THE EFFECT OF ADSORBED GASES ON THE SURFACE TENSION

T H E EFFECT OF ADSORBED GASES ON THE SURFACE TENSION O F WATER BY SHANTI SWARUPA BHATNACAR

The experiments of Stockle and Meyer1 on the surface tension of mercury in the atmosphere of various gases and in vacuum indicate that the adsorbed gases have a definite effect on the value of the surface tension. Freundlich* is of the opinion that the differences between the static and the dynamic determinations of the surface tension of mercury are due to the adsorption of gases by mercury. In a thorough review of the problem, Bancroft3 emphasizes the part played by adsorbed gases on the .value of surface tension. These observations are, however, applicable only to the surface tension of mercury, as there are probably no data in literature a s to the way in which the surface tension of water is affected by adsorbed gases. The yalues of the surface tension of water obtained in vacuum in presence of the vapor of the liquid only, leave much to be desired. The liquid contains a lot of adsorbed gases, while the capillary tubes or the spherical disks employed for the determination of the surface tension have large quantities of adsorbed gases sticking hard to their surfaces. It was in order t o obtain a value of the surface tension free from these effects that the following investigation was undertaken. free from these Later, when the apparatus for determining ‘I’ effects had been designed, a few experiments were performed t o see how the adsorption of various gases affected the value of T for water. Seleotion of Method Out of the numerous methods for determining surface tension, there is only one which can be convenient1.y tried in vacuum and that is the classic “capillary tube” method. 1

2 3

Freundlich: Kapillarchemie, 86 (1909). Kapillarchemie, 85 (1909). Theory of Emulsification, P a r t V I I I . Jour. Phys. Chem., 20,

I

(1916).

E j e c t of Adsorbed Gases o n Surface Telzsion of W a t e r 717

This method is, however, subject to defects of inaccuracy and lack of sensitiveness, and although it i$ possible to minimize these defects by using a number of capillary tubes, i. e., by trying the “capillary multiplier method,” it is very difficult, if not impossible, to remove adsorbed gases from the surface of such delicate and difficult-to-handle stuff. The dangers of destroying the uniformity of the bore of the capillary, and of producing distortion in the tube on strong heating, are numerous. Besides, the error due to the presence of adsorbed gases in the body of the glass will be greater in the capillary method than in the disk method, for, proportionate to the amount of water in contact a larger area of glass-surface is exposed to the liquid in the “capillary multi- ’ plier” method than in the disk method. The analytical problem of evaluating surface tension by the disk method, i. e., from a knowledge of the forces required to detach a circular disk from the surface of a liquid had been solved approximately by Laplace and Poisson, and advocated as an experimental method by Gay-Lussac, Buys-Ballot,2 Merian, and Hagen4 Fergusonj has found that the solution of Laplace is faulty for disks of radii less than 7 or 8 cm. Ferguson, on analyzing the problem, gets

where P is the pull required to detach the disk, A its area, g the gravity constant, and p the density of the liquid and the r the radius of the disk. This result is in agreement with that of Laplace’s if the term i; r 2 be suppressed. Whether such a suppression is legitimate or not depends upon the relative values of Y and a. The effect of this third term in the equation, as pointed out by Ferguson, is appreciable, and can be negligible only when 2

Quoted by Laplace, op. cit., 54. Pogg. Ann., 71, 1 7 7 (1847). Ibid., 73, 485 (1848). Ibid., 77, 449 (1849). Phil. Mag., ( 6 ) 25, 507 (1913).

S h a d Swarupa Bhatiaagay

718

the disk has a radius of more than 7 or 8 cm. Perguson has discussed at length the disadvantages, both practical and theoretical, arising from using a flat disk,l and he has suggested an ingenious method to minimize these defects by substituting for the disk, a segment of a sphere partly immersed in the liquid, so that the liquid touches the sphere round a small circle. The analysis of the forcesactingon the sphere in contact with the liquid brings out the equation

where m = weight required to detach the circular disk from the surface of water. R = Radius of the sphere. p = Density of the liquid. Finding m.,we can easily obtain the value of a2 from the equation T 8 2 gP

where T is the surface tension and g the gravity constant. Thus we can evaluate T from a knowledge of m. Use is made of this mathematical analysis by Ferguson in experiments described here, and a modification of Ferguson's apparatus suitable for the requirements of this investigation has been employed to find the value of the surface tension. Description of the Apparatus and Experimental Work The one obvious inconvenience in Ferguson's method was the balance. In order to avoid a big balance, a suitable spring was selected and used as a Jolly's balance, which easily gives accurate values to four significant figures. Now it is not possible to use a large sphere of radius from 7 to 8 cm. with a fine sensitive spring without risking elastic fatigue of the spring.2 The next thing, therefore, was to find a suitaLOC.cit. The spring used did not sutier from fatigue of elasticity for weights ranging up t o 28 gms. The total pull in grns. has never exceeded 2 2 gms. in these experiments. 1

2

Egect of Adsorbed Gases on Surface Teizsioiz of Water 719 ble substitute for the sphere. A flat disk could not be a fit substitute for this.sphere for reasons pointed out by Ferguson‘in his paper on “The Solid Sphere in Contact with Liquid.” A segment of a quartz sphere was therefore obtained from one of the best opticians here, and carefully tested for any irregularities of surface. It was found that the surface of curvature was quite regular.. In the centre of the spherical disk was cemented a millimeter quartz scale, after the disk had been carefully cleaned with alternate‘washings of ” 0 3 , KOH, alcohol, and boiling distilled water. The hook at the free end of the scale was then connected t o the spring which hung freely in the glass tube C (Fig. I ) , provided in the centre of a tightly-fitted

Fig.

I

brass cover B on a stout cylindrical jar of glass A. The brass cover was further perforated for admitting a tin condenser T and for passing any gases through the delivery tube D, and for connecting the whole apparatus to the air pump and manometer at a. A blank experiment was performed with the disk LOC.cit.

720

Shanti Swarupa Bhatnagar

in order to find out the approximate length to which the vessel containing water had to be lowered to detach the, disk from the surface of the liquid. This helped in choosing the length and size of the screw S, which passed through the centre of a fine brass support B ’ and was attached at one end to a lever L of the form shown in the diagram. The end pieces of the lever were of iron so that the screw could be raised or lowered a t will by means of an electromagnet from outside the jar. The nickel-plated dish was cemented to the brass lever L and the perfect hemisphere attached to the spring balance hung in the centre of the disk as shown in the diagram. The tin condenser T was attached to the still as shown in the diagram. The latter consisted of an inverted distilling flask, the end of whose delivery tube was thinned down in the form of a jet and slipped into the tin condenser. The whole apparatus was arranged as shown in Fig. I , which is self-explanatory. The opening at D was closed by means of a solder when readings in vacuum were taken. The brass cover, B, was made air-tight by means of a preparation of rosin and beeswax in equal quantities. Very careful experiments were made to calibrate the screw in order t o calculate lengths in terms of the turning of the screw. A cellul6id scale divided in 344 mm was pasted round the jar to find out the actual length if the screw moved even to a fraction of a complete round. The method employed for calculating length correct to four significant figures to which the disk was lowered in order to detach the disk from the surface of water and for converting the pull in terms of the weight, is given later. The methods for finding out the radius of curvature of the spherical disk are also described later. In order to take readings for evaluating the surface tension in vacuum the apparatus was exhausted by a good Toepler’s pump, and all the precautions described later were observed. The manometer readings were 0.006 mm. When the adsorbed gases from the sphere had been removed as much as possible by heating the sphere in a coil of resistance wire by electric current, the water was then distilled through

Eflect of Adsorbed Gases on Surface Teiision of Water 7 2 1 the tin condenser by applying a very low flame to the inverted distilling flask, The condensed water was allowed t o trickle in the nickel dish till it was very nearly 'full. The distilling was then stopped and the water was allowed to stand till it acquired the constant temperature of the thermostat. The sphere was next brought in position by adjusting the screw head S' by an electro-magnet and the disk was gradually raised by means of the electro-magnet till the water surface just touched the apex of the sphere. This was easily noticed by a sudden swing of the spring scale downwards. Next the position of the pointer attached to the lever was noticed. The screw was then lowered by means of the electro-magnet, and the number of rounds being noted all the while till the disk was detached from the liquid surface. The number of rounds and the fractions calculated by noting the position of the pointer on the scale when the disk was just detached gave the measure of the pull in centimeters. This was repeated several times and consistent readings were obtained. For obtaining a number of consistent readings one precaution is necessary. When the first reading has been taken the disk attaches a drop of water with it which should be removed by allowing the disk to stand for some time in vacuum and letting the drop evaporate by a further vigorous motion of the air pump and by drying i t further in the heated coils. For working this apparatus in the atmosphere the tube a t D was opened to allow in a known quantity of the required gases from a reservoir with suitable arrangements for the drying and purification of the gases in question. The precautions observed in each case are treated under. separate headings. For repeating the observation, the whole series of precautions had t o be taken over again, and the quantity of gases admitted was always kept equal. No results were obtained to find the change of T with change in time. The next step was to convert the value of pull exerted in terms of weight. This was done by very carefully adding weights to the disk till they produced an equally long lowering of the spherical disk. A travelling microscope was used to see that the lowering pro-

722


duced by the addition of weights was exactly identical to that produced by the pull due t o surface tension. The weight thus obtained gave us the wz of the equation. The experiments were tried in vacuum and in an atmosphere of air, carbon dioxide, hydrogen, carbon monoxide, and nitrogen, and the special precautions in individual cases are described under the experimiintal data of these individual cases. The following are the general precautions which have been taken throughout the work.

PRECAUTIONS I. The Purification of Water From Absorbed Gases and Other Impurities I n order to see that the water contained no absorbed gases the following precautions were taken : ( a ) The water was distilled in vacuum. ( b ) A fresh sample of such distilled water was used in different experiments. After making a set of observations under one condition the water.was siphoned aff through the opening at D and a fresh sample distilled over again for immediate use in the next set of observations so that the water left little to be desired regarding freedom from absorbed gases. In order to remove the possibilities of dissolved and volatile impurities from the water, the following precautions were taken: ( a ) A sample of laboratory distilled water was redistilled through a Liebig’s condenser, and the water examined for possible impurities of silicates and ammonia. The presence of both the impurities was noticed. ( b ) I n order to free the distilled water completely from volatile nitrogenous organic compounds it was necessary to redistil i t after adding solutions of potassium permanganate and caustic potash. The examination of the water so distilled showed it to be perfectly free from ammonia, and gave no coloration with Nessler’s solution. (c) The presence of silicates though very slight, must

Eflect of Adsorbed Gases o n Surface Tension of W a t e r 723 be due to the glass condenser which was used in the blank experiment. This was replaced later by a tin condenser. A sample of the tin of which the condenser was made was analyzed and found t o be free from lead, arsenic, and zinc. On redistilling water through this new condenser the silicates also disappeared. 11. Selection of the Dish The dish d , in which water is distilled in order t o contain as little of occluded gases as possible, should have the following qualifications : ( a ) It should be smoothed on the lathe in order to remove all points and angularities for the gases are more readily adsorbed a t points than at a smooth surface. ( b ) *Itshould be made of a tough poreless metal. (c) It should be made of a substance which is not acted upon by water and the gases (air, carbon dioxide, etc.) under the conditions of the experiment. In these experiments a very carefully lathed and smooth nickel-plated nickel dish was employed, which was carefully cleaned for impurities such as grease, dust, etc., and finally carefully dried by heating by an electric current in vacuum. 111. The Cleaning of the Spherical Disk The edges of the spherical disk had been very ,finely rounded by an expert optician. The disk was then carefully cleaned by alternate washings of nitric acid, caustic potash and alcohol, and was further boiled for 15 minutes in distilled water. The adsorbed gases were removed by heating it in a coil of resistance wire in high vacuum. This heating was continued for several hours. IV. Other Precautions In order t o remove moisture from the-apparatus-a big bottle, J, full of concentrated sulphuric acid and several bottles of calcium chloride and other dehydrating agents, were included through ( a ) near the pump. V. Temperature Control The temperature was kept constant in a thermostat.

724


CALCULATIONS Calibration of the Screw The screw S was calibrated according t o the following methods : The outer jar, A, with the brass stand B and the screw and the lever L in position were arranged as shown in the diagram (2). The nickel dish was cemented to the lever I, and the stand on which the jar A was placed was levelled by m o v i n g t h e bottom screws of the stand. Water was then put in the . dish and a light float.(Fig. 2 ) made by blowing a thin flat bulb a t the end of a fine capillary tube was used as an index. The cross-wires of the telescope of a cathetometer were fixed on the mark M on the float, and the position of the two marks Fig. 2 on the end pieces of the lever were also noticed. The electro-magnet was then brought into play, and the distance through which the mark M moved when a number of complete half-rotations of the screw took place was measured by the cathetometer. The following table gives the results obtained: l/g Position of gos. nofofthe the the mark +: t u rscrew

Position of the mark M after the turning of Difference the screw I--

I

16.8515 16.725 16.6 16.35 16.104

16.725 16.600 16.345 16.104 15.729

Value in cm of the lowering produced by one comp. rotation

0.1265 0.125 0.255 0.246 0.375

0.1265 0. I 2j

0.1265 0.123 0 . 125 Mean 0.125

Egect of Adsorbed Gases on Surface Tension of Water 725 Thus by noting the complete rotation of the screw it is possible to find the lowering of the level of the water accurately to four significant figures. If, however, the screw, instead of making a complete rotation, stands somewhere say at onefourth of a complete rotation, the problem becomes a little difficult. I n order to find out the value of the lowering of the screw in cm for even a part of fraction of a complete rotation, a , thin celluloid scale was pasted round the jar A and the circumference was divided into 344 equal parts so that the movement of the pointer of the screw to any part of the scale indicative of the slightest movement of the screw 0.125

could be calculated, one part being equal t o - = 0.0036 344 cm, so that this afforded a very good and easy method of finding out the pull in terms of length by the simple counting of the complete rotation and part rotations of the screw. The pull was checked frequently by a cathetometer and gave concordant results. Measurement of the Radius of Curvature The measurement of the radius of curvature presented some difficulties. The sphereometers which were available did not fit the spherical disk. They were always a little too big for it. Consequently the following three methods were employed : I. The convex surface of the spherical disk was silvered in the laboratory, and the radius of curvature measured according t o the f o l l o w i n g ,q method : Let 0 (Pig. 3) be the center of the reflecting sur- x. face, and OC the axis. Suppose two objects, A’, A”, placed a t equal distances on each side of OC, and a t 4” the distance 0 from 0. Fig. 3 Images of these two points will be formed by reflection at

~-

726

Shanti Swarzlpa Bhatnagar

points a’,ut’, on the axes OA’, OA”, such that (calling the points where the axes OA, OA’ cut the spherical surface C’, C”. I

I

2

A’C‘

a‘c’

OC

or I ---=

A’C‘

1

2

a’~’

OC

and I I - -2 = A//CU OC

Now the points being very distant, and therefore C’A’ very nearly equal to CX, we may assume that the straight line a’”’ cuts the axis OCX a t a point u.

and for the size of the image, we have

-a’aK _A’A‘’

-

ou OX

Hence, if CX = A, OC = Y, A’A” = I,, Cu = x, and a’“‘ X, we get from (I)

=

Hence, 1

1

1

1

A+;=;--

r

. r- + _ A- - r - - n : A X

‘ ’

and therefore x- = -r - x , A r f A

and X = r-- - x _

I+ r From these two equations x=-

+ x

. “I,

Ar andX zA r

+

A-

= X-

A

Y = ___

2A

+r

Place a finely divided scale S”S’ immediately in front of the reflecting surface (but not so as to prevent all light falling on

EfS’ect of Adsorbed Gases on Surface Tensioiz of Water 7 2 7 i t ; i. e., place i t horizontally to cover nearly half the reflecting surface), and observe the images a’, a’’, and the scale S”S’by means of a telescope so that its object-glass shall be as nearly as possible in the middle of the line AA’. The length L’L” intercepted by the scale in this experiment was very carefully measured by means of a travelling microscope, and the points A’A“ were lighted by means of a single-filament electric lamp. Let the object-glass be at X, let L’L” = I. Then joining Xu’, X a ” , we get1

I-

X

:.

= -X-L-’ -

Xu’ A

A A+%

Lr

1 = -

A + x X 2 m

or

or y=-

2 A1

L-

21

To make use of this method to find the radius of the curvature of the surface, place &e surface opposite to, but a t some distance from, a window. Then place horizontally a straight bar of wood, about half a meter in length between the surface and the window, fixing it with its ends equidistant from the surface, and at such a height that its reflection in the surface is visible t o the eye placed just below the bar, and appears to cross the middle part of the surface. Fix a telescope under the centre of the bar with its object-glass in the same vertical plane as the bar, and focus it so as to see the image reflected in the surface. It is best that the whole of the bar should be seen reflected in the surface. To accomplish this two well.defined patches of light from two single-filament lamps, the reflected images of which can be seen clearly, are allowed t o fall through two points in the bar. Cf. Practical Physics by Glazebrook and Shaw.


728

Now place against the reflecting surface a finely graduated scale, arranging i t so that one edge of the image of the reflecting filament mark is seen against the divided edge of the scale: This distance between the two images is measured by means of a travelling microscope. This gives us the length 1 of the above formula. Measuring the length of the bar or the distance between the two sources of light gives us I,; and measuring with a tape the distance between the reflecting surface in the center of the object-glass of the telescope, gives A. The following are the data: Distance between the two points in the travelling microscope : 0.516 0.516 0.516 Mean, 0.616 cm. Distance between the two filament lamps, 8 3 . 7 5 cm. Distance between the reflecting surface and the object-glass 401.485 cm.

Substituting these values we get R. -

2Al

L-2--I

- 2 X 0.516 X 401.485

8 3 . 7 5 - 1.032

=

82.718

=

-

1.032 X

401.485 82.718

5 . 0 0 7 cm.

11. Sphereosoopio Method In principle this method is essentially identical with the sphereometric method. The only difference is that there the rotation of a needle indicates the radius of the curvature so that if the rotation of the needle produced by a known curvature is noted, the curvature of any other surface can be calculated from the readings of the rotation produced in the sphereoscope. The best way to accomplish this is t o get a number of readings by known movements by the legs of the sphereoscope, and to obtain a curve from these readings. The following readings were taken by adjusting the instrument against a fine screw gauge. A large number of readings were taken to obtain a good curve. The R can be read di-, rectly from the curve which can be obtained from the equation for the spherometer by plotting the following readings of the instrument :

-

E f e c t of Adsorbed Gases on Surface Tension of Water 729 Readings on the instruments

Repetition

-14.80 -13.35 -1 I . 85 -10.42 - 9.00 - 7.67 - 6.28 - 4.80 - 3.33 - I .92 - 0.45 I .oo

-14.72 -13.28 -I I . 82 -10.42 - 8.98 - 7.68 - 6.30 - 4.88 - 3.50 - 1.93

Readings on the instruments

Repetition

2.43 4.00 5.40 6.85 8.30 9.63 11.58 12.82 14.20 15 * 75 17.23 18.93

- 0.00

0.90

3.37 3.82 5.30 6.80 8.22 9.65 11.20

12.68 14.20 15.58 17.18 19.02

111. The lens was further tested by the Professor in charge of the Opthalmic Department of the local Medical College, and the value of R obtained by the ophthalmometer was exactly 5.007 cm. EXPERIMENTAL DATA AND CALCULATION OF T THEREFROM The following observations were taken a t 15' C : IN KO. of obser vations I 2

3 4 5

6

7

s Mean

VACUUM'

No. of complete half turns of t h e screw

No. of part :urns of the screw

32 32 32' 32 32 32 32 32 32

I43 142.8 140.8 140.5 144.5 143.2 140.8 140.3 141.8

Final value of the pull in cm

4 . I41 I

2.066

Now substituting the value of m and R and the equation m

=

4apa2 i R -

1

-- 3

ildsorbed gases removed a t 0.006 mm.

ialueof pull in termsof mass

3

4.1411 K

and

p

in

Shaizti Swarupa Bhatnagar

730 We get

22

4.1411 = 1 X - X o.999a2 7

i

da

5.007-

x

5.007 2 3 3

)

Solving this equation by Hornel’s method of approximation, we get a? = 0.0735. Now a2 E T gP

where g is the gravity constant. . ‘ T = 0.0735 X 0.999 X 979.7 =

71.30 dynes.

Sp e ci a1 Pree aut ions The same as given above. IN CARBON MONOXIDE NO.of observations I 2

J 4

5

6

7

Mean

No. of complete half turns of the screw

No. of part turns of the screw

32 32 32 32 32 32 32 32

323 322 329.7 322.6 323.8 321.8

of pull Final value of Value of the pull in cm in terms mass

321.8

324.01

2.1301

Now substituting the value of m and R and the equation

4.237 T

and

p

in

we get 22

4.237 = 4 X - X 0.999 X a2 7

Solving this equation by Hornel’s method of approximation, we get a2 = 0.07527. Now

T

a2 E-gP

where a2 is the gravity constant.

E#ect of Adsorbed Gases on Surface Tmsion of W a t e r 731 .*.

' I '= 0.999 X 979.7 X 0.07527 = 73.00 dynes.

Special Precautions I . The gas was prepared by the action of concentrated sulphuric acid on formic acid. 2 . The gas before being stored in the reservoir passed through two wash-bottles-one containing water and the other caustic potash. 3 . The gas from the reservoir was connected to the apparatus through a spiral tube in order to arrest particles of water which may splash off with the gas. IN

No. of observations I 2

3 4 5 6

7 Mean

NITROGEN

No. of complete half turn:: of the screw


32 32 32 32 32 32 32 32

323 322 329.7 322.6 323.8 321.8 321.8 324.0

Final value of Value of pull of the pull in cm in terms mass

2.13


7.237 ir

and p in

we get 22

4.237 = 4 X - X 0.999 X a2 7

-.

Solving this equation by Hornel's method of approximation, we get a2 = 0 . 0 7 5 2 7 . Now 512 E

T gp

when g is the gravity constant. ... T = 0.7527 X 979.7 X 0.999

=

73.oodynes. -

'

Shan ti Swarupa Bhatn agar

732

Special Precautions I . The gas was prepared by heating ammonium nitrite in a strong round-bottom flask. 2. The gas as i t issued out from the flask through the delivery tube passed through two wash-bottles, before being finally stored up in the gas reservoir for use in the instrument for determining surface tension.

NO. of observations I L

3 4

5 Mean

No. of cam)lete half turns of the screw


Final value of 7alue of pull of the pull in cm in terms mass

32 32 32 32 32 32

2.07

4.227

Now substituting the value of m and R and in the equation we get 22

4 . 2 2 7 = 4 X - X 0.999 X a2

7

Solving this equation by Hornel’s method of approximation we get a2 = 0 . 0 7 5 1 . Now T 51.2 E gP

when g is the gravity constant. :. T = 0 . 9 9 9 X 979.7 X 0.0751

=

72.83 dynes. -

Special Precautions I . The gas was prepared by the action of dilute sulphuric acid on zinc. 2 . The gas before entry into the apparatus passed through

E j e c t of Adsorbed Gases on Surface Tension of Water 733 a wash-bottle containing water, in order to dissolve all soluble impurities. 3. The wash-bottle through which the gas bubbled was connected by means of a fine spiral tube in order to arrest the water which may splash off with the gas. 4. The precautions described for caibon monoxide were also all observed with hydrogen. EXPERIMENTAL DATA AND CALCULATION OF T THEREFROM The following observations were taken a t I 5 O C. I

I N AIR No. of ob-

servations I 2

3 4 5

Mean


screw

32 32 32 32 32 32

322 329 322.6 320.8 322.4 324

2.133 cm.


4,242 cm. K

and

p

in

I _

we pet

Solving this equation by Hornel's method of approximation we get a 2 = 0.07535. Now T a2 gp

where g is the gravity constant. T = 0.999 X 979.7 X 0.07538

=

Special Precautions The same as given above.

73. dynes. I


734

NO.of observations



Final value of Jalue of pull the pull in cm in terms of mass

I

I 2

3 4 5 6

7 Mean

32 32 32 32 32 32 32 32

152

150.8 1 5 1 .o 150.4 151 .o 150.8 152.8 151.2

4.2?8

2.08


Atmosphere Vacuum Hydrogen Nitrogen Carbon Monoxide Carbon Dioxide Air

4.228 T

and p in

Value of T at 15' C. 71.3 72.83 73.00 73.00 72.85 73.' '

Effect of Adsorbed Gases on Surface Tension of W a t e r 735

It is interesting to note that the increase in value of T for water in gases is proportional to the rise in their densities, in all cases excepting for carbon dioxide. A conspicuous lowering of T in case of COa may be explained on the solubility of the.gas in water. Stockle and Meyer’s data on the value of T for mercury show a similar discrepancy with COZ. In general, their results also indicate a rise in the value of T for a corresponding increase in the density of the gaseous atmosphere. It has been endeavored throughout to exclude adsorbed gases as far as possible. It is intended to get still better results by using a Langmuir pump, by heating the disk to a still greater temperature, and by substituting for the brass lid a glass lid which can be sealed and made perfectly air-tight. 2. A slight modification has been proposed in the optical method of determining radius of curvature of curved surfaces by making use of single filament lamps and travelling microscopes for greater accuracy. 3. Results with substances like olive oil which have practically zero vapor pressure are reserved for a later contribution. It appears that when all gases are excluded from the apparatus, the pull in case of olive oil is much greater than when gases are present. This tends to show that perhaps the pull indicates the interfacial tension between quartz and oil. It is premature to draw a conclusion from this slender and solitary observation. An instrument is under construction in which a pull greater than those observed in the present investigation can be measured, and it is intended to obtain results in high vacuum, with all adsorbed gases removed as far as possible, with olive oil and other such materials having practically zero vapor pressure. The author takes this opportunity of thanking Professor J. M. Benade for his help in constructing the apparatus and for the benefit of his expert advice in high vacuum experiments. Physics Department, F . C. College, P u n j a b University, Lahore, and University College, London

THE EFFECT OF ADSORBED GASES ON THE SURFACE TENSION

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