TRANSPARENT PROJECTIONS of LECTURE EXPERIMENTS WILLIAM J. CONWAY Fordham University, New York Citj
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HERE are many instructive experiments of firstyear chemistrythat cannot be performed as lecture demonstrations because they must be carried out in such small vessels that the more remote students cannot follow the course of the experiment. Other experiments move too slowly when performed in the usual manner to hold student interest. The author has found difficulty in demonstrating the activity series of the metals, supersaturation, certain specific properties, and so forth, to large groups of students. Some experiments of this type have been improved for classroom use by being demonstrated in an opaque projector,' but the inaccessibility of the apparatus when in position in an opaque projector, the heat of the lamp, probability of fumes injuring the instrument, and the poor visibility considerably limit the use of an opaque projector for this type of experiment. Accordingly an attempt was made to develop a method whereby the ordinary transparent lantern slide projector might be used with only slight modifications of the instrument. The principle of the scheme lies in the use of glass cells thin enough to permit good definition of the contents when used in a lantern slide projector. The only modification of the projector that is necessary is the removal of the slideholder and the substitution of a wooden frame to hold the cells. The cells we have used are easily made from standard lantern slides (8.3 cm. X 10.2 cm. X 0.125 cm.) and vary in thickness from 3.75 mm. to 7.5 mm. The cells are made by separating two of the lantern slides with strips of glass (about 1 cm. wide and 1 to 5 mm. thick). The strips are obtained from the customary type of thin cover glass for lantern slides, or heavier glass depending upon the required thickness of a cell by first 1 FILLINGEX, H. H., J. CHEM.EDUC., 8, 1852 (1931).
cutting a scratch in the glass plate with a glazier's glass cutter. Then the strip is broken off by using the ends of the index fingers as a fulcrum near one end of the mark previously made with the glazier's instrument. Three of the strips are then placed on a lantern slide as shown in Figure 1 and covered with another plate. The binding material, which was selected after discarding a host of others, is polyvinyl acetate.= This is first dissolved in acetone to make a twenty-five per cent. solution and applied to the slides so as to make contact with the separatory strips (cf. Figure 1). The acetone is allowed to evaporate a t room temperature, the cell is assembled and placed under a small weight in an oven a t 10O0C. for ten minutes. When the polyvinyl acetate softens (9S°C.) the weight presses the walls of the cell against the separating strips, and, on cooling, a water-tight cell results. Occasionally it is found that due to imperfect binding between the plates and the strips a cell will leak. This can be overcome by first drying the cell after testing it and then painting the edges with the solution of polyvinyl acetate. In some experiments the heat carried by the beam of light is a serious disadvantage. This is conveniently removed by placing a second cell containing a one per cent. solution of copper sulfate between the first cell and the source of light. The water for the solution should be boiled previously to remove dissolved air which otherwise forms interfering bubbles. To demonstrate the inoculation of a supersaturated solution by projection, the best salt seems to be sodium acetate. A forty per cent. solution of C.P. sodium acetate is heated to about 85'C. until all the salt dis-
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Vinylite, Grade AYAA manufactwed by Carbide and Carbon Chemical Corp., New York City.
solves. The warm solution is then transferred by pipet to a cell heated by partial immersion in warm water. After cooling, the cell containing the supersaturated solution can be freely transported without crystallization taking place. When ready to perform the experi-
beam of light to warm the solution again and thereby redissolve the crystals. Another lecture experiment of elementary chemistry which is particularly adaptable to this mode of presentation is the one usually performed early in the school year to illustrate specific properties, uiz., the solubility of wool and the insolubility of cotton in alkali. When 2-cm. square pieces of wool and cotton yam are placed
ment all that is necessary is to place the cell in the projector, focus the meniscus, and inoculate. Immediately, long, beautifully shaped, needles form, growing completely across the screen (Figure 2).s Since the cell is only 1.25 mm. thick, the illusion of three-dimensional focusing of the crystals is produced. The rapidity of the crystal growth is striking, and observations are
made that are usually overlooked when carried out in the customary macro scale, e. g., that the rate of crystal growth is proportional to the degree of supersaturation. It is also effective to remove the cooling cell after crystallization has taken place to allow the heat of the
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a The photograph shown in Figure 2 was taken with a Leica camera, 3B. Fa.
in a cell containing sodium hydroxide and projected without the use of a cooling cell sufficient wool quickly goes into solution, even though the solution is heated only by the heat of the light, to show that the wool fabric is disintegrating and that the cotton does not dissolve. During the course of this experiment the whole class can follow the test and can see the action of the alkali on a single fiber. The projection greatly increases the sensitiveness of the test, makes the action seem much more rapid, and is comparable to an observation, made microscopically, by individual students. The electrochemical series of the metals has always presented difficulties to the author when it seemed advisable to demonstrate it to a large class. However, when several cells are successively placed in the projector and to each a representative metal is added the great diierence in the degree of turbulence of the solution caused by the evolution of hydrogen gives a rough estimation of the activity of the metal. The difference in the activity of Mg, Al, Zn, Fe, Sn, Pb can be shown also by inserting wire or small shot into a divided cell containing the same concentration of acid. A cell may be divided to make a number of small compartments by placing two or three strips of zlass in a cell parallel to d of ~ i & r e1. Care must be taken to clean carefully the surface of the metals just before use and to handle
the cleaned metals with forceps. This precaution is added lead acetate to make a ten per cent. solution. necessary because the action is greatly magnified, The cell is filled by pouring the solution through a funthereby necessitating equalization of factors affecting nel made by drawing out a test-tube so that the stem will fit in between the plates of the cell. When the projector is approximately twenty feet from the screen a slide is magnified to about six feet by eight feet. This enables one using the above concentration to cause the growth of a lead tree, the projected image of which is eight feet tall, in less than ten minutes. Figures 3, 4, and 5 are photographs of lead trees as projected on the screen. Figure 3 was produced slowly, i. e., the tree grew to the bottom of the cell in approximately fifteen minutes, whereas in Figures 4 and 5 twenty-five volts were used a t the start to form the crystals rather slowly for the trunks and branches, then fifty volts to produce the crystals more rapidly but much smaller to
the speed of the action. With the above experiment it is also desirable to demonstrate an example of "catalysis" due to electrolytic action.Vbis is accomplished by adding a wire of pure zinc to reagent grade sulfuric acid. The slow reaction is accelerated by placing platinum foil in contact with the zinc and all students can see the formation of bubbles on the platinum surface in preference to the zinc. It is also effective to illustrate here the difference in the apparent degree of ionization of acids. This is accomplished by adding an aluminum wire to a sulfuric acid solution in a cell. Attention is drawn to the rate of reaction by noting the turbulence of the solution. A solution of sodium acetate is added replacing the sulfuric by acetic acid and thereby retarding the speed of the reaction. For the projection of the growth of a lead tree a cell is made with separating strips 1.25 mm. thick. This cell differs from the other cells inasmuch as a lead strip is used as the bottom of the cell. The lead strip, b, in Figure 1 is used as the anode with a protruding end to facilitate the attachment of the positive wire of a dry battery or other source of current. A piece of platinum foil to act as the cathode is inserted in the upper part of the cell. The lead strips for the bottom of the cells were made by folding lead foil over on itself a number of times to make a thickness of something greater than that which was necessary, then placing in a large vise to squeeze it down to 1.25 mm. We have also hammered down one-eighth-inch sheet lead to the required thickness and planed the surfaces in a vise. This thin cell has the advantage of producing a tree of lead in which the crystals are kept in an approximate plane, hence making it possible to keep the entire tree in focus during its formation. A five per cent. agar agar solution is prepared and filtered to remove clots. To this is ' F o m ~ s G.. , "Lecture experiments in chemistry,'' P. Blakiston's Son & Ca., Inc., Philadelphia, Pa., 1937, p. 467.
adorn the branches with foliage. Figures 4 and 5 vary considerably from Figure 3. This variation shows the wide variety of effects that are possible. It seems to the author to be a distinct improvement over the older method of growing the lead tree in a Petri dish. This technic has been presented in the hope that the method may be helpful to others for the demonstration of certain experiments and to make them clearly visible to each member of a large class. The experiments described in this paper are simple and are typical of others upon which work is now in progress and which will be reported a t a later date.
