JOURNAL OF CHEMICAL EDUCATION
868
JULY, 1928
PROJECTION OF BROWNIAN MOVEMENT N. HENRY BLACK, HARVARD UNIVERSITY, CAMBRIDGE, MASSACWSETTS
Probably many teachers of physics and chemistry are accustomed to demonstrate the Brownian movement by means of a water suspension of carmine on a slide under a compound microscope. This method makes i t necessary for each student in turn to adjust the instrument so that he can observe the motion of the tiny particles. All teachers of large classes have long wanted some means of projecting this movement in order that the whole class or section may observe the fascinating phenomenon on the screen. Ever since the English botanist Robert Brown, in 1827, observed this irregular motion while examining with the microscope a liquid containing some pollen grains, various scientists have been studying the phenomenon and improving the methods for observing it. It was in 1910 that Perrin published his micrographs of a colloidal solution of mastic and calculated, on the basis of his experimental data, Avogadro's number.' In 1923 Ehringhaus of Gottingen described his experiments with the Brownian movement in various suspensions and also his method of projecting this ~ have followed his procedure in general, but motion on the s ~ r e e n . We have made some improvements in the technic which we hope will be useful to other teachers. Apparatus The first essential for microprojection work of this sort is a powerful source of light which is concentrated, as nearly as possible, at one point. We use for this purpose a 90' arc lamp ( A ) ,as shown in Figure 1, in which the special cored carbons are about 5 millimeters in diameter and coated with ~ o p p e r . ~ Of course, one might set up an heliostat so as to use sunlight, but the high-powered incandescent lamps will not serve as a source. It is very convenient to have an automatic clock feed for the arc lamp, but this is not necessary. Next we use as a condenser an aplanatic collector (C) of about 5 centimeters focal length; in front of this a water cell (W) about 5 centimeters in thickness, with parallel glass sides; then a double convex lens (L) of about 20 centimeters focal length. At the other end of the optical bench the compound microscope is set up on a stand, so as to be in the usual vertical position, and a t such a height that the center of the mirror (M) is exactly on the optical axis of the projection apparatus. Under the stage of the microscope we insert the dark-field condenser
' "The Brownian Movement and Molecular Reality," J. Perrin.
Translated by
F. Soddy, 1911. A. Ehringhaus, Kollmd Zeilschrijt, 32, 19 (1923), and Die Naturvissenschajte+z, 42, 1923. a These can be procured from E. Leitz, Inc., 60 East 10th Street, New York.
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FIG. 1.-Diagram t o show the arrangement of apparatus for microprojection A , arc lamp; C, aplanatic condenser with irir diaphragm; W, water cell t o absorb heat; L, auxiliary converging lens; M, mirror under microscopic stage; Con., dark-field cardioid condenser; Sus., rutile suspension on quartz plate; 0, oil-immersion Icm; E, eyepiece; P, right-angle prism; and S, cardboard screen.
(Kardioid Kondensor by Zeiss, 1925). The paraboloid dark-field condenser will not work for this experiment. On the stage we place a fusedquartz plate or chamber which is provided with a circular groove and especially prepared by the Zeiss firm for this sort of work. On this plate we put a drop of rutile suspension (Ti02).4 This can be prepared by grinding the mineral for twenty minutes or longer in an agate mortar and then allowing it to settle for some time and pouring off the liquid containing the finer particles in colloidal suspension. It is, however, essential for this experiment not to use too small particles. The use of a centri-
PIG.2. Zsiss OFTICAL BEKCHFOR ~IICROI~ROJECTION qR. Winkel, G. m. b. H., Gottingen, Germany, prepares "Rutil Suspension zur Demonstration der Brownschen Molekularbewegung," according to A. Ehringhaus.
fuge helps in grading the particles. Over this we place the ordinary thin cover glass. Doubtless, it would be better to use a thin quartz cover glass. For an objective we use a '/,z-inch oil-immersion objective (0) with an iris diaphragm (Zeiss 90 N. A. 1.25). It is necessary to use an objective which is provided with this diaphragm. As an eyepiece (E) we use the lowest power eyepiece that comes with the microscope (Zeiss No. 1). Above the eyepiece we use the ordinary right-angle 45" glass prism (P) for throwing the image on a vertical screen (S). For this purpose we find that a large piece of white cardboard is the most satisfactory because it gives a very opaque, white, smooth surface. We find that the "Translw" screen gives excellent results only when the observer is directly behind it. Manipulation
It is best first to adjust the apparatus by observation through the miaoscope with the. eye in the usual way. 1701 this we use the illumination of the projection apparatus, but it must be somewhat reduced in intensity (a c u r r e n t of p e r h a p s 10 amperes), and a piece of white p a p e r should be inserted between the arc and the microscope. It is necessary to make sure that the projection apparatus is exactly in line and that the beam of light hits the middle of t h e reflecting mirror under t h e microscope stage. Adjust the condenser (C) so that the light fills t h e convex lens (L) and then move the latter so as to apet an image of the carbon approximately a t the microscope. Then turn the mirror so that it throws the light exactly into the condenser. These adjustments are made easily with the Zeiss optical bench, shown in Figure 2. Next, raise the dark-field condenser until its surface is level with the top surface of the stage; place a drop of distilled water on top of the condenser and then put on the quartz plate with its drop of rutile suspension and cover glass. It might be worthwhile to note here that
scrupulous cleaulmess is absolutely essential, and one must be constantly on the lookout to prevent air bubbles. Next, place a drop of cedar oil on the cover glass. Then with the coarse adjustment lower the microscope tube until it makes contact with the drop of oil, and then very slowly lower i t until it seems to touch the cover glass. Apply the eye to the eyepiece and with the fine adjustment raise the objective until the particles are brought into view. When one has once made this preliminary adjustment, he can then proceed to improve the conditions by raising and lowering the cardioid condenser. Generally, it will be necessary to bring the condenser as near as possible to the quartz plate. Also adjust the lever attached to the cardioid condenser and the iris diaphragm in the objective. Begin with the diaphragm closed, and open i t more and more. When these adjustments have been made, place the prism on the eyepiece and darken the room. It must be very dark indeed, because the amount of light which is reflected is very small. It takes about five minutes for the human eye to become sufficiently sensitive to see the particles to the best advantage. By placing the screen a few feet away from the microscope, one can gradually get the focus and readjust the various parts of the illuminating system, such as the fine adjustments of the position of, the arc and the exact position for the condenser and mirror, in order to attain the highest intensity. Then move the projection screen farther away and gradually increase the current up to 20 or 25 amperes. This will enable one to project the little particles so that they can be seen by a class of 75 to 100. It will be necessary to cover the whole projecting apparatus, except w ~ ~ & the arc lamp, with a black oil-cloth box to prevent movable bott o m which leakage of stray light.5 serves to illusAfter one has had some experience with the apparatus, trate the kihe can make the necessary adjustments and set it up in d',:',": ten or fifteen minutes, so that i t is an entirely practical to a motor. experiment to perform before elementary students in chemistry, provided one has suitable equipment. One surely gets a much more correct as well as real picture (Fig. 3) of the Brownian movement in this way than one does from the usual diagrammatic drawings which are so often found in textbooks. The Theory Involved We have all learned by experience that it is not sufficient to perform The author is much indebted to Dr. Egan Lorenz, an International Fellow in
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the Jefferson Physical Laborstory, for his personal assistance in the manipulation.
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striking experiments before our students; we must also make sure that they see the significance of the experiments. I n this experiment we are trying to help the student to visualize molecular motion, but unless we take special care, they will get the idea that they have seen molecules in motion. We must somehow make it plain that these "small visible colloidal particles are knocked about by colliding with the invisible molecules like a football in the midst of a crowd of invisible player^."^ We have found it very useful a t this point to use a mechanical model which was designed by Pohl of Gottingen. The essential part of the
FIG. 5.-POHL'S
APPARATUSFOR PROJECTING THE MECHANICAL M O D E L ILLUSTRATING M o ~ E c u ~ nAND n BRO-IAN MOVEMENT
apparatus (Fig. 4) consists of a tall metal box with glass windows a t front and back. The box is provided with a piston a t the bottom which moves rapidly up and down as it is driven by a crank attached to a small motor. In this box are placed small steel balls and also two larger wooden balls. As the piston moves up and down, the tiny steel balls dance about in the space above, as we suppose the molecules of liquids and gases behave according to our kinetic theory. The wooden balls represent the colloidal particles which are subjected a t all times to an enormous number of collisions with the rapidly moving molecules by which they are Washburn's "Principles of Physical Chemistry." p. 111. 1921.
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surrounded. This motion can be conveniently projected by means of the optical bench shown in Figure 5.' This whole apparatus was designed by Prof. R. W. Pohl of the University of Gettinpen. Germany, and is manufactured by Spindler and Hoyer of the same place.
