GLASS—-JACKSON. 259 
One is tempted to compare this effect of alkali on the copper oxide 
and boric anhydride mixture with that of water on copper sulphate, 
which, in the anhydrous state, represented by the formula CuSO,, 
is white. The addition of water sufficient to give the composition 
CuSO,.H,O leaves the substance still white; but with more water 
the well-known blue copper sulphate CuSO,5H,O is produced. 
Without going so far as to call this an example of hydrolysis by 
water, it may not be too much to speak of the development of color 
as indicating in CuSO,.5H,0 a greater tendency to the formation of 
blue copper hydroxide than is possible with the smaller mass of 
water in CuSO,.H,0. 
The notion that there is an analogy here with the progressive de- 
velopment of color in glasses and borates with increase of alkali may 
be suggested, but with reservations. Still, the changes from brown 
to violet in the case of nickel, from pink to blue in the case of cobalt, 
and the progressive development of the color of copper, all brought 
about by increasing the proportion of alkali, do seem to point, if 
not to a definite separation out of the oxides of these metals, to some- 
thing like it in the sense that with very little of the alkali present the 
oxides of the metals may be playing a basic part, but are turned out 
by more of the stronger base (the alkali), and may be either freed 
or caused to play the part of acids to the alkali. The study of a wide 
range of coloring agents in glasses has furnished some facts which, 
from a chemical point of view, lend plausibility to the notion and 
others which seem to need a great deal of interpretation to support it. 
As an idea it has been useful in suggesting methods of producing 
as well as preventing color in glasses. More facts, however, must be 
accumulated for a fuller and more correct shaping, in its physical 
and chemical aspects, of one of the many interesting problems con- 
nected with glass. 
