The PROJECTION
of
CHEMICAL onto the
SCREEN
OSCAR W. RICHARDS Research Department, Spencer Lens Company, Buffalo,N e w
T
HE importance of lecture demonstrations is no longer questioned in natural science; the problem resolves to choosing the best demonstrations and the most effective method of presenting them to the students. The advantages of the usual lantern slide projector have been described recently by Conway.' A more convenient instrument for many purposes is the lecture-table projector of the type that projects a vertical beam, which is reflected by means of a mirror onto a screen above the instructor's head. The "Delineascope," an example of this type of projector, is illustrated in Figure 1. The holder for the lantern slide is horizontal so that various materials may be placed on the clear, glass-covered opening and projected. With this projector transparent, semi-transparent or translucent materials and other materials which are meaningful in silhouette may be shown on the screen. The field covered is nearly three by three and onehalf inches. The usable depth amounts to about five millimeters. However, if detail is not important throughout the depth, i t is possible to project the image of a standard half-inch test-tube so that the color and the presence or absence of a precipitate will show on the screen. Tubes which are oval in cross-section, as used by the bacteriologists, may be obtained in either lime or pyrex glass and are particularly convenient for this type of demonstration. No spe$ial equipment is demanded for this work beyond that ordinarily found in the laboratory. A convenient type of dish for chemical experiments is the ordinary Petri dish, obtainable in diameters of two, three, and four inches. The vertical projector is very convenient when one wishes to present the student a near view of some routine experiment. By placing a drop of material to be tested and a drop of indicator on the platform, the color change indicating alkalinity or acidity appears on the screen. The formation of a precipitate is of great importance in many chemical ezzperiments. For instance, if one boils a test-tube of Fehling's solution, i t can be held slantwise over the stage of the projector, the projection lens focused high enough to show i t on the screen, and the student sees that its color is blue and is free from precipitate. On adding a bit of sugar and boiling again, the test-tube may be shown to con1 CONWAY, "Transparent projections of lecture experiments." THISJOURNAL. 16,314-16 (1939).
York
tain a reddish brown precipitate proving the presence of a reducing sugar. From routine demonstrations it is only a step to teaching some of the fundamentals of microchemistry. Instead of depending upon textbooks or demonstra-
FIGURE I.-MEDICINEDROPPER MICRO-FILTRATION DEMONSTRATED WITH A SPENCER MODELB DELINEASCOPE
tious which could be shown only to a few students a t a time, i t is possible to make the demonstration with the projector before the entire class. Most of the methods of applying reagents2 can be demonstrated on the projector if one uses half of a Petri dish and focuses the lantern sharply on the surface or on the materials used. The various procedures of filtration can be demonstrated. The material containing a precipitate may be
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' C ~ OAND T MASON, "Handbook of chemical microscopy," John Wiley & Sons. Inc., New York City, 1931, Vol. 11. ix
411 pp.
+
placed in a Petri dish, a strip of filter paper placed with one end in the solution and the dish tipped so that the liquid material runs through the strip and accumulates as a filtered drop a t the other end (Figure 2). An interesting experiment for freshman chemistry is to mix a little tooth paste with water and filter it in this manner and then add a drop of silver nitrate to the clear filtrate. Immediate precipitation of silver chloride shows that the tooth paste contains salt. A filter stick may be made by placing filter paper in the open end of a medicine dropper (Figure 1). It is possible if one makes a micro-burner by drawing out a medicine dropper to a fine point, to demonstrate micro sublimation on the stage of the lantern. The
stage of the projector and allowing the heat from the lantern gradually to drive off the water. Diffusion may be illustrated by taking a layer of about three millimeters of distilled water in a Petri dish and adding a drop of methylene blue or other dye
F I G W ~ E ~ . - N E E D L E CRYS~ALS F R O M POTASSIUM CONTRASTED WITH S M A L L P Y R A M ~ D A LCRYSTALS F R O M S O D I ~ XWITH URANYL
ACETATE
.
FIGURE2.-FILTRATION
WITH A
STRIP OP PAPER
wide use of some types of micro reagents may be conveniently shown. An illustrative 'example would be the use of uranyl acetate as a test for sodium and potassium (Figure 3). Drops of sodium and potassium salts are placed in separate places in a Petri dish on the table of the projector, and then the uranyl acetate is added to each. In a short time the crystals are large enough to show on the screen the characteristic differences of the salts of these two metallic elements. Many of the general principles may be shown conveniently. For instance, if one has a few Syracuse watch glasses, small stender dishes, or other small dishes filled with material of different refractive index, by immersing crystals in them one can show the effects of the index of refraction and how it is determined by the immersion method. The Becke line may be shown. Crystal forms can be shown if one chooses crystals which are not larger than about five millimeters in their greatest dimension. A broken crystal dropped into a saturated solution will repair itself before the eyes of a student and then grow appreciably during the lecture period. Water of crystallization may be demonstrated by placing a sodium sulfate crystal on the
.
in the center of the dish. Thg heat effects of convection currents from the heat of the lantern are interesting as well. Or, one may put a crystal of a colored chemical into the solution, or one may lay a test-tube slantwise across the field and then drop the colored crystal into the bottom of the test-tube. More elaborate types of diffusion may he shown by taking a thin layer of gelatin in a Petri dish and putting a drop of Congo red on one side and of eosine on the other. When the activity is too slow to show all in one lecture period, the dish may be kept and placed on the screen by means of the projector a t successive lecture periods. I t is not difficultto keep a quantitative record by turning tlie mirror of the projector so that it throws the image onto the blackboard or the lecture table within easy reach and tracing the outline of the stained region on a strip of paper. At the next lecture period, another student may add the outline and in that way a permanent record of progress is obtained. Liesegang rings may be demonstrated by taking two milliliters of saturated potassium bichromate and one hundred milliliters of six per cent. gelatin. This is placed in a thin layer in the bottom of the Petri dish and after it is se< a drop of twentyafty per cent. :silver nitrate is added to the center. The gradual formation of the precipitation rings shows clearly on the screen. Membrane formation may he demon-
212 strated by adding a drop or two of a potassium ferrocyanide solution to three per cent. copper sulfate. If a small amount of a strong sugar solution be added, then the difference from osmotic pressure may be demonstrated on the drops within the membrane. A great many of the conventional experiments on surface tension may be demonstrated before the entire class a t once. The commonest demonstration is to float a needle on distilled water and then sink the needle by adding some surface-active chemical such as a small amount of soap solution. One of the more interesting surface tension experiments is the wellknown mercury amoeba. The crystallizing dish is half filled with ten per cent. nitric acid; a drop of mercury about a centimeter in diameter is added and a crystal of potassium bichromate is placed about a halfcentimeter from the mercury. As soon as the bichromate dissolves and reaches the mercury the latter shows active movement similar to that of an amoeba. The color of the diffusing solution of bichromate shows clearly and the mercury shows effectively in silhouette. This spectacular experiment usually leads to considerable discussion among students after the lecture is over. Determination of the hydrogen-ion concentration by means of a colorimetric method is easily demonstrated with the projector. A piece of glass such as a lantern-slide cover glass or the bottom of a Petri dish is coated with a very thin coating of vaseline. ,Then drops of buffer solution may be placed on this and a drop of indicator added to each. The drops stand up and may be seen on the screen-like colored glass beads." A drop of the unknown is placed beside the series and a drop of indicator added. The comparisons are then made with the colors. The swelling of gelatin may be demonstrated in buffer solution. The gelatin may be cut into discs with the aid of a cork borer, or a convenient source of pieces of the same size is the caps of some of the smaller gelatin capsules sold by the druggist. If the buffers are placed in small dishes it is possible to show several of them on the screen a t one time. Otherwise, they may be shown successively. This experiment may also be made semi-quantitative by having the student keep a record of the amount of swelling of the capsule a t the differenthydrogen-ion concentrations. Many experiments may be made with various colloids by using a thin layer of the colloid in a suitable dish. Mutual precipitation of one colloid by one of opposite charge may be readily demonstrated, or the colloid may be precipitated by placing two electrodes in the solution and passing direct current through. The effects of acids and bases on oil-water emulsions are readily demonstrated. Emulsifying agents and detergents can be illustrated in this manner. Colloidal gliadin is useful for many experiments and is readily made in the laboratory by taking an alcohol extract of flour and pouring it into distilled water to
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~ R I C A A R D S"A . rapid method for determining calorimetrically the pH of small amounts of fluids," Science, 68, 185 (1928).
give a white milky colloid of positive charge which is stable and is useful for colloidal experiments. The effect of charge on staining is demonstrated by using semi-transparent or transparent materials. The formation of lakes may also be shown. Electrochemical effects may be demonstrated readily with the vertical projector. A strip of clean metallic zinc when placed in a lead or in a tin solution brings forth the characteristic growth of these metals. Differential growth may be shown in a solution containing four per cent. potassium fenicyanide and one-half of one per cent. sodium chloride with a little gelatin or albumin added (Figure 4). A piece of iron wire shows rapid growth of crystals on being placed in the solution, while a piece of zinc wire shows little, if any, growth. However, if they are twisted together and placed in the solution, then rapid growth will take place on the
F ~ C U R4.-DIFPERENTIAL E GROWTH FROM IRON AND ZINC AND AN IRON-ZINC COUPLE I N POTASSIUM FERRlCYANlDE
zinc, and the growth on the iron will be inhibited. By cutting them apart, the original conditions are restored. This experiment usually produces considerable student discussion and sets the stage for a detailed consideration of electrochemical phenomena, including the problems of corrosion. Electrolysis may be demonstrated by placing the electrodesin solution. The demonstration of the breakdown of water is a little more spectacular if a small amount of gelatin or albumin be added to the solution to trap the bubbles as they are formed. Some exneriments mav be arranged so that the results will appear on thescreen befoye the students. By blowing the breath through a shallow layer of lime water in a small dish on the stage of the projector, the formation of carbonate is readily shown. A simple experiment mav be arranged by taking some yeast in a test-tube and bringing a tube from that around so that it dips
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under the layer of lime water. This experiment will proceed on the screen before the students and makes an interesting accompaniment to a lecture on respiration and fermentation. Polarized light lends itself to many spectacular demonstrations. A piece of Type 2 Polaroid may be placed over the stage opening of the vertical projector and another one on top of the objective. Or, the demonstration kit put out by the Polaroid Corporation may be used held in place with an ordinary laboratory ring stand. The effect of many crystals on polarized light is readily shown on the screen, e. g., medium-sized crystals of benzoic acid, aspirin, hydroquinone, tartaric acid, thymol, potassium nitrate, borax, potassium chlorate, and many others. Recrystallization of spermacetic or palmitic from oil solution and lecithin from absolute alcohol may show liquid crystals in polarized light. The growth of one of these crystals when a seed crystal is placed in a supersaturated solution on the stage is very spectacular when seen on the screen with polarized light. The importance of polarizing light for distinguishing between materials may be demonstrated by showing the effectof Cellophane and Kodapak. These transparent' layers have quite different effects on polarized light. A great many experiments may be performed in this manner including all of those included in the Polaroid kit. The experiments described have been taken frdm current books and from the experience of the writer and
are offered merely to demonstrate the possibilities of this projection lantern. Once one has become accustomed to working on the flat surface it becomes natural to turn to it for many demonstrations where one would like to bring the students up to the lecture desk. A number of experiments may be taken from many of the laboratory books and demonstrated in this manner, restricted only by the limitations of the method. The advantages are that the student's attention is focused on the demonstration and that each student can see equally well and from the best viewpoint. By use of a glass rod or pencil, the instructor may point out regions of special interest in a chemical experiment. The principles of scientific methods are more readily discussed and demonstrated by means of experiments made on the projector, and many experiments may be made into permanent records by tracing outlines of changing size as mentioned before. Another method which is very effective and interesting is to let the students photograph the results with a medium-fast film and lens. It is usually possible to obtain a picture of the screen with an exposure of about a twenty-fifth of a second. With the exception of a few experiments which require deeper layers or which must be held upright, the lecture-table type of vertical projector is better adapted for most experiments in chemistry and has many real advantages over the more conventional type of lantern-slide projectors.
