T H E VARIATIOS OF T H E CAPILLARY ACTION O F SOLUTIONS KITH TIME* BY H. M . TRIMBLE
I n a recent paper Rashburn and Bigelow' have shown that the surface . tensions of certain aqueous solutions, as measured by the capillary rise method, vary with time. Using solutes which are more volatile than water, they found a marked increase in this property. Their explanation of the phenomenon is essentially as follows: The height to which a liquid rises in a given capillary tube is determined largely by the cohesive forces which the molecules in the support'ing meniscus exert; that is, by the surface tension of the liquid. For a mixture of two liquids the effective surface tension is undoubtedly a function of the composition of the surface; a composite of the cohesive forces of its components. If the solute is, as in the cases which they investigated, more volatile than the water in which it is dissolved, it will escape to the air of the room a t a more rapid rate than will the water. This loss of solute can be replaced only in part by diffusion from the main body of the solution, since diffusion in the liquid state proceeds relatively slowly. As a result of bhis preferential escape of the solute, the composition of the surface changes, and with it the surface tension and the height to which the liquid rises in the capillary. Since the volatile solutes which they used act to lower the surface tension of water, the process should result in an increase in capillary height; as, indeed, it did. The interest and the possible importance of this phenomenon led the author to study it as it is manifested by various mixtures of organic substances.
Apparatus The capillarimeters which were used were of the three types shown diagrammatically in Fig. I . Of these, types A and B have been described by Washburn and Bigelow. (Loc. cit.) Type C was constructed to provide for the sweeping away of the vapors by means of a stream of air, as they issued from the capillary tube; thus, in effect, eliminating the portion of tubing which was above the capillary tube in the other two forms. Its nature will be clear from the diagram. A11 tubing through which measurements upon the positions of meniscuses were to be made was chosen so as to be uniform in diameter, and free from striations or other optical defects. Measurements were made upon a selected portion of a millimeter scale placed within the larger tubes, and upon threads of mercury in the capillary tubes; and those tubes were chosen for which these measurements, in different positions, were const'ant to within * o . o z millimeters. -1comparator of standard make was used in this work. It was found later that the possible error int'roduced in the variation abooe men* Contribution from the Chemistry Laboratory of the L-niversity of Michigan. ' E. R. Xashburn and S.L . Bigelow: J. Phys. Chem., 32. 32I-j3 (19281.
H. M. TRIMBLE
I212
tioned was well within the limits of accuracy of the experiments as a whole. The mercury threads used in examining the capillary tubes were then expelled and weighed and the average radii of the tubes were calculated. With soft glass capillaries there was no great difficulty in finding portions as long as one to three centimeters which were satisfactorily uniform. Such portions were marked by rings etched in the glass and used in subsequent experiments. Two pieces of Pyrex capillary tubing were found, each about five centimeters in length, which were very uniform throughout. They were used in the major portion-of the work which is here discussed. A
6
C
FIG.I Capillarimeters.
Since it was desirable to avoid the necessity of correcting the height of the meniscus for the capillary rise in its tube, small tubing could not be used in the larger arm of the capillarimeter. Calculation by the method of Lord Rayleigh' shows that, for a liquid whose specific cohesion, a*, is 7 , in a tube of radius 14 millimeters, the correction involved is less than 0.02 millimeters. Since no liquid or mixture of higher capillary constant than about 7 was to be used, the large arms of the capillarimeters were made of tubing of this size. After being built into capillarimeters, the capillary tubes were further tested for constancy of internal diameter by determining the heights to which carefully purified benzene rose in them a t various places in the uniform portions a t 2 0 O C . They were found to be uniform within the limits previously set, and the diameters as calculated from the capillary rise agreed very well with those previously determined. Lord Rayleigh: Proc. Roy. SOC.,92A, 184 (1915).
VARIATIOK O F CAPILLARITY O F SOLUTIONS WITH TINE
1213
All experiments were carried out in thermostats provided with plate glass windows. That portion of the front window through which the positions of meniscuses were to be read was tested and found to be very uniform. The large meniscus was illuminated by the method of Richards and Coombs.’ Readings were made by means of two cathetometers provided with low power microscopes mounted upon micrometer slides. By means of these instruments it was possible to read directly to 0.01and 0.00; millimeters, respectively. Care was taken to have both the large and the small meniscus clearly in focus a t the time of making a measurement. Materials All the organic substances used were the best standard “C.P.” commercial products. Each liquid, with one exception, was dried over a suitable dehydrating agent and fractionated just before using, rejecting large first and last portions Pentane, which had been prepared from petroleum in the laboratories of the Eastman Kodak Company, was not further purified. The Experiments Cleaning of Capillarimeters. All capillarimeters were cleaned by washing them out with alcohol; allowing them to stand over night or longer filled with freshly prepared aqua regia ; and then, finally, washing them thoroughly with boiling conductivity water. The liquids were always drawn out of a capillarimeter through the capillary arm with the aid of a filter pump. After washing, the capillarimeter was dried in a hot air bath, drawing filtered air through it repeatedly during the process. After this treatment the apparatus was almost invariably found to be satisfactorily clean. Experimental Procedure. Mixtures of organic liquids for the experiments were made up by weight as wanted. Since the composition of a mixture of volatile substances may be slightly changed in pouring from one vessel to another, the flasks in which the solutions for study were prepared were converted into “wash bottles” by inserting rubber stoppers provided with pressure and delivery tubes; and this device was used in transferring the liquids. Such an amount was introduced into a capillarimeter as would bring the meniscus in the capillary tube to some desired position within its uniform portion. The capillarimeter was then put into the thermostat in the proper position and left for twenty to thirty minutes to take the temperature of the bath. The experiment was then started. The liquid was drawn out of the capillary tube by applying suction through a rubber tube attached t’o the outlet tube of the larger arm, and thoroughly mixed by successive blowing and drawing. During this process, which was continued for one or two minutes, the walls of the capillary were well cleared of liquid by drainage and by evaporation. When the mixture was allowed to run up the tube again, no droplets formed above it if it was satisfactorily clean. Such droplets must be avoided if the experiment is to be successful. Richards and Coombs: J. Am. Chem. SOC.,37, 1656-76 (1915)
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H. hl. TRIMBLE
The cathetometer had previously been brought into position and leveled. Its cross-hair was now brought to the approximate position of the small meniscus a t its equilibrium height. The agitation was then repeated as above described, the meniscus was allowed to run up the tube about thirty seconds before the time set for beginning the experiment, and the cross-hair of the microscope was quickly brought to exact tangency with the lower edge of its image. The meniscus was then made to oscillate about its initial position by alternately applying the finger to the opening of the capillary arm and removing it, until the second hand of a watch used in timing the experiment was just passing its .zero position, when the agitation was stopped. The changes, if any occurred, always started immediately, and the readings were taken a t suitable intervals thereafter. I t was found that in successive experiments, carried out as above described, and start'ing again with the same mixture, the results were reproducible t'o within about one percent. Much more consistent results were gotten, however, if the experiment was repeated with a fresh mixture after recleaning the capillarimeter. This was done in the great majority of cases. All experiments were carried out a t 2 j" C unless otherwise stated. Efects of Tube Length, and of Tube Size. If the preferential evaporation theory be accepted as a satisfactory explanation of the phenomenon under consideration; then the rate of diffusion of the vapors away from the capillary meniscus will determine the rate a t which the surface energy of the solution changes. Other things being equal, it would seem that this diffusion and the corresponding rise of liquid in the capillary tube, should be more rapid the shorter the tube above the meniscus, and the greater its cross sectional area. This matter was the first to be investigated. For reasons which will be explained below, it was found to be impossible, in general, to express the results of experiments in the usual terms, that is, in terms of dynes per centimeter, or in terms of the capillary constant, a*. Instead, the height of the meniscus in the capillary above that in the large tube as it was found in the experiments has been used in almost every case. This height is called the "capillary rise" in the discussion which follows. In a few preliminary experiments it was found that mixtures of ethyl ether with benzene, with toluene, and with acetone all showed consistent increases in capillary rise with time. Studies of the effect of tube lengt'h were carried out with all of them in capillarimeters of type X. The experiments a t 2 j"C with an ether-toluene mixture in which the mol fraction of each was 0.j were typical. The results, in terms of capillary rise in a tube of radius 0.214mm. for various tube lengths, are shown in Fig. 2 . Each curve represents the mean of the data found in a t least two,-generally more, experiments, whose results agreed to within 0 . 2 5%. Tube lengths were measured from the bottom of the capillary meniscus to the top of the capillary tube. I n each case the wide tube above the capillary had its influence upon the phenomeon, of course, but this was not taken into account a t this time. AS will be seen, the rate of rise of liquid in the tube increases with decrease in tube length. Experiments in capillary tubes of radii 0.167 and 0.246 mm., and
VARIATION O F CAPILLARITY O F SOLUTIONS WITH TIME
1215
with mixtures of ether with benzene and of ether with toluene, a t various concentrations, gave, qualitatively, the same result. The effect of tube size was next studied. Capillarimeters of type h were prepared, using small tubes of radii 2.54, 1.05, 0 . 7 7 , 0.43, 0.35, 0.246 and 0.167 mm., respectively. The results with an ether-acetone mixture, in which the ether was present at a concentration of 0.412, in terms of mol fraction, are shown in the curves of Fig. 3. The capillary meniscus was, in these
Erne in minutes.
FIG.2 Effect of tube length.
experiments, started a t a point 5 mm. belowtheupperend of the capillary tube. Rates of rise as found with the various capillarimeters could not give comparable curves, since the tube sizes were different; and so the capillary rises a t the start, as well as the rates of rise due to changes in the surface forces, would be quite different for different tubes. I n this case hr, the product of the capillary rise by the radius of the tube, has been plotted against time. This product is employed here simply for convenience in representing the data, and must not be confused with 8 2 , the specific cohesion of the mixture of liquids. It cannot be taken as characteristic, in general, of the capillary activity of any mixture which is evaporating, as will be explained below. It is noteworthy that the tube of radius 2.54 mm. failed to show an appreciable rise a t all. Results very similar to these were found with mixtures of ether and toluene and of ether and benzene. Other tube lengths were also tried with all these liquids, with much the same results.
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H. M. TRIMBLE
I n the sets of curves of Figs. z and 3, it will be observed that the uppermost ones are abnormal in form. I n these experiments it was observed that a t the time when the abnormality appeared, the liquid had risen to the top of the uniform capillary tube, and was spreading into the larger tube above. Effect of Densities, Volatilities, and Surface Tensions of the Components. A study of the change of capillary height with time was next carried out with mixtures of pairs of organic liquids, chosen so as to give rather wide variations in density, volatility and surface tension; other factors, which it seemed, should greatly influence the nature and extent of the phenomenon. Capil-
larimeters of type A, of nearly the same dimensions, were employed. The capillary tube in one of them had an average radius of 0 . 2 13 mm. and that in the other had a radius of 0 . 2 1 6 mm. The capillary meniscus was, in every case, started I O mm. below the top of the capillary tube. The changes found in a few typical cases are shown in Fig. 4. These curves are all drawn to scale, and so they show the relative magnitudes of the changes in the various cases as found. The nature of these changes will be discussed and, a t the same time, explained with the aid of Table I, in the light of the theory of preferential evaporation given above. Columns 2 , 3, 4 and 5 of the table give, respectively, the vapor pressures, densities, surface tensions, and capillary rises of the pure liquids named in a tube of radius 0.214 mm. a t 2 5 O C . When a mixture of ether and toluene is allowed to evaporate in one of these capillarimeters, the ether escapes preferentially from the capillary meniscus because of its greater volatility. Its escape results in an increase of surface energy accompanied by a rise, since thereby the surface is enriched in toluene, which has the higher surface tension. This change is shown in
VARIATION O F CAPILLARITY O F SOLUTIONS WITH TIME
1217
TABLE I CS* CeHjCH3 (C2Hs)20 CH3COCH3 cc1,
P mm. Hg.
dnb0
Y
h
361
1.2559 o.8j91 0,7099 0.7863 I 4835
31.5 28.0 16.7
23 ' 9 31 . o 22.4 27.7 15.8
27
526 226 1I4
22.8
26.3
curve I of type A. The effects with two other such pairs are shown in curves 2 and 3. The differences in vapor pressure and in surface tension of the components of these mixtures are less than in the first case, and the capillary rises are correspondingly less rapid and less extensive. Again, carbon disulfide has a greater vapor pressure than has toluene; and so escapes preferentially from the surface of a mixture of the two. This removes the substance of greater surface tension, and so the height of the column falls. This change is shown in curve B. It will be noted that the rate
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H. M. TRIMBLE
of fall is surprisingly low, considering the differences in vapor pressure and surface tension between these two substances. Curve C illustrates another kind of behavior, where the capillary height first increases, comes to a maximum, and then decreases. A fourth type of change, in which a fall to a minimum would be succeeded by a rise might be expected. It has not been found in the course of these experiments. Mixtures of benzene with toluene and of carbon tetrachloride with chloroform showed no change whatever in these experiments, probably because the liquids of these pairs are very similar in their properties. Most of the mixtures studied gave a change of capillary height, with time, of type A. The pairs ether-carbon disulfide, ether-nitrobenzene, and etheramyl alcohol, gave much more pronounced increases in capillary height than any of those whose behavior is shown in Fig. 4. I n each of these cases ether, with high volatility and low surface tension, is paired with a substance of low volatility and comparatively high surface tension. Pentane, which is much like ether in density, volatility and surface tension gave changes nearly as pronounced as did ether in mixtures with toluene, carbon disulfide and acetone. Every mixture whose more volatile component had also a lower surface tension than the other substance in the mixture showed a change of this kind, provided that the two did not differ greatly in density. I n no case was a rise greater than 5 mm. found in the course of IOO minutes. All these curves displayed a decreasing slope as time went on; but a condition of equilibrium was reached only after a very long time, if ever, as the capillary height was found still to be increasing slowly in a number of cases, with different pairs of liquids, even after the expiration of 50 hours. Mixtures of carbon disulfide with benzene, chloroform, and carbon tetrachloride also showed a decrease in capillary height, with time, of the same kind as is shown in curve B of Fig. 4. Mixtures of pentane and of acetone with carbon tetrachloride; and of ether with chloroform showed the same rise to a maximum, succeeded by a fall as that shown in curve C of Fig. 4. Mixtures of the various pairs of liquids named above in other proportions were also used. The tube sizes, the tube lengths and the temperatures were varied somewhat in different experiments. The results found differed from those which have been given in degree in every case, but never in kind. These results are, with the exception of curves of type C, just what anyone with the data before him might have anticipated. I n experiments with many binary mixtures formed from some 2 5 organic liquids, predictions of changes in capillary height from the vapor pressures and surface tensions of the components were amply verified in a qualitative way in nearly all cases. The effect of the difference in density between the components of a given mixture upon the phenomenon was the cause of much uncertainty in any attempts a t quantitative prediction. This effect is most apparent in the case of the mixture of ether and carbon tetrachloride whose behavior is shown in
VARIATION O F CAPILLARITY O F SOLUTIONS WITH TIME
1219
curve C of Fig. 4. Here the capillary height at first increases, due to preferential escape of ether from the surface. After a time, however, the increase in weight of the solution, due to loss of this specifically lighter component, first balances the increase in surface energy and then exceeds it in its effect; and the curve passes through a maximum and then falls. Similarly, the escape of the specifically heavier carbon disulfide from its solution in toluene, the mixture whose behavior is shown in curve B makes the mixture progressively lighter, and so undoubtedly acts to retard the rate of fall of the meniscus. Mixtures of ether with any of the other substances named must become specifically heavier as the ether evaporates, and this change undoubtedly acts to decrease the rate of rise of the liquid in the capillary. Thus, the effect of differences in density between the components upon the nature of the phenomenon under consideration is never negligible, and it may be of prime importance in the behavior of some mixtures. Convection Currents in Capillary Tubes. It was believed, in the earlier part of this work, that the process by which the components of a mixture reach its surface involves only simple diffusion, unaided by any kind of mechanical stirring. If this were the case, then the alteraFIQ.5 tion of the density of the solution with time could be Convection curdescribed in terms of some such generalization as Fick’s rent* in Law of diffusion, and the data might yet prove susceptible to mathematical treatment so as to make possible quantitative predictions. But one afternoon it was found, while working with a mixture of ether and acetone, that a tiny mote of some foreign substance had gotten into the liquid just below the capillary meniscus. There i t remained, moving around and around in a path such as is shown in Fig. 5 . This behavior was quite unexpected, so it was followed with great interest, and in detail. The time required for it to traverse this path was about one second. .The rise in the capillary tube went on in this experiment in quite the usual manner, giving a normal curve. The meniscus was twice drawn down to reform the surface, and each time the usual rise took place; the particle meanwhile keeping up its regular motion. It was still present and actively in motion when the experiment was terminated after eighteen hours. The occurrence of such a mote in a mixture is, of course, quite accidental. It happened in two other cases, however; once with a mixture of ether and toluene and once with a mixture of carbon disulfide and ether. I n both cases the mote behaved in the same manner as did the one first studied. These motes served to show, by their motion, the motion of the liquid in which they were suspended. It seems clear that specifically heavier liquid was flowing down the outer portion of the liquid mass from the region where the meniscus made contact with the walls of the tube, and where the most active evaporation was in progress. At the same time, liquid which was specifically lighter was flowing up the middle of the tube to the meniscus
M
1220
H. 11, TRIMBLE
and along it to the zone of evaporation a t the edges. The amplitude of the motion of the motes in the vertical sense was never greater than about twice the radius of the capillary tube, but the convection currents which it indicat,ed may well have extended much farther down into the solution. The tube in which these convection currents were first made manifest was 0.214 mm. in radius. The other tubes were only slightly larger. I t would be interesting, if feasible, to extend these studies to much larger capillaries. I t seems probable to the author, however, that the only difference between the phenomenon as between small and large tubes would be that in the latter the convection currents would be more complex, and the liquid would be actively stirred to much greater depths. This stirring tends to maintain uniformity of composition in the liquid, probably to a greater extent the larger the tube. It seems, then, that we have here the explanation of the fact that the rate of change of capillary height with time falls off rapidly with increasing size of tube. It was found, in the experiments described above, that in a tube of radius 2 . 5 4 mni. there was no appreciable change in the height of the meniscus in I O O minutes. I n a tube of this size, then, we have reached t,he condition where convection currents will reduce the rate at which the substance of higher surface tension is concentrated in the supporting surface t o almost zero. The abnormality of the uppermost curves of Figs. z and 3 may be explained similarly. This mixing by convection currents also explains why changes such as are shown in curves B and C of Fig. 4, where one would be led to expect extensive effects, are, in reality, very limited. Most important of all, however, so far as expressing the results goes, these experiments show that the composition, and so also the density of a solution which is evaporating in a narrow capillary tube changes in an unknown manner and to a depth which cannot be accurately determined. Evidently, too, the composition of the escaping vapors also varies in an unknown manner. The surface tension of such a solution, then, cannot be calculated by means of the usual formula,’ y = 4 rhdg. Seither can results be expressed in terms of specific cohesion. The product r h cannot be “specific” for the solution. In the first place, its composition varies from moment to moment. Then, too, as has been seen, solutions at the same stage in their evaporation do not give a t all the same values of “h r” in capillary tubes of different sizes. For these reasons results have been expressed in terms of “capillary rise” almost entirely in this paper. ilmount of Liquid lost i n Erperiments. The amount of liquid lost by evaporation in any of these experiments is certainly very small. This was proved by measuring the actual rat,e a t which the meniscus fell in a capillary tube closed a t the bottom and filled with liquid. The vapor was allowed to escape freely to the air of the room. I n experiments a t z j”C, less than 0.04 grams of pure ether was lost from a tube of radius 0.73 millimeters in the 1 It should be remarked a t this point that the densities of the solutes in the mixtures investigated by Kashburn and Bigelow, in which preferential evaporation was believed to be responsible for the changes which they found were but little different from that of water.
VARIATION O F CAPILLARITY O F SOLUTIONS WITH TIME
I221
course of six hours. Mixtures of ether with toluene and with carbon tetrachloride showed a much less rapid escape of vapor from the same tube. The meniscus a t the start was two centimeters below the top of the tube in every case. Eflect of Closing Tubes. If a U tube with a ground glass stopper is sealed on in such a manner that it connects the two arms of the capillarimeter, so
FIG.6 Volatile substance distilling to liquid.
that the system is completely closed, changcs in capillary height with time can be prevented. Rubber tubing cannot be employed, as it is useless in preventing the escape of organic vapors. If a tube is closed after some change in capillary height has taken place, the meniscus slowly returns to its original position. If the tubes are closed from the time of agitation no change ever takes place. This repetition of certain experiments by Washburn and Bigelow confirms further the correctness of their finding; namely, that the phenomenon under consideration is due to the effects of preferential evaporation.
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H. Y. TRIMBLE
Effect of Evaporation from a Side Tube to the Surface of Pure Liquid. Washburn and Bigelow found that when a capillarimeter of type B is used, pure water being placed in the capillarimeter and the volatile solute in the U tube side arm, the volatile liquid distils to the capillary meniscus and the meniscus falls. This experiment has been repeated with organic liquids; and it has been found possible to duplicate their results, with modifications such as would be expected for these substances. Thus, ether and various other liquids distilling from the side tube to the surface of certain substances of higher surface tension causes the meniscus in the capillary tube to fall. This behavior is shown in curve A of Fig. 6. Ether, when it distils from the
Time in minutes. FIG.7 Effect of sweeping away vapors.
side arm to carbon tetrachloride, causes a decrease in the height of the liquid in the capillary tube, a t first; followed by a rise, as shown in B of Fig. 6. Evidently this rise is due to the dilution of the specifically heavier carbon tetrachloride by ether; resulting in a decrease in the weight of the column of liquid in the capillary tube. There is also further evidence that active mixing by convection such as has been described above goes on in these experiments; for carbon disulfide on distilling from the side tube to toluene causes, not a rise, as might be expected from its high surface tension; but a fall. Here the effect of its greater density is again apparent. This is shown in curve C of Fig. 6. Effect of the Large Tube abozle the Capillary. The effect of the large tube above the capillary upon the rate of change of capillary height was next investigated, using a capillarimeter of type B and a solution which contained toluene and ether in equal proportions in terms of mol fraction. In the first experiment dry nitrogen was passed down the side arm of the capillarimeter and up the central large tube. The meniscus in the capillary tube was I I mm. below its top. This gas current swept away the vapors coming from the capillary tube; and so, in effect, eliminated the tube above the capillary. The capillary height increased as shown in curve I of Fig. 3 . S e x t the liquid
VARIATION OF CAPILLARITY OF SOLUTIONS WITH TIME
1223
was drawn down, the capillary meniscus was re-formed, and an experiment was made without passing air. The capillary height increased as shown in curve 2 of Fig. 7. As will be seen,, the rise is more rapid and more extensive when the vapors are swept away from the mouth of the capillary tube. The rate a t which the gas is passed apparently does not influence the rate of rise, provided that it is not passed too slowly. This experiment was repeated with similar results using various pairs of liquids which gave, normally, a rise with ti me.
92 JI
30 29
4
28
aLl 2 7
Y
P)
26
P 25 *+
d
~g2r E 23
,d
3 22 0
20
40
60
lime in minutes. FIG.8
Effect of concentration.
Ether-toluene mixtures.
Another device for sweeping away the vapors from the mouth of the capillary tube and so, in effect, of eliminating the large tube, was found in capillarimeters of type C. Khen they w r e in use, the dried gas was passed through the side tube and up around the capillary, and so out. Reversing the direction of the gas current gave the same effect. These capillarirneters gave results which were much the same as those found with capillarimeters of type B. I t is very interesting to note that when the vapors are not swept away the rise of the liquid with time cannot be accurately represented by any of the simpler curves; but when they are swept away the rise with time during the first hour is represented closely by a parabola. This fact was established by
I224
H. M. TRIMBLE
many experiments, using capillarimeters of both types, B and C. Its explanation is not immediately apparent. Evidently, however, for the best study of the alteration of capillary height with time for mixtures of liquids, the disturbing effect of a tube other than the capillary itself should be eliminated. E$ect of Concentratzon. The effect of the concentration of the more volatile component upon the rate of rise was then studied, using capillarimeters of type C. Mixtures of ether and toluene were again used, since, of all the pairs whose behavior has been studied, this one seemed to give the most consistent results. The changes for mixtures containing various concentrations of ether by mol fraction are set forth in the curves of Fig. 8. The numbers a t the left indicate, in each case, the mol fraction of ether in the solution a t the start. As will be seen, these curves are of the usual type. The rise is more rapid and more extensive for intermediate concentrations of ether. The fact that these mixtures, in two cases, showed greater capillary heights during the latter part of the experiment than did pure toluene, is to be explained as due to the fact that the mixtures at that time are specifically lighter than pure toluene. The tube length a t the start of the experiment was 20 millimeters in every case. Summary and Conclusions Experiments, using the capillary rise method with mixtures of organic liquids confirmed the conclusion, reached by Washburn and Bigelow, that the variation of the surface tension of a mixture of liquids with time is due to preferential evaporation of the more volatile substance. The change which occurs may be a rise, a fall, or a rise succeeded by a fall of the column of liquid in the capillary tube. The rate and magnitude of the change depends upon the size and length of the capillary tube, being greater the less the tube length and the smaller the tube. If these factors are held constant, the nature of the change may be roughly predicted from a knowledgeof thesurface tensions, volatilities and densities of the components of the mixture. This work shows, again, that evaporation must be completely prevented if trustworthy data upon the surface tensions of mixtures, one or both of whose components is volatile, are to be obtained. This is, perhaps, the most important conclusion which can be drawn from the investigation.
