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the contracted stress field catalyzed by flaws. Lowering of the test temperature appears t o increase appreciably the elastic strain energy available to accelerate the fracturing. From this view, the possible role of microconstruction of the medium is dual in character. It may either act t o provide flaws or catalyzing agents or t o alter the availability of strain energy. These two characteristics would be expected to have different temperature dependencies. The temporal development of flow markings and fractures, when viewed in their simpler aspects, strongly suggest a process which is at least grossly analogous to the nucleation of a new from a parent phase. I n fact, Fisher, Hollomon, and Turnbull ( 7 ) have ventured the speculation that the origins of fracturing and plastic flowing may be generally regarded as a nucleation phenomena. The discontinuous, fine structure illustrated here represents a more complex series of processes than that suggested by the delay time experiments. If the simpler aspects of plastic flowing and fracturing may be regarded as nucleation phenomena, then these advance nucleations of fracture and flow elements may be analogous to the advance stimulation of recrystallization nuclei which has been described by Burgers (3) in studies of the recrystallization of aluminum. I n general conclusion, the illustrations cited here provide a number of new and interesting features of the development of macroscopic flow and fracture markings. At present it appears that some of these may be useful from the model point of view in extending understanding of the microscopic processes of plastic flowing. The nucleation of new flow and fracture elements in advance of primary flow and fracture may prove to be of particular importance. If a similar process does exist on the microscopic scale it may well be the major source of slow speed dislocation multiplication and hence the answer to the very old question of how dislocations multiply and pile up to form the macroscopically observed flow marking. ACKNOWLEDGMENT
The writer wishes to acknowledge many stimulating and clarifying discussions with G. R. Irwin and E. Orowan on numerous aspects of the general topic of this paper. Special acknowledgment is due W. H. Charch, E. I. du Pont de Nemours & Co., Inc., for gift of Mylar film.
LITERATURE C I T E D
(1) Andrade, E. N. da C., Proc. Roy. Soc. (London), A84, 1 (1911). (2) Bridgeman, P. W., J. Applied Phys., 17, 225 (1946). (3) Burgers, W. G., Nature, 157, 76 (1946). (4) Clark, D. S., and Wood, D. S., Am. Soc. Testing Materials, P , o c . ,
49, 717 (1949). (5) Cottrell, A. H., Proc. P h y s . SOC.( L o n d o n *) ,60, 30 (1948).
(6) Eshelby, J. D., Ibid., A62, 307 (1949). (7) Fisher, J. C., Hollomon, J. H., and Turnbull, D., J . Apuplaed * Phys., 19, 775 (1948). (8) Frank, F. C., Proc. P h y s . SOC.( L o n d o n ) ,60, 46 (1948). (9) Ibid., A62, 131 (1949). (10) George, W., P h y s . Rev., 73, 1262A (1948). (11) George, W., and Irwin, G., MIT Plastics Symposium, available on microfilm, June 1950. (12) George, W., and Irwin, G., P h v s . Rev.,73, 12308 (1948). (13) Gough, H. J., Hanson, D., and Wright, S. J., Trans. R o y . Soc. ( L o n d o n ) ,A226, 1 (1928). (14) Irwin, G. R., “Fracturing of Metals,” p. 147, Cleveland, Am. Soc.‘ Metals, 1948. (15) Kauffman, J. W., and George, W., J . Colloid Sci., 6, No. 5, 450 (1951). (16) Kelvin, Lord (Thomson, W,), “Math. and Phys. Papers,” Vbl. 1, p. 97, Cambridge, England, Cambridge University Press, 1882. (17) Kies, J. A., MIT Symposium on Plastics, available on microfilm, June 1950. (18) Kies, J. A,, and Hauver, C., unpublished. (19) Kies, J. A., and Sullivan, A. M., J . Metals, 188, 1090 (1950). (20) Kies, J. A , , Sullivan, A. M., and Irwln, G . R., J . Applied Phys., 21, 716 (1950). (21) Love, A. E. H., “Treatise on Mathematical Theory of Elasticity,” 4th ed., p. 221, New Yolk, Dover Publishing Co., 1944. (22) MoLean, E., Hauver, C., and George, W., unpublished. (23) Orowan, E., MIT Conference on Fatigue, in press, June 1980. (24) Orowan, E., 2.Physzk, 89, 634 (1934). (25) Polanyi, M., 2. Physsk, 89, 660 (1934). (26) Smekal, A., Glastech. Ber., 23, No. 3, 57 (1950). (27) Smith, H. L., and Ferguson, W. J., N a v . Research Lab. Progiess Rept., 11 (April 1950),unclassified. (28) Smith, H. L., Kies, J. A., and Irwin, G. R., Phys. Rev., 83, 872A (1951). (29) Taylor, G. I., Proc. R o y . SOC.( L o n d o n ) , A145, 372 (1934) (30) Volterra, V., Ann. Bco2e norm. (Paris),Ser. 3, 24, 401 (1907). (31) Wood, D. S., and Clark, D. S.,33rd Annual Convention Am. Soc. Metals, to be published. (32) Yoffe, E. H. (nee Mann, E.), Phil. Mag., 42,739 (1950). (33) Zappfe, C. A., Trans. Am. Soc. Metals, 41, 396 (1949). RECEIVED for review Deoembxr 21, 1951.
ACCEPTEDApril 3, 1052.
The Photographic latent Image As a Nucleation Process from the Solid Phase J. H. W E E E K O D A K RESEARCH LABORATORIES, ROCHESTER, N. Y .
T h e nucleation process, which occurs in the individual grains of t h e photographic emulsion, is referred t o as the formation of t h e latent image. T h e latent image and i t s formation are discussed from t h e standpoint of t h e electronic and ionic properties of crystals. T h e m i n u t e size of t h e latent-image nuclei, together w i t h t h e discrete structure of t h e photographic emulsion, makes possible t h e tremendous amplification factor which is inherent in t h e photographic process.
T
HE macroscopic image obtained on a photographic emulsion is
initiated by a brief exposure t o a geometrical pattern of light and is brought up t o the visible stage by a subsquent chemical development process. This paper shows t h a t the photographic process depends on a primary nucleation mechanism t h a t occurs in the solid crystalline grains of the emulsion under the action of light. The nuclei thus formed act t o ‘%rigger” the chemical reduction of a grain when i t is placed in a so-called developing solution. The minute size of these nuclei which initiate developJune 1952
ment, together with the grainy structure of the emuIsion, makeer possible the tremendous amplification factor which is inherent in the photographic process. By development, the original nuclei are magnified some billionfold t o produce the black and white image ordinarily observed. The nucleation process, which occurs in t h e individual grains of the photographic emulsion, is generally referred to as Iatent-image formation. Other aspects of the action of light include the direct photolysis of silver halide, whereby gross overexposure produces a
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visible image directly. The growth of photoproduct nuclei in this case can be followed under the electron microscope. ACTION O F LIGHT
Structure of Photographic Emulsion. From microscopic studies it is knoyn that the photographic emulsion consists of a suspension of tiny crystals of silver halide (usually silver bromide with small percentages of silver iodide) in gelatin. A photomicrograph (2500 magnification) of the grains of a typical negative
Figure 1.
~~~
silver bromide and occurs in the grains of a photographic emulsion and also under certain conditions in gelatin-free silver bromide. An example of direct photolysis in silver bromide on a groes scale is shovm in the electron micrograph ( 7 ) (50,000 magnification) of Figure 2. This shows a silver bromide grain, which had received a strong exposure t o light and was then fixed. An outline of the original grain in the gelatin can be seen, as well as a number of specks of metallic silver. These specks are less than 200 A . in diameter, and appear t o be cubic structures with about 125,000 atoms each. I t has been shown ( 5 )further through x-ray examination that the first yieible trace of an image in silver bromide gives an x-ray diffraction pattern identical with that of metallic silver. These results correspond t o direct print-out by light and require exposures t’housands of times greater than that for producing the latent image that induces development. The importance of the direct photolysis experiment lies in the fact that, radiations which produce this effect also will produce the latent image. From continuity considerations it is logical to assume that this effect is merely a continuation of the process involved in latent-image formation and that the latent image is also composed of silver.
Photomicrograph (2500X) of Grains of Photographic Emulsion
emulsion is shown in Figure 1. In the ordinary commercial multilayer emulsion, there are approximately 1019 grains per sq. em. of plate area, and the average areas of the grains vary from about 0.1 X 10-8 sq. em. for low-speed positive-type emulsions t o 1.0 X 10-8 sq. em. for high-speed negative emulsions. The grains of the ordinary negative emulsion consist mainly of flat plates of triangular and hexagonal forms. When light of a wave length that is absorbed by the silver halide falls on an emulsion, some of the grains, the percentage number depending upon the amount of the exposure, are altered so that they acquire the capacity for reacting with certain reducing agents known as developers-grains that have been affected by the light are reduced completely t o metallic silver by the developer, while the other grains are not reduced a t all. This developable state produced in a Eingle photographic grain by the action of light is called the “latent image.” Although experiments have been carried out t o detect the latent image directly, thus far none of these has been decisive, and development is still the only means for certain detection of the latent image. The latent image is not directly observable because of the minuteness of the change occurring in a single grain in exposure. The most sensitive grains are made developable by the action of a few quanta, perhaps of the order of 4 to 20 quanta. Assuming quant u m equivalence, this means that only a few atoms are affected and it is not surprising that a coagulation process is required for these atoms to be effective in initiating development. Latent Image Composed of Silver. Because the latent image is detectable only by development, a process which in itself destroys the latent image, information about the latent image is mainly of an indirect nature. Silver bromide darkens with excessive exposure t o light, bromine gas being liberated and metallic silver in the colloidal state being left behind. This deoomposition takes place under the action of light adsorbed by
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~
Electron-Microscope Photograph (50,OOOX) of Photolytic Silver Particles
Figure 2.
Silver bromide exposed t o ultraviolet light, fixed
The latent image itself cannot be extracted by chemical reagents in sufficient quantity to be analyzed. Nevertheless, it ha5 been established ( 1 ) that the latent image can be destroyed by oxidizing agents such as chromic acid, free halogens, or substances that ’readily give up halogens, and by most strong oxidizing agents such as potassium persulfate and potassium permanganate. Localization of Latent Image. I t is known (8) from microscopic examination of exposed and developed photographic plates that
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NUCLEATION-From Solids the individual grains of the emulsion act as the units in exposure. The observed fact is that individual grains of an exposed emulsion either develop completely or not at all. I n Figure 3 is shown a photomicrograph (2500 magnification) of a partially exposed and fully developed emulsion. It may be seen t h a t for the specific exposure conditions used here only a small fraction of the grains is developed. Those grains which start to develop do so completely, while the remaining grains remain unchanged. With a higher exposure, a larger fraction of grains would develop, thereby giving increased density with increasing exposure.
substances in gelatin and in showing that they consist of traces of organic sulfur compounds, of which allylisothiocyanate is typical. It was shown t h a t the presence of these substances t o the extent of a few parts per hundred thousand is sufficient t o render a gelatin active. These materials combine with silver bromide, forming complexes, which, in the presence of alkalies, decompose t o form silver sulfide. From these researches it was shown t o be very probable that the sensitivity centers present on the photographic grain before exposure consist of minute specks of silver sulfide, which are formed during the process of emulsion manufacture. Other work has furnished supporting evidence t h a t the sensitivity specks consist of silver sulfide. For example, it has been shown that the sensitivity centers are affected by hydrogen peroxide and by sodium arsenite in much the same way as by light and that they can be destroyed by chromic acid. As a result of these experiments and many others, there emerged what came t o be called the concentration speck theory of latentimage formation, which in a large measure is a description of observed facts. According t o this theory, which was p,ut forward in 1925 by Sheppard, Trivelli, and Loveland ( I S ) , it is assumed t h a t the light active in latent-image formation is absorbed a t random points all over the grain, but that the silver atoms comprising the latent image are formed only a t points adjacent t o the silver sulfide sensitivity specks. The only function attributed to the speck in this theory is t h a t of concentrating the photoproduct formed during exposure. Though many other theories had been previously
Figure 3. Photomicrograph ( 2 5 0 0 X ) of Partially Exposed and Fully Developed Photographic Grains T h e author is indebted t o R. P. Loveland, Eastman Kodak Co. Laboratories, for t h l s photomicrograph
The study of grains in the early stages of development has been of great importance in acquiring information about the latent image. Hodgson ( 4 ) was the first to show that the development of an exposed grain begins a t one or a few discrete points on the surface of the grain and proceeds from these points t o the entire grain. A photomicrograph of fully exposed and partially developed grains is shown, in Figure 4. Much work has been done with experiments of this type, and the general conclusion that has been reached is that the photolytic products that initiate development are concentrated a t specific points on the surface and are not spread uniformly through the grain, this despite the fact that the light which acts to produce the latent image is absorbed over the entire grain. Sensitivity Centers. A great deal of experimentation has been carried on to determine the nature of these presensitivity centers which act to concentrate the action of light at certain discrete points, and an important step forward was made in 1925 by Sheppard ( I d ) . Prior to Sheppard’s work, it was known that some gelatins would produce sensitive emulsions, while others would not. It was natural t o attribute these differences t o chemical constituents contained in the gelatins. Working along these lines, Sheppard, by means of a series of chemical analyses of the acid deliming liquors obtained in the process of gelatin manufacture, succeeded in isolating the sensitivity-promoting
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Figure 4. Photomicrograph (2500X) of Exposed and Partially Developed Photographic Grains
proposed for the latent-image formation, the concentration speck theory summed up the basic facts better than any other theory. While it did not give an adequate account of t h e mechanism involved in the process, this theory came to be generally accepted. C U R R E N T THEORY O F L A T E N T - I M A G E F O R M A T I O N
In 1938, a more explicit theory for the formation of t h e latent image was put forward by two English physicists, Gurney and Mott (3). This theory was in good accord with the factual part of the concentration speck theory and indeed strengthened the concentration speck theory by supplying an underlying mechanism for it that was based on sound ideas in solid-state physics. This theory makes use of the specific properties of photoconduct-
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ance and ionic conductance, both of which the silver halides possess to a strong degree. The joint operation of these two properties acts as a nucleating mechanism to produce the latent image. Photoconductance. If a crystal of silver bromide is placed between electrodes, with the crystal held a t liquid air temperature, no current will flow through the crystal as long as it is dark. However, if the crystal is illuminated, a current will flow, and it can be shown that the current sets in immediately with the illumination. Experiments have shown conclusively that this current is an electronic current. LERO POTENT\AL E N E R G Y
1 ,
/-Wi
A q 0 C CRYSTAL
A g SPECK
Figure 5. Schematic Energy-Band Diagram for Electrons in Silver Bromide Crystal i n Contact w i t h Silver Speck
If the methods of quantum niechanics be applied t o the motions of electrons in crystalline solids (11), the electronic behavior of different crystals can be distinguished by the energy-level diagram of the electrons of that solid. It turns out that electrons in crystals cannot have any energy value whatever, but that they are restricted to certain allowed bands of energy levels. Depending upon whether these energy bands are completely filled or not and upon the degree t o which the bands overlap, the electronic properties of the crystal arc different. For example, an insulating crystal is one in which there are no partially filled bands and in which the lowest empty band is separated from the highest filled band by a considerable energy gap. The electrons in a full band cannot conduct electricity because they cannot change their energy states under ordinary electric fields. If, however, an electron has its energy state raised in some way t o one of the levels of the otherwise empty band, it will behave as a free conduction electron. According to this theory, photoconductivity in an insulating crystal- results from an electron having its energy state raised by light absorption from a lower filled band of the crystal into an upper empty band, referred t o as the “conduction band.” The two relevant energy bandfi for this process in the silver bromide crystal are indicated on the left side of the diagram of Figure 5 . An electron in the conduction band of the silver bromide crystal may be considered t o be shared by all the silver ions of the lattice and as continually moving about from one silver ion to another with thermal energy. Such electrons behave like free conduction electrons in a metal, and, under the influence of an electric field, give rise t o electronic conduction. The lifting of an electron from t h e lower filled band t o t h e upper empty band leaves a vacant electron energy level (positive hole) in the lower band, and this hole can also move in an electric field. The movement of the positive hole occurs by electronic replacement conduction. The location of a positive hole corresponds physically to the position of a bromine atom, and t h e movement of a positive hole takes place by the shift of an electron to this bromine atom from an adjacent bromine ion. Thus, the movement of a positive hole is equivalent to the movement of a bromine atom and affords the mechanism whereby bromine atoms can move to the crystal surface and escape during exposure. There is always the possibility that a free electron in the conduction band and its positivehole counterpart in the lower band will recombine and transform the crystal back t o the insulating state. This type of recombination does occur t o some degree and lowers the efficiency of the photographic process. From the experimental side, photoconductance (9, 1 0 ) is
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studied by placing the crystal between electrodes with a known potential between them and measuring the total current through the crystal on illumination. In the case of the silver halides, the crystal must be held a t low temperature t o suppress the ionic current, which otherwise would mask the electronic current completely. A voltage-current curve obtained in this way for silver bromide under constant illumination is shown as curve 1 of Figure 6. It is seen that the curve rises a t first linearly with voltage, and gradually approaches a maximum saturation value at high voltage. For an understanding of this curve, it must be realized that the contribution t o the electric current by each electron released by light depends not alone upon its charge but also upon the fractional part of the interelectrode distance traveled by that electron while it is free. The horizontal part of the curve results from the fact that, a t high voltages, all of the electrons released by light are pulled all the way t o the positive electrode, thereby producing saturation. At lower voltages, it must be assumed t h a t a certain number of the electrons become trapped in borne manner before reaching the electrode. This effect has been extensively studied in the silver halide crystals, and it has been found (6) that crystals which contain colloidal particles of silver require higher clectric fields for saturation.
Figure 6. Current-Voltage Curves for a Given Number of Absorbed Quanta 1. 2.
Pure silver bromide Silver bromide w i t h colloidal m e t a l
A current-voltage curve for a crystal containing colloidal silver produced by gross overexposure to light is shown by curve 2 in Figure 6. The conclusion t o be drawn from this type of experiment is t h a t colloidal silver specks act as traps for the photoconducting electrons and thereby reduce the average distance traveled by these electrons. The trapping of electrons by colloidal metal particles can be understood from the energy diagrams of the metal and silver halide crystal in contact, as shown on the right side of Figure 5 . If the energy of a conduction electron in silver bromide lies in the conduction band, and if the bottom of this band lies above the highest filled conduction level of the metal by an amount, As,as shown, then an electron passing from the silver bromide to the metal will descend through the potential step, AE, and become trapped. I n simple terms, the silver speck constitutes a foreign body in the silver bromide crystal to which an electron can become attached with a certain binding energy. Ionic Conductivity. The other property of silver bromide which is important for the latent-image process is ionic conductivity. Measurements of ionic conductivity of the silver halides have been made by a number of workers and it is known that these materials exhibit relatively large conductivity even a t room temperature. Tubandt ( 1 4 ) and coworkers showed by a series of experiments that the conductivity of the silver halidesis due principally t o the movement of the silver ions through the crystal. The conductivity changes rapidly with temperature and is about 10-8 ohm-’ cm.-l a t room temperature. Until fairly recently it was not clear how silver ions could move through the crystal lattice so readily as indicated by the magnitude of the observed currents. It is now known t h a t ionic currents can occur in a crystal only if there is a certain amount of disorder among the elements of the crystal. If all the ions in a
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crystal were in a perfectly ordered array, there could be no ionic movement. However, if a small percentage of the ions are displaced from their regular lattice positions, a degree of freedom is available for ionic movement. The type of disorder of the ions in a crystal depends upon the nature of the ions of which it is comprised and t h e binding forces between the ions. According t o present theory, the ionic conductivity of the silver halides rem l t s from silver ions dislodged from their regular lattice positions and removed to interstitial positions. Aq’
Br-
Aq’
Br-
Aq+
Br-
AqC
Aqt
Aq+
Br-
a
b
C
d
Br-
f
Aq’
Br-
BrBr-
Aq+
H Aq+
8r-
Aq+
Br-
Aqf
Br-
0
Br-
Aq+
0r‘
Figure 7.
0 ~ - Aq+ AS+-Aq+ 0r-
BrAq+
Frenkel Model for Electrolytic Conductivity i n Silver Bromide
This type of disorder, which was first proposed by Frenkel (Z), is illustrated in Figure 7. That the silver ions and not the bromine ions are pushed into interstitial positions is a result of the smaller size of the silver ion. The ionic conductivity of silver bromide is ascribed t o the movement of .these interstitial silver ions and to the movement of the vacant silver ion sites in the lattice. How this occurs is shown in Figure 7. The interstitial ions move either t o other interstitial positions or into vacant positions in the lattice. When the latter takes place, the holes are said t o move by “replacement conduction.” Nucleating Process in Latent-Image Formation. According to measurements of photoconductivity in silver bromide, for each quantum of light absorbed by a silver bromide grain there is released one electron, which can move about freely in the crystal. Furthermore, evidence has been given to show that these electrons can be trapped on metallic specks in the silver bromide crystal. If there is a minute amount of silver in colloidal form present initially in a grain, electrons liberated by light will be trapped on the silver specks to charge them negatively. The electrostatic field set up within the grain by this negative charge will attract mobile silver ions in the neighborhood and they will move t o the speck and join with the electrons t o form silver atoms. I n this way, the speck can grow by one atom of silver for each quantum of light absorbed. Of course, if there are several specks of silver present, all of them will grow in the mme manner, but it is readily seen that this nucleation of the photoproduct occurs by light t h a t is absorbed all over the grain. The latent-image problem must be distinguished from the above described case of direct photolysis in that it must be explained how it is possible for the silver speck t o begin to grow. To account for this in the case of the latent image in a photographic grain, it is assumed that the silver sulfide specks di8covered by Sheppard act as electron traps in the same way as colloidal silver specks. If the silver sulfide specks can trap electrons and become negatively charged, then silver ions will be drawn t o these specks and the silver of the latent image can form in the same way as was outlined in the case for the colloidal silver specks. A schematic view of the process of latent-image formation is shown in Figure 8. I n the diagram a, a crystal of silver bromide is shown being illuminated. The action of light produces a gas
Figure 8. Schematic Model of Formation of Latent Image i n a Single Photographic Grain
of free electrons in the crystal which will persist for a certain time. If there are some pre-existing specks of silver sulfide en the grain surface, as indicated, then electrons will be trapped by these specks and charge them negatively as shown in b. While the processes of electron liberation and trapping are occurring, a simultaneous movement of the disordered silver ions In the crystal will be going on, as shown in c. Thus, as the electrons charge the silver sulfide specks negatively, the silver ions will be drawn to the specks t o form silver atoms, as shown in d . I n this way it is clear how the specks can grow by one silver atom for each quantum of light absorbed, and the latent-image specks can continue t o grow until one of them is large enough t o act as a nucleus in development. LITERATURE CITED
Bullock, E. R., “Chemical Reactions of the Photographic Latent Image,” Monograph 6, Rochester, N. Y.,Eastman KodakCo., 1927. Frenkel, J., 2.Physik, 35, 652 (1926). Gurney, R. W., and Mott, N. F., Proc. Roy. SOC.( L o n d o n ) ,A164, 151 (1938).
Hodgson, M. B., J . FrankZinInst., 184, 705 (1917). Koch, P. P., and Vogler, H., Ann. Physik, 77, 495 (1925). Lehfeldt. W..Nachr. Ges. Wiss. Gdttinaen, Math. Physik Klasse, 1, 171 (1935).
Mees, C. E.K., “Theory of the Photographic Process,” 1st ed., p. 142, New York, Macmillan Co., 1942. Mees, C . E. K., and Sheppard, S. E., “Investigations of the Theory of the Photographic Process,” p. 69, London, Longmana, Green, and Co., 1907. Mott, N. F., and Gurney, R. W., “Electronic Processes in Ionic Crystals,” 1st ed., Chap. IV, Oxford, Clarendon Press, 1940. Pohl, R. W., Proc. Phys. SOC.(Extra Part) No. 274, 49,3 (1937). Seits, F.,“Modern Theory of Solids,” 1st ed., New York, McGraw-Hill Book Co., 1940. Sheppard, S. E., Phot. J., 65, 1 (1925). Sheppard, S. E., Trivelli, A. P. H., and Loveland, R. P.,J . Franklin Inst., 200, 51 (1925). Tubandt, C., in Wien, W., and Harms, F., “Handbuch der experimentalphysik,” Vol. XII, Part 2, p. 135,Leipzig, Akademische Verlagsgesellschaft, 1932. RECEIVED for review December 13, 1951.
n
IT. June 1952
ACCEPTEDFebruary 21, 1952.
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