Second case: if a crack takes place in a solid mass of glass, chrystal, or ice.
Third case: if lamellæ of transparent stones become separated.
Second condition.—If a surface of glass or a polished stone is breathed upon.
Third condition.—The combination of the two last; first, breathing on the glass, then placing another plate of glass upon it, thus exciting the colours by pressure; then removing the upper glass, upon which the colours begin to fade and vanish with the breath.
Fourth condition.—Bubbles of various liquids, soap, chocolate, beer, wine, fine glass bubbles.
Fifth condition.—Very fine pellicles and lamellæ, produced by the decomposition of minerals and metals. The pellicles of lime, the surface of stagnant water, especially if impregnated with iron, and again pellicles of oil on water, especially of varnish on aqua fortis.
Sixth condition.—If metals are heated; the operation of imparting tints to steel and other metals.
Seventh condition.—If the surface of glass is beginning to decompose.