GLASS COLORING. Mr. G. Bon temps has shown that all the colors of theprismatic spectrum might be given to glass by the use of the oxide of iron in varying proportions and by the agency of different degrees of heat ; and that all the colors are produced in their disposition in proportion as you increase the temperature. Similar phenomena were observed with the oxide of manga nese. Manganese is employed to give pink or purple tint to glass, and also to, neutralize the slight green given by iron and carbon to glass in its manufacture. If the glass colored by manganese re mains too long in the melting-pot or the annealing-kiln, the purple tint tuns first to a light brownish red, then to a yellow, and afterwards to green. White glass in which a small proportion of manga nese has been used is liable to become light yellow by exposure to luminous power. This oxide is also in certain window-glass disposed to turn pink or purple under the action of the sun's rays.
M. Bontemps has found that similar changes take place in the annealing oven. He has determined, by experiments made by him on polygonal lenses for M. Fres nel, that light is the agent producing the change mentioned • and the author ex presses a doubt whether any change in the oxidization of the metal will explain the photogenic effect. A series of chro
matic changes of a similar character were observed with the oxides of cop per ; the colors being in like manner regulated by the heat to which glass was exposed. it was found that silver, al though with less intensity, exhibited the same phenomena; and gold, although usually employed for the purpose of im parting varieties of red, was found by varying degrees of heating at a high temperature and recasting several times to give a great many tints, varying from blue to pink, red, opaque yellow, and green. Charcoal in excess in a mixture of silico alkaline glass gives a yellow color, which is not so bright as the yel low from silver, and this yellow color may be turned to a dark red by a second fire. Mr. B. is disposed to refer these chromatic changes to some modifications of the composing particles rather than to any chemical changes in the materials employed.