THE PROJECTION OF LECTURE EXPERIMENTS* HARRIBTI H. F~LINGBR. HOLLINS COLLEGE, V~GINIA
A projection lantern for opaque objects has been found useful i n projecting lecture experiments. Details often seen by only a small portion of the class when experiments are carried out on the lecture desk i n the usual fashion may be made clearly visible to every member of a large class. Some of the lantern's uses, as demonstrating the relative degrees of ionization of strong and of weak electrolytes as s h m by conductiwity of electricity and chemical reaction experiments, the suppression of ionization of weak electrolytes as s h o w by indicators and decreased chemical activity, fractional p e cipitation, and double refraction of Iceland spar are discussed, and the experiments described.
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For several years the author, by using an opaque projection lantern of the balopticon type for the projection of lecture experiments, has successfully made the details of such experiments clearly visible to each member of a large class. Experience has taught that even in the best arranged lecture rooms, those students a t a distance from the desk often miss the main point of the experiment through failure to see the most significant details of the reaction. For instance, even though relatively large quantities of material are used, the evolution of gases, relative rates of evolution of gases, color changes. etc., cannot be seen by more than a small portion of the class when such experiments are carried out in the usual fashion a t the lecture desk. In an attempt to overcome such difficulties it occurred to the writer that such experiments might be carried out to advantage in a lantern and the magnified image projected upon a screen. The results have been very successful with the simple experiments for which the lantern is constantly used. In the hope that the idea may prove helpful to others a few of the experiments which have been carried out easily and effectively by this means are briefly described. The experiments have been purposely set up without any special arrangement of lantern or any additional equipment of the usual lecture room other than a small piece of mirror. The experiments used have been kept simple in the belief that the least elaborate set-up which will demonstrate the principle in question is the most satisfactory because i t gives the student a better opportunity to direct his attention to the reaction rather than to the apparatus required to manipulate it. By inserting small ammeters and voltmeters in the lantern and projecting them, the small pointing needle and the scale of inexpensive pieces of apparatus are converted to dimensions of several feet. The ammeter is used in making a rough study of the relative effective degrees of ionization of acids and bases as indicated by differences in conductivity of solutions
* This paper was presented in part before the Division of Chemical Education of the A. C. S. at Richmond, Virginia, April, 1927. 1852
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of equivalent concentration. Normal solutions of hydrochloric, phosphoric, tartaric, and acetic acids and normal solutions of sodium and ammonium hydroxides are generally used in this experiment. The voltmeter is used in discussions on batteries, or the generation of electricity by chemical action. The ammeter or voltmeter is the only portion of this set-up which is placed in the lantern, the remainder of the apparatus being on the lecture desk. If the lantern is one that must be placed some distance from the desk, wires for attaching the electrical apparatus can be permanently run from the desk to the lantern. Differencesin effective degrees of ionization as indicated by rateof chemical action easily lend themselves to being carried out in the lantern. For instance, if small lumps of marble of equal size are placed on separate watch crystals, very low-form crystallizing dishes, or flat-bottomed petri dishes, in the lantern and small amounts of 6N hydrochloric, phosphoric, and acetic acids, respectively, are added separately to the samples of marble, very marked differences in the rate of evolution of carbon dioxide can be noticed in the images of the reactions which are projected side by side on the screen. The action of acids on other salts and on metals can be used similarly to illustrate the same point. If the elements are chosen from different positions in the Electromotive Series of Metals and used with the same acid, these experiments may be used to indicate differences between the activities of the metals. To demonstrate the effect of particle size on the rate of chemical action, zinc graded in several sizes from powder to quite coarse granular particles and 6N hydrochloric acid may be used. These experiments rather obviously indicate that the lantern permits a more careful study of several experiments a t one time than would a series of reactions in test tubes or test glasses even to a small group. It is the only successful method which the author has found for showing the details of such experiments to large groups of students. Suppression of ionization of weak acids and bases as indicated by the color change of an indicator or by decreased chemical activity may be shown easily in the lantern. The suppression of ionization of ammonium hydroxide by a soluble ammonium salt with phenolphthalein as an indicator and the suppression of the ionization of acetic acid by a soluble salt of the acid as indicated by the color change of methyl orange may be used for this purpose. A piece of plain white paper instead of the mirror should be placed under the watch crystals containing these materials. A sample of the solution and indicator should be placed by the side of the one in which the suppression of ionization is taking place for reference in comparing the color change of the indicator as the suppression proceeds. The action of acetic acid and of the acid in the presence of one of its soluble salts toward marble can be used to show the effect of suppression of ionization on chemical activity. The difference between ammonium hydroxide
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JOURNAL OF CHEMICAL EDUCATION
SEPTEMBER, 1931
and ammonium hydroxide plus a soluble ammonium salt as precipitants for insoluble hydroxides can also be shown in this connection. The details of fractional precipitation and fractional dissolving experiments can clearly be shown to large groups by carrying out such experiments in the lantern instead of on the lecture desk. For instance, if silver nitrate solution is added in several separate portions to a mixture of rather dilute solutions of a chloride and a chromate, the white precipitate of the silver chloride first formed will give place to a mixture of white and red precipitates as successive portions of silver nitrate are added to the solutions. Or, on addition of a solution of a chloride to the red precipitate which has been drained free from supernatant liquid, the red silver chromate will he dissolved* and give place to a white precipitate of silver chloride, thus illustrating one of the methods of dissolving insoluble compounds. The dissolving of lead iodide with a solution of an oxalate illustrates the same point. Dissolving through complex ion formation can also be nicely demonstrated by the action of a solution of ammonia on an insoluble cupric salt. To illustrate the principle of fractional dissolving a solution of ammonia can be used on a mixture of silver chloride and silver sulfide. As a miscellaneous collection of experiments the following are briefly mentioned as suggestive of the range of application of the lantern experiments. The effectof light on silver chloride can he clearly exhibited as the light in the lantern fairly quickly causes the silver chloride to darken. The effect of camphor or soap on the surface tension of water can easily be demonstrated. If a large crystallizing dish containing a small amount of water is placed in the lantern, small bits of gum camphor added to the water, and the image of the whole focused on the screen, the pieces of camphor will dart about over the screen as if they were animate matter. Or, to show the effect of soap on the surface tension of water, add a drop of oleic acid to a dilute solution of ammonia. For demonstrating quickly the double refraction of Iceland spar, use some highly colored object as a colored plate of bright line spectra. A certain line or lines can he shown without the spar and then shown with the crystal of spar lying over the colored lines. As viewed through the crystal each line will appear double. Liesegang rings in silicic acid gel in small test tubes can also, with the aid of the lantern, be shown very nicely to large groups. A few general points to be borne in mind in setting up the experiments in the lantern are listed below for the convenience of those who wish to use any of the experiments herein mentioned or others which these may suggest: I-Use watch crystals or low-form crystallizing dishes for experiments. 2-For best results if the materials to be used are somewhat opaque, set
* Obviously, the term dissolve as here used refers entirely to chemical reactions and, therefore, is rather inaccurately used in the strictest sense of the term.
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the apparatus on a mirror placed in the lantern. However, if the materials are entirely in solution and color changes only are to be noted the best results may be obtained by setting the apparatus on a piece of white porcelain or plain white paper. 3-Because of the heating effect of the lamp in the lantern the light should be turned on only as needed. 4-A dropping pipet of the medicine dropper type, or a longer one, has been found most convenient to use in adding small quantities of liquid to a piece of apparatus already in the lantern. 5 S i n c e three-dimensional objects are to be projected i t may be necessary to change the focus of the lantern slightly during the experiment in order to show all of the details of interest. 6-The "layer of reagents" in the pieces of apparatus used should be kept as thin as possible. The experiments above outlined are mentioned only as examples of classes of experiments which may by means of the lantern easily be shown to large groups. These will doubtless suggest many more to the reader as well as call attention to an additional use for as expensive a piece of apparatus as an opaque projection lantern.
