THE EXHIBIT AS A SUPPLEMENTARY METHOD-CRYSTALLOGRAPHY

BY HAROLD J. ABRAHAMSSimon Gratz High School, Philadelphia, Pennsylvania

Of necessity many important fields of science must be omittedfrom the high school curriculum in favor of those which aretoday considered of primary importance either in themselvesor as disciplines. So vast has become the body of fact and theory

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FIG. 1

in those sciences which are included in the high school coursethat many closely allied fields must be omitted even though theymay have intimate contact with the major fields. Not only arethey omitted from the curriculum, but the individual sciencesfind it impossible to include adequate treatment of such corre-lated fields even though they may be an essential part of thebackground of more than one of the sciences taught.A case in point is crystallography, an understanding of which

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is of value not only in chemistry, but in physics, biology, andother sciences. A brief discussion of the fundamental principlesof crystallography seems entirely omitted nowadays from thevarious courses in high school science. Potentially, both fasci-nating and soundly instructive it is slighted by teachers and text-books alike. Yet frequent reference is made in the classroom tothe characteristic shapes of the polymorphus modifications ofsulfur, iodine, carbon, ice and many other elements and com-pounds. The crystallographic terms applied to substancesstudied in high school chemistry would be far more meaningfulagainst a background provided by at least a minimum knowl-edge of the simple concepts of this interesting field.

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A series of exhibits might prove helpful in furnishing a back-ground in certain of these overlapping fields and in partiallymeeting the restrictions imposed by limited curricula and therapidly growing body of scientific knowledge. Rotation of suchexhibits has the double advantage of including more materialand of maintaining greater interest. It would appear that anexhibit which is extended for more than about a month becomesso much a part of the surroundings that it loses much of its value.A recent exhibit of crystal forms arranged for high school

students has served to make many of them conscious of theworld of beauty to be found in naturally occurring geometric

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forms. Such interest and enthusiasm once aroused may becarried over into the classroom, making students richer invocabulary, more careful in visual discrimination and more ableto recall the crystal form of certain chemicals. It may indeed bethat some, at least, of this enrichment will have lasting effectsin the direction of improved habits of careful observation, wordcontrol and esthetic appreciation.The exhibit here discussed, consisting of three parts, was ar-

ranged in the following manner. Upon the middle shelf of a glasscase were placed six celluloid models, representing the six crys-tal systems. Upon the upper shelf a group of chemicals, chosen

PICTURE 2

for the size and perfection of their crystals, were so arrangedas to correspond in crystal systems to the celluloid models im-mediately below them (upon the middle shelf). A number ofminerals, arranged in the same manner on the lower shelf com-pleted the exhibit.Thus chemicals, model and minerals belonging to the same

crystal system, being in the same vertical column, were readilyassociated by the observer. Another feature of the exhibit was asmall magnifier focused upon some sodium chloride and soplaced within the case as to be observable from the outside.

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Much interest and surprise were expressed by ninth and twelfth-year students alike, the unexpected cubical shape of commonsalt being striking even though other parts of the exhibit mightnot be comprehensible.Above the case was placed a plaque labeling the exhibit as

"Nature’s Vagaries" and bearing on typewritten sheets thefollowing information, aimed to be both striking and instruc-tive :

One of the most beautiful, though baffling, phenomena in Nature, is theformation of crystals. Men of Science are unable to explain satisfactorilywhy substances choose one form, rather than another, in which to crystal-lize. Yet, incomplete as our knowledge of crystallography (the science ofcrystals) is, it has proved both interesting and useful.

Chemists, physicists, metallurgists, biologists, and others who practicethe technical arts and sciences make wide use of a knowledge of crystals.Crystallography even enters into legal trials. There are records, in bothfact and fancy, which show that people, incorrectly accused of crimes,have been saved from punishment by the testimony of crystallographers,given in courts of law.A well known story tells of a man who was about to be convicted for the

poisoning of his uncle, in order to inherit the latter’s fortune. It was shownin court that the nephew had recently purchased and hidden away some ofthe same poison of which the uncle died. Although this evidence seemedvery conclusive, the defendant was acquitted of the murder charge, en-tirely because a chemist succeeded in showing that the particles of poisonfound in the dead uncle^s body belonged to a different "crystal group"from those in the bottle. (The same substance sometimes crystallizes indifferent forms.)

All crystals may be divided into six main groups, depending upon thelength of each of their three axes* and the angles at which these intersecteach other. The names of these groups and the facts about their axes fol-low:

1. Isometric, or regular system: Three equal axes, at right angles toeach other. Example�cube.

2. Tetragonal system: Two equal axes, and one longer or shorter, all atright angles to each other.

3. Orthorhombic or Rhombic system: Three unequal axes, all at rightangles to each other.

4. Monoclinic system: Two axes at right angles and a third perpendicu-lar to one and inclined to the other.

5. Triclinic system: Three axes, all inclined to each other.6. Hexagonal system: Four axes, three equal axes in the same plane in-

tersecting at angles of 60°, and a fourth at right angles to all these.

* Axes are imaginary lines, so drawn, that the crystal surfaces are arranged symmetrically aboutthese lines. Opposite ("similar") faces will therefore be parallel to each other, intersecting the respectiveaxes at correspondingly equal distances. The inner and outer models of each pair are so constructed asto possess a common set of axes in every respect, but a host of other geometrical forms which wouldmeet the definitions of a given crystallographic system can be imagined both within and without thesecelluloid figures. A complete discussion of the measurement of interfacial angles by the reflectiongoniometer or of how to ascertain the axial angles and ratios would be beyond the scope or purpose ofthis exhibit, and is left for student "research."

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Each of these groups is represented by a celluloid model in this showcase.Note that there are really six pairs of models, each model consisting of oneinside another. The purpose of this is to show you that the shape or sizeof a crystal is relatively unimportant in classifying it. The important thingis the relationship of the axes to each other, with respect to length andangles. Thus you see that two crystals of as widely different appearancesas the one inside the other may belong to the same group, because theiraxes are identical. In these models the axes have been painted red, tomake them more visible. They are, however, only imaginary lines.Many surprises await you, if you will focus a good magnifying glass upon

familiar substances. Note, for example, the almost perfect cubes of lowlytable salt (sodium chloride) shown under the magnifier in the exhibit.Examine other substances in this way at your first opportunity.

PREPARATION OF THE CELLULOID MODELSThe first step in the preparation of these models consisted in

making the six sets of axes out of steel rods using stock 3/16inch in diameter. The rods of each set were fastened togetherby being forced into suitably drilled holes in a small block ofsteel. The axes were then painted red, to increase visibility.

Celluloid sheeting, .01 inch in thickness, was then placed overthese axes in such manner as to construct octahedrons (eight-sided solid figures, or two pyramids placed base to base. Forthe hexagonal system the model was, of course, a dodecahedron).The axes were allowed to protrude about } of an inch at eachend, to permit the next figures to be built up around them.White tape was pasted along each edge of the figures, to accentthe shape and increase visibility.The last step consisted in building hexahedrons around the

octahedrons (for the hexagonal system, an octahedron aroundthe dodecahedron.) For this purpose 0.03 inch celluloid wasselected, because of its greater rigidity and strength. After per-forating with a cork-borer at the correct places, the sides weresupported on the axes by passing the latter thru the perforationsof the celluloid and then fastening the sides together by meansof cellulose tape. The transparency of this kind of tape was idealfor the purpose, as it did not obscure any part of the insidemodel. Furthermore, opaque tape would add to the alreadylarge number of lines and thus cause possible confusion.Each model (really a pair of models, one inside the other)

was then placed upon a black wooden base, with bevelled frontedge for suitable label;

CHEMICALSThe chemicals used in the exhibit were prepared by the usual

method of crystallizing upon a thread hung into a hot saturated

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solution of the substance in question. Cooling was made to takeplace very slowly by immersing beakers in hot water. By suchmeans large crystals of copper sulphate, magnesium sulphateand other salts were obtained. Sodium chloride, perfect enoughfor exhibition purposes can be taken directly from a package oftable salt. Alums of various types may be readily prepared bycrystallizing from mixtures of hot saturated solutions of theunivalent and trivalent sulphates. The only substance in thispart of the exhibit which cannot be made in the laboratory iscarborundum, but this was used only because it was available,attractive and exemplified a substance made by an unusualmethod. In choosing chemicals for the exhibit account shouldbe taken of efflorescence and deliquescence on the part of certainsalts. In this exhibit efflorescent salts were satisfactorily sus-pended in small, wide-mouth bottles having a layer of water atthe bottom and a plug of cotton or a stopper in the mouth. Theiodine was covered with a small, flat, glass jar to prevent sub-limation.

CHEMICALS AND MINERALS IN EXHIBIT

System Chemicals Minerals1. Isometric or Sodium chloride (rock salt; Fluorite

Regular table salt under magnifier) GalenaPotassium bromide GarnetPotassium iodide PyritePotassium aluminum sulphateAmmonium aluminum sulphateAmmonium ferric sulphate

2. Tetragonal

3. Orthorhombicor Rhombic

4. Monoclinic

5. Triclinic

6. Hexagonal

Magnesium sulphate

IodineSulphur (rhombic)

SugarBoraxFerrous sulphateFerrous ammonium sulphateFerrous ammonium oxalate

Copper sulphate

Sodium nitrateCarborundum

Zircon

Sulphur

GypsumOrthoclase

Rhodonite

QuartzCalcitePictures of snowHematite

In addition to the advantages indicated above, of the use ofexhibits for the presentation of such subjects as the one hereindiscussed, several other advantages will readily come to mind.

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One of these is a proper recognition of the wide variation in theabilities of the student body. Such a method gives opportunityfor interested students to enhance their knowledge of a subjectnot generally treated in class because of the limitations imposedby the presence of a large proportion of less capable students.The lack of ability and interest which would be evidenced bythe inferior students would react to the disadvantage of theentire class. For many, perhaps the majority of the students, aclassroom treatment of crystallography would be difficult, bur-densome, and dull. Illustrated in the manner indicated the factsand concepts are comprehensible, more readily assimilated andnot loaded upon a curriculum already burdensome to manystudents. For certain types of students, a very few to be sure,it can become the starting point for special assignment and"research." Lastly, it may be that "lack of compulsion" maywork its subtle psychology upon those who would not otherwisebe interested in crystals.

EXPLOSION-PROOF PREPARATIONS APPROVEDFOR REMOVING ADHESIVE TAPE

Two preparations for use in removing adhesive tape have been approvedby the American Medical Association^ council on pharmacy and chemis-try as an aftermath of the recent explosion in which two Purdue Universityfootball players lost their lives.With kerosene and gasoline, commonly used to remove adhesive tape,

there is always danger that nearby flame or heat may cause an explosionsuch as killed the two Purdue players, the medical association warnscoaches, trainers and the public.One of the approved substances consists of 98 per cent dichlormethane

and has no fire hazard, since it is non-explosive and non-inflammable.The other is a mixture of 60 per cent carbon tetrachloride and 40 per

cent naphtha with a small amount of oil of sassafras. Such a mixture mayburn but will not explode under ordinary conditions.With any preparation of this type, the American Medical Association

warns in the forthcoming issue of its Journal, there may be some dangerassociated with the removal of large quantities of tape in small rooms with-out proper ventilation. Carbon tetrachloride fumes are poisonous in highconcentrations. This, however, is a minor danger and far less hazardousthan the use of gasoline anywhere near a source of flame or heat.The old method of pulling off the adhesive is painful, pulls out hair and

may even remove the skin, giving opportunity for secondary infection."Coaches and trainers of football teams will do well to equip training

quarters with plenty of modern improved solvents so as to eliminate thedanger of catastrophe such as that which has thrown a somber atmosphereabout the current football season,^ the medical journal recommends.

�Science Service