the wombatcave

Ruby glass owes its red color to the presence of colloid gold. I show you three specimens which are "solid solutions" of gold in three very different and characteristic degrees of dispersion (demonstration).[1] The first is an almost clear and but slightly yellow mass of glass. This is obtained immediately after dissolving the solid gold salt in the glass. There is obtained in this way a molecularly dispersed solution of the gold in the glass, and one which, in consequence, is ultramicroscopically empty. The second preparation is the ordinary ruby glass in which the gold is contained in a colloid state. The third specimen is deep blue by transmitted light and orange brown by reflected light. The specimen is also distinctly turbid. It springs from a failure in glass manufacture in that, presumably through a too long heating of the glass, a coagulation of the red gold particles to the more coarsely dispersed blue particles has taken place — just such a change as I showed you in an aqueous dispersion medium when I coagulated the red gold (produced through reduction of gold chlorid by tannin) to blue gold through the addition of acid. These same facts as illustrated in the case of glass prove of what little importance is the kind of dispersion medium and how much depends upon the degree of dispersion in determining the variations in color in this substance.

[1]: Different specimens of gold ruby glass were kindly placed at my disposal by POPPER AND SONS of New York