided by I O to give the corresponding “C./g. values. On a volume basis, however, these charcoals are not so good as the coconut. Before concluding, it is desired to point out an important factor which has not yet been touched on in this discussion. I n the course of this work, after much experience in the manipulation of the apparatus had been acquired, it was found that some charcoals when poured into benzene caused a much more rapid rise in temperature than others. The more active charcoal in this respect usually was found to be better in all other respects. In several cases the time required for the total rise in temperature after mixing was noted with a stopwatch; the following experiment is typical: Rise in Temperature Charcoal 20 Oak 43AbOak
Sample cc.
Time Sec.
5.35
21.4
5.00
14.0
“C.
2.66 2.46
Heat
0.023
of Wetting “C./cc. 0.50
0.035
0.49
“C./cc.
Chlorpicrin Retentivity mg./cc.
76 115
HEAT O F WETTING O F CHARCOAL AND ACTIVITY
I409
These two samples with the same heat of wetting show a much different activity when dropped into the benzene, and the sample which produces a more rapid evolution of heat also has a higher retentive capacity. This and a few similar experiments may be explained by the assumption that the heat of wetting measures the surface area of the charcoal but not the activity of the surface. This explanation is in accord with a statement of Lamb and Coolidge20who found that “the heats of adsorption of vapors are very nearly the same on inactive as on active charcoals of the same kind.” This is equivalent to stating that the activity of the surface does not affect the total evolution of heat when vapors are adsorbed by charcoal. Unfortunately, further work on this problem was discontinued before the results obtained were sufficient to give any definite conclusions.
Conclusions The literature has been reviewed on the determination of the heat of wetting of charcoal. Data are presented which agree with statements of other authors that the heat of wetting in benzene is a good rapid test of the value of charcoal as a gas adsorbent. However, it is shown that the heat of wetting is not a function of the adsorption capacity of charcoal, but of the capacity for firmly held vapor, or retentivity. It is probable that the heat of wetting in benzene, and the retentivity of adsorbed vapor, are a measure of the surface area of charcoal. Edgeuood Arsenal, Edgewood, M d . August 1, 19SO. 20
Lamb and Coolidge: J. Am. Chem. Soc., 42, 1146(1920).
