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(10) Langsdorf, A., Jr., thesis, RIassachusetts Institute of Technology, 1937. (11) Nielsen, C. E., and coworkers. Phys. Rev., 81, 324 (1951). (12) Nielsen, C. E., Needels, T. S., and Weddle, 0. H., Rev. Sci. Instruments, 22, 673 (1951). (13) Owen, G., and Hughes, A. L , Phil. d f a g . , 14, 528 (1907); 15, 746 (1908). (14) Regener, E., "Festschrift der Technischen Hochschule Stuttgart zur Vollendung Ihres Ersten Jahrhunderts 1829-1929," pp. 331-5. (15) Schaefer, 1 ' . J., IND.ESG. C H E X ,44, 1292 (1952). (16) Bhntt, R. P., Rev. Sci. Instruments, 22, 730 (1951). (17) Shutt, R. P., and coworkers, private communication. (18) Thompson, J. J., "Applications of Dynamics to Physics and Chemistry," pp. 162-75, London, hlacmillan Co., 1888; "Con-
duction of Electricity in Gases," V d . I, 3rd ed., pp. 32534, London, Cambridge University Press, 1928. (19) Tohmfois, G., and Volmer, M., Ann. Phusik, 33, 109 (1938) (20) S'ollrath, R. E., Rev. Sed. Instruments, 7, 409 (1936). (21) Volmer, Max, "Kinetik der FhatJenbildung," Dresden, T. Stemkopff, 1939. (22) Volmer, Max, and Flood, H., Z. physilc Chenz., 170A, 273 (1934). (23) Volmer, Max, and Weber, A., Ibid , 1196, 277 (1926). (24) 137i150n,C. T. R., Proc. Eo2oy. Soc. (London), h87, 277 (1912); Proc. Cambridge Phil. Soc., 8 , 306 (1895); and many other publications.
(25) TTilson, J. G., "Principles of Cloud Chamber Tcchnlque," London, Cambridge University Press, 1951. RFCEIVHD for review December 21, 1953. ACCEPTIDMarch 1 4 , 1952
Formation of Ice Crystals in Ordiaa Nuclei-Free Air VINCENT J . S C H A E F E R GENERAL ELECTRIC RESEARCH LABORATORY, SCHENECTAPY. N . Y.
Previgus experiments showed t h a t at -39" C. large numbers of ice crystals f o r m whem a n a t t e m p t is made t o produce a supercooled cloud. Recent studies using ordinary arrd nuclei-free air t o explore t h e factors t h a t control t h e formation of ice crystals a t various tmperatures are described. T h e results indicate t h e relationship of freezing and f r a g m tation nuclei t o t h e initial formation of ice crystals. T h e relationship of these findings t o t h e formation of snow i n t h e atmosphere is briefly mentioned.
T""
'
4 formation of ice crystals in the atmosphere has been studied for many years. With a rapidly widening interest in the physical and chemical nature of the atmosphere due to the importance of such subjects as experimental meteorology and air pollution, an even greater emphasis is now directed toward reaching a better understanding of the interrelationships among suspended particles, water vapor, clouds, temperature, radiation, and convection. I n 1946 several experiments were described ( 3 , 16) which indicated that a t about -35' C. ice crystals formed spontaneously in air saturated with respect t o water. More precise experiments by Cwilong ( 4 ) indicated this critical temperature t o be -41.5' C. and by Schaefer (11) to be -38.9" C. More recently Rau (10) stated that he had cooled liquid water to -72" C., although these results are now questioned ( 2 ) . Another investigator ( 9 )reported supercooled water droplets a t - 6 3 " C., and others (8) suggested that they failed to detect a critical temperature.
RESULTS O F EARLIER EXPERIMENTS
The initial approach ( 1 6 ) to determining the critical temperature utiliLed an inverted U-shaped copper bar with one end immersed in liquid nitrogen and the other projected into a cold chamber held a t -20" C. The rod was well insulated and its temperature controlled with a Nichrome wire heater. The temperature of the tip which was exposed in the c d d chamber was measured by an embedded thermocouple. The temperature at which a dense mass of ice crystals streamed from the end of the copper rod was determined to be about - 3 5 " C. -4 later check of the contact between the thermocouple and the copper rod suggested that this temperature might be several degrees too warm. The second experiment involved the use of a copper box containing a source of water vapor. The temperature of the box was varied from - 35 O to -45' C. and the cloud was observed through portholes in the lid. In every instance as the temperature of the chamber was lowered, the supercooled cloud was observed to per1300
sist until the chamber reached a temperature of -39" C., when all the particles became ice crystals and many new crystals a p peared. Similarly, as the chamber w m slowly warmed from -45" C., the first traces of supercooled cIoud appeared at the same temperature threshold. An exact temperature measurement was difficult because the chember used had a volume of about 36 liters, and it was not feasible t o mainttaimm it in a truly isothermal condition. ilnother approach t o more precise determination of this critical transition temperature of ice utilized solidified pellets of pure substances such as 1,2-dichloroethane (melting point -35.3' C.), n-octyl acetate (melting point -38.5' C.), and mercury (melting point -38.89 (3.). It was found that solidified mercury produced a dense stream of ice crystals which disappeared as the mercury melted. RECENT EXPERIMENTS
ON
TRANSITION PHENOMENON
The vapor diffusion type of continuous cloud chamber utilizing
a strong temperature inversion as devised by Langsdorf ( 7 ) and modified by the writer seemed to offer a new approach to this problem. Because it is possible t o exercise exact control of the temperature and the atmosphere within the closed chamber, a number of different experiments may be conducted in sequence to establish the various parameters which may bear on the spontaneous formation of ice crystals. EXPERIMENTAL APPARATUS AND PROCEDURE
The chamber used is illustrated in Figure 1. The base of the chamber is cooled with fragments of dry ice acked around it. The top plate is permitted to reach an equiEbrium temperature between the room and the cold base. This becomes about 15" C. when the room is 25" C. A water supply which lasts for several days is held in contact with the top plate and consists of a cellular structure of aluminum which holds water in a multitude of tiny vertical cells. The walls of the chamber are glass plates cemented with sealing wax. A deep cold zone is
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NUCLEATION-From established by using copper walls soldered to the base late The 0" C. level in the chamber is determined by the de&: to 'whlch the chamber is positioned in the insulated box containing the dry ice. Under equilibrium conditions, the 0" C. level within the chamber is about level with the top edge of this box. Figure 2 shows a typical temperature profile as measured with a sheath-type copper-copnic thermocouple 1/32-inch in diameter. I
I
HOLES FOR THERMOCOUPLE ETC. TOP PLATE SILICONE PUTTY SEAL WATER SUPPLY PLATE GLASS WALLS
1000 WATT LAMP
Gases
these fast-moving droplets, tiny, brilliantly illuminated particles appear which seem to hang in the air motionless for a moment before they start t o fall. Observed under 10 x magnification, these points of light appear like stars, which actually rise a distance of 1 t o 2 mm. before falling. At this moment, the stars appear t o disintegrate and clusters of 4 t o 50 shiny ice crystals fall like a miniature display of fireworks. When photographed by time exposure using modulated light, the falling velocity of these newly formed crystals is found to be about 0.3 cm. per second with less than 10% variation between their individual velocities. The small she of the crystals and their very low temperature have thus far prevented the preparation of good photomicrographs of the crystals. By using the replica method (16),the evidence thus far gathered shows that the water droplets freeze into spherical or slightly angular particles, while the spontaneously formed crystals are very small hexagonal columns.
-------
P
TOP OF CHAMBER------
I
35 -
K
% BOTTOM PLATE SAMPLING ROD
a
b I
ec. P
Figure 1.
Schematic
Diagram of Chamber
Continuous
Cloud
To start an experiment with a minimum of delay due to the presence of a large number of condensation nuclei in the laboratory air, liquid nitrogen is ordinarily used to flush the chamber. This, however, is not an important feature of the phenomena to be described, as the "rainout" effect will eventually produce nuclei-free air in the sealed chamber. Distilled water was used in the water reservoir and, as a general rule, equilibrium conditions occurred within an hour or two after dry ice was put in contact with the heat sink. To assist in observing the cloud and to obtain photographs, a 1000-watt water-cooled capillary, mercury discharge lamp was used. To increase contrast, the walls of the copper box were coated with dull black lacquer. Subsequently, stri s of black chiffon velvet positioned 0.5 cm. from the copper walg proved to be superior t o the paint, SPONTANEOUS FORMATION O F I C E CRYSTALS
If there is a critical transition temperature for the spontaneous formation of ice crystals, one should expect t o find a sharply defined zone in the chamber, above which would be only supercooled water droplets and below this level nothing but ice crystals. Depending on the quantity of moisture passing down the chamber, the number of crystals that form should bear some relationship to the moisture supply. Both effects are observed. If the moisture supply is high, water droplets form in the upper part of the chamber due either to a sdpersaturation phenomenon or to the presence of condensation nuclei, pass through the 0" C. isotherm without change, and then a t a very sharply defined level produce a dense mass of ice crystals having a concentration in excess of 104 crystals per cc. If the supply of water vapor is reduced, a remarkable phenomenon occurs a t this same level in the chamber. This is most striking if the concentration of condensation nuclei is in the range of 10 to 50 per cc. Droplets form in the warm zone of the chamber and reach a falling velocity of 1 to 2 cm. per second as they pass through this critical temperature zone. When illuminated with a beam of parallel light and viewed a t right angles, it is possible to see that they flash over to ice crystals aa they fall through this level. Less than a second later at this same place and in the wake of some of June 1952
30-
0 I
25-
cm. 20-
z
rBOTTOM 01 -70
1
-
I
-60
I
-50
OF
CHAMBER7
4 , -40
TEMPERATURE
-30 4 0.
OF
-20
,4
,
,
I
-10
0
10
20
AIR IN CHAMBER
Figure 2. Typical Temperature Profile in Continuous Cloud Chamber
With the large temperature gradient thus far employed in conducting the experiments, it has not been possible t o obtain an exact determination of the temperature a t the critical transition level. The sheath-type thermocouple indicates it to be -338.5" i 0.5' C. POSSIBLE EXPLANATION O F CLUSTER FORMATION
It is believed a t present that there is a reasonable explanation for the formation of these spontaneous ice crystal clusters. It is apparent that a critical temperature exists which limits the supercooling of water droplets to temperatures warmer than -39" C. The experimental evidence shows that spontaneous ice crystal formation a t -39' C. requires saturation with respect to water. No ice crystals form unless water droplets occur slightly above the transition level. AB the supercooled liquid droplets pass through this level and freeze, their passage causes a slight increase in the concentration of water molecules in the warmer, moister air which is channeled downward in the wake of the water droplet. This momentary and very local enrichment in the number of water molecules must be sufficient to exceed the critical concentration necessary for the spontaneous formation of ice crystals. The slight rise of the initially formed crystals observed a t the transition level is prob-
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ably due to the release of the heat of formation as the ice crystals form. While the possibility cannot be ruled out that the newly generated crystals are due t o the splintering phenomenon originally described by Findeisen (6) and more recently by Brewer and Palmer ( I ) , it is difficult t o see why this effect should be restricted
of a transition level a t -39' C. could be detected. The initiation of this new level is shown in Figure 5 . EFFECT O F D R Y I C E I N CHAMBER
When tiny fragments of dry ice are dropped into the chamber, another interesting effect occurs. The thin condensation line which instantly develops in the wake of the falling fragment pinches off it falls below the phase transition level at -39" C. A second later a discontinuity forms at the 0 " C. level, with a large number of ice crystals forming and rapidly falling below this level, while a tiny vortex ring forms above the 0' C. level in the water droplets formed there. The water droplets above this region then fall into the cold zone, supercool, and freeze as they reach the -39" C. level. Stages in the discontinuity formed at the 0 " C. level are shown in Figure 6. I N S T A B I L I T Y OF A I R C O N T A I N I N G H I G H CONCENTRATIONS OF DROPLETS OR CRYSTALS
Figure 3. Appearance of Beam of Light Projected through Cloud Formed i n W a r m and Cold Regions of Continuous Cloud Chamber
Another very beautiful phenomenon is the instability of air containing localized concentrations of particles. The additional weight added to the air by the sudden localized condensation of water droplets and ice crystals leads to the very rapid fall of the particles. This rate exceeds the falling velocity according to Stokes's law by a factor of 2 to 5 times or more This effect occurs in localized channels, but is probably the same phenomenon which, on a much grander scale, gives rise to mammato-cumulue clouds, long cirrus streamers, and other precipitation cell phenomena often observed in clouds which form in the free atmosphere. This rapid removal of ice crystals generated by the sudden localized chilling of moist air prevents the formation of a persistent phase transition level at the 0 ' C. isotherm.
3-second exposure
OPTICAL PROPERTIES O F SMALL WATER DROPLETS AND ICE CRYSTALS
to a very narrow temperature range. Effects which are similar to this phenomenon, which the author prefers to call fragmentation, occur a t temperatures up to nearly 0" C. The effect is without question related to the freezing of certain water droplets. It is not possible a t this time to say whether the particles are "sloughed off" the actual droplet as it freezes or whether it is an effect that occurs quite independent of the near proximity of the droplet which is responsible for the effect. Figure 3 shows a time exposure made as a beam of light illuminated the chamber at an angle of 45". The photograph was taken with the camera oriented 90" to the axis of the beam. Falling supercooled droplets appear as faint continuous lines At the phase transition level the brightly illuminated lines showing curvature are formed as the individual ice crystals form, rise as previously mentioned, and then fall. A fortuitous slight sidewise drift of the air in the chamber moves the crystals from the vertical, so they are more easily seen. Figure 4 shows s e ~ e r a lcluster showers as they fall below the transition level.
Water droplets and ice crystals when illuminated by a stiong beam of light generally appear strikingly different. Cloud droplets in particular, which are generally in the size range of 5 to 20 microns in diameter, have most of their light scattered over an angle of a few degrees in the forward direction with respect t o incident parallel light. If these droplets buddenly crystallize into single hexagonal plates, the change is very striking, since the flat crystalline faces reflect light in a specular manner. If, however,
EFFECT O F SILVER I O D I D E NUCLEI ON TRANSITION LEVEL
Vonnegut has shown (17 ) that submicroscopic particles of silver iodide serve as excellent nuclei for ice crystal formation. When introduced into a continuous cloud chamber the writer has found that if the silver iodide particles serve as condensation nuclei, they also act as freezing nuclei a t -4' C. When a concentration of silver iodide smoke of the order of l o 3 cc. was introduced into the warm zone of the cloud chamber, the droplets that formed on these nuclei shifted t o ice crystals at the -4' C. level. As they invaded the cold portion of the chamber (colder than 0' C.), a new phase transition level developed at the -4" level, which persisted as long as the concentration of ice crystals remained great enough to cause the freezing and sublimation of the water drops and vapor. During this period no trace
1302
Figure 4.
Formation of Clusters of Ice Crystals a t and Immediately below -39" C. Isotherm Photographed a t an angle of 30'
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water. No alteration of the test unit wab necessary except t o substitute benzene for water in the liquid holder a t the underside of the warm plate. The development of supercooling, the seeding effect with dry ice, and the development of a phase transition level at -42" C. have been observed. Another effect which has also been observed with water involves an interesting 5-second period of condensation. The effect with benzene is more spectacular than with water. At 5-second intervals a layer of droplets about 1 cm. thick and extending all across the chamber suddenly appears and falls as a coherent mass through the transition level. At a given instant three such bands are visible: one in the supercooled region, one in the transition zone, and one below it, each separated by a clear space of about 1 cm. The mass of particles falling into the transition zone gives rise to a number of new spontaneously formed crystals, but continues falling into the region below. As the crystals become visible in the lower region, they have an entirely different optical appearance, since they are now crystalline. Figure 5. Ice Crystals Formed by Freezing of Cloud Droplets Containing Silver Iodide a t -4°C.
SIGNIFICANCE OF SPONTANEOUS I C E CRYSTAL F O R M A T I O N I N RELATION f0 T H E ATMOSPHERE
Photographed a t a n angle of 30°
Apart from the interesting light which these experiments throw on the problem of spontaneous crystal formation, the effects described may be of considerable significance in relation to the formation of cirrus clouds and ice crystal fogs in the free atmosphere.
the crystals form as hexagonal columns, the effect is much lesa striking, so that a certain amount of familiarity with the appearance of such crystals is necessary to detect the change. If the droplet freezes and retains its spheroidal shape, there is virtually nothing t o alter the optical appearance; therefore, other techniques must be employed t o establish the change thathasoccurred. The easiest method to establish whether or not water droplets are present is to permit tiny fragments of dry ice to fall through the chamber. Wherever supersaturation with respect to ice occurs many new crystals appear and grow, because the partial vapor pressure of water is greater than ice a t all temperatures below 0 O C. If ice crybtals are present a t a concentration in excess of 50 crystals per cubic centimeter, the newly formed crystals immediately evaporate. This effect appears as a pinching off of the condensation trail. A good example of this effect is shown in Figure 7 . The end of the trail in this instance coincides with the -39" C. level. The optical method for differentiating between cloud droplets and ice crystals also fails when the particles are of the same order of size as the wave length of the light used for illumination. This type of particle is most commonly obtained in expansion chambers and is characterized by high-order Tyndall spectra. Another feasible method is to remove the particles with a suitable conduit to a warmer environment such a9 - 10' C. and then allow them t o come in contact with a supercooled film of a 2% solution of polyvinyl alcohol in water spread on a thin plastic membrane (14). No effect will be observed if the droplets are supercooled water. If the particles are single or compound crystals, they will produce specific effects as they seed the supercooled film. Thus, a hexagonal plate will form a six-rayed dendrite, while a hexagonal column will produce one with four rays. A spheroidal or asymmetric ice particle produces a peculiar rounded crystal in the supercooled film. TRANSITION PHENOMENA W I T H BENZENE DROPLETS A N D CRYSTALS
Many of the interesting phase transition phenomena observed with water between 0" and -39" C. may be duplicated with benzene (CsH6)between 5.5" and -42" C. It may be that these are universal effects whenever supercooling and crystallization are characteristic properties of a substance that has a reasonably high vapor pressure. As benzene has a melting point of 5.5 O C. and forms a rhombohedral crystal, it lends itself nicely for comparative purposes with June 1952
Figure 6. Effect of Tiny Fragments of D r y Ice Dropped through W a r m and Cold Regions of Chamber Note vortex ring along upper part of condensation trail which forms a t 0' C leve;. Photographed a t angle of 20
Most cirrus clouds, as well as ice crystal fogs, develop in air having a temperature in the neighborhood of -40" C. I n six Project Cirrus flights a considerable effort was directed toward obtaining photographic evidence of the appearance of the top of cirrus clouds. Despite the various irregularities seen from below, the tops of the clouds as a yule were extremely flat. I t is well known among meteorologists and others interested in weather phenomena that cirrus cloud formation is often associated with the overrunning of cold air by a warmer tongue of moist air. Wherever the moisture conditions in the warm, overriding air reach saturation with respect to water and the colder air below has a temperature of -39 C. or colder, ice crystals will form spon-
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NUCLEATION-From
Gases On the basis of the results obtained in the continuous cloud chamber, it is apparent that crystals form a t -39” C. without recourse to the presence of ice crystal fragmentation (12) or splintering (1)phenomena, freezing nuclei (18), or sublimation nuclei (IS). In fact, the evidence seems to be clear that the air must be saturated with respect to water and contain either condensation nuclei or sufficient moisture to form spontaneous water drops. If these two conditions are not met, the multiplication phenonienon will not occur. \In this respect, the results are in agreement with ideas advanced recently by Mason and Ludlam (8). Observations recently reported by Dingle and Nexsen (6)are alsoexplained by the phenomena described. Additional experimental results concerning spontaneous nucleation phenomena with respect to the water-ice phase will be described in a subsequent paper. LITERATURE CITED
Brewer, A. W., and Palmer, H. P., Nature, 164,312 (1949). Brewer, A. W., and Palmer, H. P., Proc. Phyls. Soc. B64, 765-73 (1951).
Cwilong, B. M., N a t u r e , 155,361 (1945). Cwilong, B. M., Proc. Rou. SOC.(London). A190, 137 (1947). Dingle, -4.iY., and iiexsen, W.E., J . Meteorol., 8, 365 (1951). Findeisen, IT., Meteorol. Z., 57, 201 (1940); 60, 145 (1943). Langsdorf, 4.,Rev. S e i . Instruments, 10, 91-103 (1939). Mason, B. J., and Ludlam, F. H., Progress in Physics, 14, 147
Figure 7. Pinch-Off Effect When Condensat i o n T r a i l of Ice Crystals Passes i n t o Region of Ice Crystals Photographed a t angle of 20’ above horizontal
taneously a t the inversion interface. The number of primary crystals that form will depend on the concentration of condensation nuclei and ice nuclei in the moist air mass. The number and size of the secondary crystals that form will probably be some multiple of the effective number of condensation nuclei. Since these conditions for ice crystal formation are of a marginal nature, the variability and often unique appearance of true and false cirrus clouds may be closely related t o these spontaneous crystal formation phenomena. It is likely that the concentration of supercooled water droplets a t the transition temperature of -39” C. is of primary importance in the formation of cirrus crystals. Ice nuclei in the overrunning moist air will operate t o prevent the multiplication effect described in this paper.
(1951).
Pound, G. &I., aiid Madonna, L. A., Division of Industrial and Engineering Chemistry, A x . CHEM.Soc., Nucleation Symposium, Evanston, HI., December 1951. Rau, W., Schriften deut. A k a d . L u s t . , 8 , 65 (1944). Schaefer, V. J., Bull. Am. Meteorol. Soc., 29, 175-82 (1948). Schaefer, V. J., “Compendium of Meteorology,” p. 221, Boston, American Meteorological Society, 1951. Schaefer, 5’.J., J . A p p l i e d Math. and P h v s . (ZASIP), 1, 153-211 (1950).
Schaefer, V. J., J . Jfeteorol., 6 , 283 (1949). Schaefer, V. J., Science, 93, 239 (1941); Nature, 149,81 (1942). Schaefer, V. J., Science, 104, 457 (1946). Vonnegut, B., C h e m . Revs., 44, 227 (1949). Weickmann, H., “Die Eisphase in der Atmosphare,” Rept. an3 Trans. No. 716, Volkenrode, Ministry of Supply, Great Britain. RECEIVED for review January 4, 1952. A C C E P r E D .%gril 4 , 1952.
Liesegang Rings of Ammonium Chloride W.
H. J O H N S T O N
AND
PETER J. M A N N O
P U R D U E UNIVERSITY, I A F A Y E T T E , IND.
R e c e n t experiments are reported on t h e phenomenon of Liesegang rings of a m m o n i u m chloride, including isothermal studies, experiments w i t h x-rays, surface studies, and a n investigation of t h e effect of water. Discoveries of a cloud chamber effect and t h e necessity of water vapor as a catalyst are described.
A
LTHOLGH many workers have studied the formation of Liesegang rings in gels (I), the phenomenon of rhythmic precipitations in the gas phase has received little atfention. One such example, however, is the rhythmic precipitation of ammonium chloride produced by ammonia and hydrogen chloride into opposite ends of a tube, as reported by Koenig ( 3 ) and Hedges ( 3 ) . Recently, Spotz and Hirschfelder measured the time of formation of a ring, and by diffusion theory that a 1000-fold supersaturation enists €or both gases prior t o precipitation (4). on the The presentpaper sulllmarizes further phenomenon of Liesegang rings of ammonium chloride including isothermal studies, experiments with x-rays, surface studies, and 1304
an investigation of the effect of water. The discoveries of a cloud chamber effect and of the necessity of water vapor as a catalyst are described. Experimentally, the vapors from sohtions of 1.5 and 2 F‘ ammonium hydroxide and 10 F hydrochloric acid were allo.l\-ed to diffuse into the opposite ends of 50- or 100-cm. lengths of 3 mm. borosilicate glass tubing. Under these conditions the rings formed near the center of the reaction tube. The isothermal runSWere done a t 25.0” 0.1’and 35.0” =I=0.1” C. The cloud chamber experiments were done with 10 mg. of radium and with a hfachlett AEG 50 x-ray tube operated at 40 kv. and 20 ma. with a tungsten target and beryllium window. The surfaces tested were clean borosilicate glass, Desicote, and powdered ammonium chloride. The dry runs were done with phosphorus as drying agentsfor the pentoxide and magnesium hydrogen chloride and ammonia, respectively.
*
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