MUSEUM REACTIONS* Although the present tendency is toward individual instruction in the chemical laboratory, the experimental lecture is still an integral feature of most elementary chemistry courses. The average student cannot possibly gain as much from an experimental lecture as he could from properly planned individual assignments covering the same ground. If the group of students is large most of them are located so far from the scene of action that they cannot see the details of the set-ups. They form the habit of awaiting a flash of light or a loud noise or a word from the instructor as the signal that the expected has happened.
* Paper read before the Division of Chemical Education at the April, 1925, Meeting of the A. C.S .
No amount of explanation, either before or after the demonstration can mkke up to them for the lack of contact with the apparatus and materials. During a lecture the instructor cannot consider each individual in the class. He must consider the class as a whole. Admittedly, therefore, a t the close of the lecture, the poorer students are suffering from mental iudigestion and need supplementary instruction. Very shortly after the lecture, he must clear away his set-ups in preparation for the next class, and unless the lecture apparatus is kept intact in the museum there is practically no opportunity for any student to become acquainted with either the apparatus or the details of the reactions at his leisure. The actual experiment should in some way be made to live in the mind of every student. Many reactions may be harnessed so as to proceed more or less uniformly for considerable lengths of time. One need only assemble the necessary apparatus and materials, start such a reaction going and leave it for inspection. Meanwhile the poorer students may study the "experiment" a t length, until each one has mastered it. The principle involved and any unusual features may be emphasized by means of placards (there are crosssection diagrams of any parts of the apparatus which are not plainly visible). The instructor may even carry out a very definite assignment by including on the placards carefully chosen questions and references for library work or consultation. In any case healthy discussion is stimulated among the onlookers. Sometimes it may seem wise to bring the set-up to the lecture table to repeat the experiment before the whole class. The tenn "Museum Experiment" seemed appropriate for these unattended demonstrations of reactions which have a long "shelf life." The requirements are obvious: There should be a visible change; i t should be continuous or recurrent; the reaction as well as the apparatus should be. simple enough in principle to permit clear, concise exposition by means of charts; the apparatus and materials shonld be comparatively inexpensive on account of "shoplifting" and there should be a minimum of danger to property or observers in case of accident. While these museum experiments may very effectively supplement formal instruction in the laboratory, they probably possess a greater value in stimulating popular interest in chemistry. They have proved to be extremely successful in connection with chemical "expositions," "shows," "fairs" and "open honses." Although static exhibits require least attention, experience has shown that they do not attract the casual visitor as do those which are advertised in some dynamic fashion. Nor are displays of products and apparatus ever as successful as actual demonstrations of reactions or processes. If upperclassmen, graduate students or instructors are available to act as demonstrators no other device is necessary. In their absence, however,
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each exhibit must be its own "barker." By deiinition, the museum experiment, with its accompanying charts, can play this role very effectively. Once attention is attracted, real interest is not hard to arouse, since there are no fictitious values in chemistry. I shall confine myself to the description of a few tested museum experiments, without quoting "lines" for the placards. The experiments have been selected from various sources, individual credit for which I have not attempted to give. I t is assumed that the reader is familiar with the following books: Newth, Chemical Lecture ExperimentsLongmans; Benedict, Chemical Lecture Experiments-MacMillan; van Klooster, Lect. and Lab. Expts. in Physical Chem.-The Chem. Pub. Co. The last-named author in particular, gives valuable references to the original literature. As cheap materials whose contiuuous flow is assured, air, water and illuminating gas present the greatest possibilities for the average laboratory, although the employment of other substances is limited only by one's ingenuity in devising set-ups. A comparatively simple experiment offers an almost unlimited field for instntction. The following are a few illustrations. Combination of Gases A . Recifirocal Nature of the Combustionofllluminating Gasin Air (Fig. 1) A lamp chimney is closed at both ends with pieces of asbestos board. a is an elbow tube which serves as the gas inlet, b and c are short lengths of 0.5 cm. tuhing. With everything in place except b, the chimney is filled with gas which is then lighted at G and at the hole for b. The tube b is now inserted and carries the flame d inside the chimney.
B. The Bunsen Burner Prim'ple 1. Construction of the Burner (Fig. 2) A glass model is constructed from a straight piece of tubing a, having a diameter about 2 cm. and a length of 3040 cm., and an elbow tube b, constricted at one end to a 1 mm. tip, which serves to admit the gas. The distance between the lower end of a and the tip of b will depend upon the gas pressure and the diameters of a and the tip, but need not be greater than 3 em. to produce a very striking appearance. 2. The Luminous Flame. Same as B-1, save that the gas tip is thrust through a stopper in the lower end of a to prevent the entrance of air. 3. Glass Blast Lamp. A small side arm is sealed about in the middle of a 10 an. length of hard glass tubing (diam. 2 an.). A one-holed stopper in the lower end admits a thin straight tube which extends the length of the larger tube and ends in a 2 mm. tip. Air is blown through this inner tube while gas is admitted through the side arm.
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4. Gasoline Blow-torch. 5. Oxy-hydrogen flame. Tanks of O2and Hz and a special burner are
necessary. Onlookers are directed to hold a piece of iron wire in the flame. The flame may be directed against a piece of CaO. 6. Burner from Gas Cooking Stove. C. Structure of the Flame 1. Ordinary burner with match-head supported by means of a wire just inside the top. 2. Ordinary flame with glass tube introduced at an angle, the end being kept just below the inner cone; the unburned gas which escapes through this tube may be burned at its top.
D. Profiagation of the Flame; Kindling Point of the Gas; Safety Principle 1. Flame is maintained at a distance from the outlet of the burner by holding a piece of wire gauze (a, Fig. 3) a t about the spot where the inner cone of the non-luminous flame would strike it, turning on the gas and lighting it above the gauze only. 2. Fisher Burners. Show 3 burners: No. 1-with grid removed and air-holes open; No. 2-with grid removed and air-holes closed; No. 3with grid in place and air holes open; No. 4--with grid in place and airholes closed. E. Enriching a Gas Distribute illuminating gas through a Y-tube to two burners. Before reaching one the gas is passed through a gas-washing bottle containing CGH~. F. Production of Solid NH4CZfrom NHs and HCZ Gases HCl is produced by dropping conc. H&04 from a separatory funnel onto a strong NaCl solution in an Erlenmeyer flask. While NHs is best gotten from cylinder of compressed gas, it may be made by dropping saturated NH4C1solution on solid NaOH in an Erlenmeyer. The two gases are led through glass tubes and caused to impinge on each other underneath a large funnel (hood). Solid N H L l collects. Osmosis A. Natural Membranes General principles and technique are described in texts of General Science, Plant Physiology and Chemistry. The following membranes have been successfully employed; skins of apples, oranges, lemons, apples, carrots, beets, eggs; bladder, intestine. Most convenient system is sugar solution: membrane; pure water. B. Arti&ial Membranes 1. Collodion bag (using sugar solution) 2. Mineral flower garden (The conventional arrangement, using a dilute
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solution of NazSiOa, into which small crystals of heavy metal salts have been dropped). Distillation
A . Distillation of Water under Normal Pressure Permanganate solution in a 1liter distilling flask, fitted with a thermometer and connected to a glass condenser and receiver.
B. Distillation of Water under Reduced Pressure Same arrangement as in A, save that the receiver is a 500 cc. distilling flask, connected tightly to the inner tube, and the side arm of which is attached to a glass water suction pump. A boiling point 30 degrees below normal is easily maintained. C. Boiling of Liquid Air 500 cc. of liquid air in an unsilvered Dewar flask will boil quietly for a t least 10 hrs. Electrolytic Phenomena
A . Gravity Cell in Glass Jar, Connected with Small Lamp
B. Electrolysis of H2S0, Solution, Collecting the Gases Subatomic Phenomena
A . Geissler Tubes, Containing Sumfiles of Different Rarijied Gases, Connected with Induction Coils B. Radium or Radium Emanation, Using ZnS to Magnqy the Effect If a small tube containing emanation is broken inside a bulb coated with ZnS, the latter will glow for some time. Ten millicuries are sufficient for a 200 cc. bulb for one day. Reactions between Solutions When two solutions react to produce a gas, an insoluble solid or some substance with a different color, the reaction may be demonstrated &ectively by the following simple scheme (Fig. 4): Two short side arms b and c are sealed onto a piece of 1 cm. tubing about 1m. long. To these are attached aspirator bottles, separatory funnels, or any convenient vessels from which two different liquids may be admitted into the long tube. Screw pinch cocks control the flow from these sources. Water from the hydrant is admitted at a and flows out at d. The rate of flow is adjusted to suit conditions, but should not of course be sufficient to back the liquid up into b and c. If a solution of Na2C03 is allowed to flow slowly into b, while a solution of CaClz is admitted at c, a white ppt. will form at this spot and a milky liquid will flow out at d. Many other com-
binations a t once suggest themselves. A third side arm may introduce still another reagent. Oxidation of Phosphorus under Water In Fig. 5 about 0.5 cc. of yellow phosphorous is placed in an Erlenmeyer flask containing water to a depth of about 4 cm. The flask is heated (not to boiling) over a low flame. The whole set-up is placed over a bed of sand. A conveniently bent tube is placed in the flask, through which a current of air may be blown onto the phosphorous.
