Lantern Slides of Actual Crystals HARRIETT H. FILLINGER
Hollins College, Hollins, Virginia
HROUGH an accidental crystallization in the good results. Probably no rule can be laid down for laboratory some time ago it occurred to the writer the selection of materials to be used. The threethat crystals might be grown on lantern-slide cover dimensional difficultiesinvolved in showing the crystals, glasses and mounted into permanent slides. This problem was presented last year to Miss Warren Williamson, a senior chemistry major, who was registered for a small piece of rather independent work. To her should go the major portion of the credit for the successful execution of the idea. She suggested some of the technics and developed the skills necessary to the preparation of the more or less permanent lantern slides herein reported. Since this preliminary work has proved promising in its results and its usefulness for demonstration in elementary courses and since it is a problem which is adapted to undergraduate work, we expect to press the study somewhat further and to
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even those such as cupric sulfate, were easily overcome by changing the focus of the lantern during the showing of the slide. The importance of making slight changes in the focus of the lantern for some slides is illustrated in the photograph in Figure 1. This slide of cupric sulfate is by no means the best slide obtained, but it is
improve the technics a t certain points. Therefore, this report' should be considered in the nature of a progress report. The slides shown in the illustrations, which are photographs of the enlarged images of the slides on a screen, are among the several dozen which to date have proved to be most successful. In the selection of chemicals for this work, certain obvious characteristics such as the solubility of the compound, the bulkiness of its crystals, its stability under light and moderate heat, possible efflorescence, and deliquescence must he considered. For instance, a chemical such as cupric nitrate which gave beautiful slides was found to be entirely too deliquescent for anything other than very temporary use. But, if handled properly, sometimes surprisingly bulky crystals such as cupric sulfate gave Also presented before the nineteenth annual meeting of the Virginia Academy of Science, Richmond, Virginia, May 2, 1941.
mentioned a t this point because, on first thought, it would seem to offer much less chance of success than such substances as thiourea or benzoic acid (Figure 2). Because of the rapid growth of cupric chloride in clusters, it does not show up so well in a photograph,
but does become very successful and interesting in the lantern. Such compounds as potassium ferricyanide, mercuric chloride, and oxalic acid (Figure 3), if handled properly, were found to be well adapted to our purpose
silicic acid gel were attempted with successful results. Figures 5 and 6 of basic mercuric chloride and a "lead tree" represent beautiful slides which after nine or ten months are in very good condition. No wholly successful method of sealing these cells has been found; a t present they are kept standing in water. A number of these cells placed in a desiccator containing water are in fairly good condition after ten months of such treatment. It may be stated here parenthetically that some of the slides made by precipitation in gel whhh were too opaque to be of use in the lantern were even more beautiful examples of rhythmic banding than those formed by precipitations of the gel in a test tube. They suggest some interesting departures in technic for colloid chemists interested in rhythmic precipitation. In the preparation of these slides no one technic proved equally successful in every case, but the several
and yielded very fine slides which are in good condition after nearly a year of handling and use. The potassium chlorate slide (Figure 4) is of interest in showing the "corner effects" brought about, no doubt, by interruptions in the growth of the crystals. These effects probably resulted from partial dissolvings due to slight temperature changes during crystallization. Several more or less successful attempts were made to make slides by growing crystals from a supersaturated solution in a very thin cell made by a method described in a former issue of this JOURNAL.^ A slide
methods which seemed most useful will be mentioned. The most sucressful one was that used in making all but two of the slidcs here illustrated. Clran rover ulasses were immersed in saturated solutions which were allowed to evaporate very slowly, thus making the crystallization from the solution take place a t a slow rate. An inexpensive glass pie plate with a second plate inverted over it as a cover proved to be very satisfactory apparatus for the work. By sliding the top pie plate a bit to one side or lifting i t with a match stick or other wedge nearly any desired rate of evaporation could be obtained. The slides were examined often after crystallization began to take place. Ohviously, it was important to remove the cover glass from the solution a t the right time. This required some skill and good judgment based on experience in working with the materials. Before the crystal growth on the glass had become too thick the cover glass was carefully lifted from the solution and drained as quickly as possible in an upright position. Such thin films of
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of sodium acetate was made in this way. Slides illustrating rhythmic banding and "metal tree" growth in -
solution which did not drain off the slide fairly rapidly evaporated and were responsible for minute crystals which were tenned "trash." Various methods of removing the solution adhering to the glass were tried, but, to date, the drainage method has given best results. However, such "trash" frequently appears, does not interfere in any way with the usefulness of the slides and often, by showing the general "structure of the growth of the crystals, enhances the appearance of the slide. I n using this method it was found that best results were obtained sometimes by having the slide barely covered with the saturated solution during crystallization, and a t other times best results were obtained by having the surface of the cover glass a half centimeter or more beneath the surface of the saturated solution. Here, again, i t is not possible to lay down any rule. Compounds whose saturated solutions were reasonably dilute were found, in most cases, to yield the best results. For the reason just mentioned water solutions of organic compounds which yielded extremely dilute saturated solutions were found to give better results than solutions of these compounds in organic solvents. To show what might be called the general structure of the growth of the crystals a second technic proved effective. One large drop or several smaller drops of the saturated solution were placed on the cover glass and allowed to evaporate quickly a t room temperature or a t temperatures somewhat above that of the room.
The crystals grew from the circumference toward the center of the drop and produced many beautiful effectswhich are well adapted to use with the lantern. A third technic was found possible in some cases, but yielded slides with what might be called an artificial appearance. I t was found possible to lift from the solution rather bulky crystals such as those of cupric sulfate and the long, sturdy crystals of potassium ferricyanide and place them on a clean cover glass. The solution adhering to the crystal, in most cases, was sufficient to cement it to the glass. The fourth method of using a cell and supersaturated solution, as previously mentioned, gave some beautiful effects, but for obvious reasons was not considered a satisfactory method for making permanent lantern slides of actual crystals. The mounting of the slides after they were clean and dry was a fairly simple matter. Drops of Duco or some such cement were put a t the corners of the slide and sometimes a t the center of the long edge and allowed to set in order to support the upper cover glass and allow it just to touch the crystals but not to rest upon them. The two cover glasses with the crystals between them were bound together with ordinary black binding tape, thus yielding a lantern slide which fits into the slide-carriage of an ordinary lantern. At the present time a student is experimenting with the use of two pieces of polaroid in connection with these lantern slides of crystals. It is hoped that some interesting demonstrations may result.
