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.
432.
First condition, first case. If two convex glasses, or a convex and plane glass, or, best of all, a convex and concave glass come in contact, concentric coloured circles appear. The phenomenon exhibits itself immediately on the slightest pressure, and may then be gradually carried through various successive states. We will describe the complete appearance at once, as we shall then be better enabled to follow the different states through which it passes.
433.
The centre is colourless; where the glasses are, so to speak, united in one by the strongest pressure, a dark grey point