Malcolm E. Kenney and Selby M. Skinner Case Institute of Technology Cleveland, Ohio
J
Hollow Lantern Slides Illustrating Crystal Structure
The structure of simple crystals can be studied with the aid of models made from steel balls but these models are generally too small t,o be useful for classroom instruction. However, the effects observable in a single layer of such a crystal model can be show11 to large classes by enclosing a layer of bearing balls in a hollow 3'/&. by 4-in. lantern slide and project,ing t,he shadow pattern. The simplest situation, spheres of a single size free to move in two direct,ions. is shown bv means of slide A (Fig. 1). When the slide is horizoiitd the balls assume random positions; when it is tilted the balls simulate a single layer of polycrystalline material (Fig 2). Such features as the method of the closest packing of spheres, the regularity of crystalline areas and the nature of
Figure 1.
Slide A I l e M ond slide B (right).
Figure 2. slide A.
Shodow pattern of
Figure 3. slide B.
Shodow pattern of
crystal boundaries, slip planes aud holes in crystal lattices are apparent after one or two trials. Recrystallization and the looseness of atoms a t crystal faces can be ill~st~rated by tapping or vibrating the slide. Slide B (Fig. 1) shows the more complicated case in which a few smaller spheres (impurities) have been mixed with larger spheres. With this slide it call be shown (Fig. 3) that the smaller balls either accumulate a t the crystal interface or occupy regular lattice sites (and thus create points of potential strain). The slides were made from '/&n. acetate sheet. Slide one contains about nine hundred l/ls-in. steel bearing balls and slide two has about three hundred '/s-in. balls and a few 3/arin. balls.
Volume 36, Number 10, October 1959
/
495
