GLASS — JACKSON. 257 
of the manganese being in the higher state of oxidation becomes very 
markedl}^ colored by an amount of exposure to rays which has no 
visible effect on the other glass in which the lower oxide of man- 
ganese only is present, seems at least to point to some special behavior 
of manganese dioxide.] 
The influence of different alkalies and the remarks already made 
on the effect of varying the relative proportions of bases and acids on 
the copper glasses bring me to a short consideration of the behavior 
of coloring agents which would generally be placed in group {h) 
as existing in glasses in a state more nearly resembling that of true 
solution than that which may be considered to obtain in the more 
colloidal solutions of gold, copper, selenium, and other substances 
such as silver, sulphur, carbon, etc., with which there has been no 
time to deal. I must confine my remarks to but few of group (6), 
and perhaps nickel and cobalt will be the most suitable to illustrate 
the effect of different alkalies and also that of varying proportions 
of one and the same alkali. 
If three similar and moderately soft glasses be made containing, 
respectively, potash, soda, and lithia as the alkalies present in chemi- 
cally equivalent proportions, and if the same amount of nickel oxide 
be present in each glass, marked difference in the colors is observed. 
The potash glass is a fine deep violet, the soda glass is almost brown, 
with only a hint of purple in the brown, and the lithia glass is a yel- 
lowish brown, with less strength of color altogether in it than there 
is in the soda glass. Similar differences can be seen in beads made 
from nickel oxide dissolved in the bi-borates of the three alkalies. 
Of these alkalies potash is the strongest and lithia the weakest base. 
The glasses mentioned would not be described as acid glasses, but as 
glasses containing a fair proportion of basic to acid ingredients. 
If highly acid glasses be made with the three alkalies and the same 
proportion of nickel oxide, the lithia glass is only slightly colored 
a brownish yellow, the soda glass is a lighter brown, with no trace 
of purple in it, and the potash glass is rather darker in shade than 
the soda glass, but a definite brown. Again, very similar results 
can be obtained in beads of the borates of the alkalies by varying 
the proportions of acid and alkali, and using the same amount of 
nickel oxide in each set of experiments. With potash as the alkali 
the proportion of boric anhydride and alkali and the concentration of 
nickel oxide can readily be adjusted to show a bead colored brown 
when cold, but becoming a definite violet when heated just below 
a dull red heat. A like change of color has been observed in experi- 
mental glasses made for studying the colors obtainable from nickel. 
With cobalt oxide as the coloring agent, the difference between the 
blue colors of potash and soda soft glasses is not very noticeable; 
but a similar lithia glass is less colored, and there is an appreciable 
