FIG. 1.—GROTTO.
[Fig. 1] is a perspective view, and [Fig. 2] a longitudinal section, of a grotto formed by crystallizing alum in a box containing jagged points covered with canton flannel, or wrapped about in various directions with coarse thread or twine. The box may be of wood or metal. It should have apertures in the top, ends, and sides. These apertures are stopped with corks while the box is filled with solution. After the crystallization the corks are removed, and the holes in the top, sides, and one end are covered with colored glass, and over the front aperture is secured a convex spectacle lens, having a focus about equal to the length of the box. When the interior of the box is illuminated by a strong light passing through the colored windows, the effect is fine.
The solution used in this case is the same as that given for the cross. After the crystals are formed and the liquid is poured from the box, the interior should be allowed to dry thoroughly before closing the apertures.
FIG. 2.—SECTION OF GROTTO.