GLASS — JACKSON. 253 
to obtain other than grays or neutral tints. With each of the 
glasses in which gold, or copper, or selenium is present, quick chilling 
of the molten glass will, as in the case of opal glasses, yield a clear 
and colorless glass, and the greater the concentration of the coloring 
agents, the more sudden must the chilling be to secure this result. 
On reheating these colorless glasses they, like the opals, " strike " 
and yield the colors which could have been obtained by slower cooling 
of the molten glasses. It may be of interest to deal with a gold 
glass a little more in detail. It is not easy to get a strongly colored 
glass with gold added, in the form of gold chloride, to the batch 
mixture of an ordinary soda lime glass. Among the substances which 
enable one to prevent gold separating from the molten glass in the 
ordinary metallic state the commonest used are the oxides of lead, 
tin, and antimony. Bismuth oxide acts similarly, and so also does 
uranyl oxide. There are physical and chemical problems of much 
interest involved in this behavior of these oxides; but it would lead 
us too far into technical details to attempt their discussion here. 
A gold glass containing oxide of tin may be chosen, because it can 
be made so that its behavior can be studied either in the furnace 
or in a blowpipe flame. With a suitable concentration of the gold, 
and very slow cooling of the melt, all the ranges from red by trans- 
mitted light to a pale blue can be observed, and if rods are drawn 
out from the pot at intervals, and examined in a beam of light, it 
will be seen that, starting from a fine deep red by transmitted light 
through the various stages of reddish and blueish purple and blue 
to the pale blue, there is progressively more and more marked scat- 
tering of the light, and the rods look more and more of an opaque 
brown color by reflected light, until in the later stages the appearance 
of precipitated gold is so marked as to leave no doubt that what has 
occurred has been a progressive aggregation of the gold into larger 
and larger particles. Microscopical examination of the glass at the 
different stages gives clear confirmation of this and of the great 
similarity in the manner of separation of the gold to that of the 
materials which give opal glasses. Kemarks made under opals and 
crystallization of glasses about the influence of changing viscosities 
apply also, in a general way, to gold glass. There is one point in this 
connection which is worth referring to. If the suddenly chilled and 
colorless glass be returned, in small pieces at a time, to a pot in a 
furnace at a high temperature, about 1400° C, the glass can be melted 
f>.nd the gold still retained in it without appreciable loss by separa- 
tion into the ordinary metallic state ; but if it be slowly heated up it 
passes through the stages of color previously described and, after 
complete fusion at a high temperature, practically all the gold will 
be found in a button at the bottom of the pot. Now, except in the 
