only indications of the different degrees of heat.—Note R.
479.
When lead is calcined, the surface is first greyish. This greyish powder, with greater heat, becomes yellow, and then orange. Silver, too, exhibits colours when heated; the fracture of silver in the process of refining belongs to the same class of examples. When metallic glasses melt, colours in like manner appear on the surface.
480.
Seventh condition.—When the surface of glass becomes decomposed. The accidental opacity (blindwerden) of glass has been already noticed: the term (blindwerden) is employed to denote that the surface of the glass is so affected as to appear dim to us.
481.
White glass becomes "blind" soonest; cast, and afterwards polished glass is also liable to be so affected; the bluish less, the green