GLASS — JACKSON. 245 
the change to the crystalline form. So it is also with a great number 
of glassy bodies, and the bulk which will retain the vitreous form can 
be largely increased since by fusion and crystallization several times 
they solidify at progressively lower temperatures, until the time 
comes when they get quite cold in the vitreous state. For instance, 
the amount of vitreous zinc silicate obtained has been raised from 
20 grams at one heating up to a kilogram by 5 fusions. The calcium 
borate 2CaO.B203 has, after several fusions, been cooled to 500° 
below its ordinary solidifying point. When it does crystallize and 
the temperature rises the recalescence is remarkably bright. Long 
continued heating a few degrees above their solidifying points 
similarly retards the crystallization of many vitreous bodies, but 
it is not so effective as the alternate fusion and solidification. 
One reason at least is not far to seek, if the two processes be 
tried so that gases evolved can be pumped off and their amounts 
measured. The process with alternations yields much more gas in 
any given time. With all the glasses, simple silicates and borates, 
which have been studied so far, the chief gas evolved has been found 
to be water vapor, and with the progressive removal of it the 
vitreous state has been found to persist more and more. Direct intro- 
duction of w^ater subsequently has been found to promote ready 
crystallization. 
In connection with the comparison of long-continued heating of a 
melt with alternate fusion and solidification, one extreme instance is 
worth noting. For a special optical glass, rich in phosphoric anhy- 
dride, an experiment was tried with ammonium phosphate to find if 
this substance could be used in the batch mixture for the glass. A 
nice, clear fluid melt was obtained, which was kept fluid for several 
hours after all traces of gas bubbles had gone. The melt was well 
stirred and cooled till it was quite viscous, when it was left to get 
cold slowly. The next morning the furnace top was found forced off, 
and resting on a spongy mass of about thirty times the volume of the 
original, glass melt. The changes occurring when solidification was 
approaching had evidently been accompanied by the evolution of a 
large volume of gas, no doubt most of it ammonia, since this sub- 
stance was smelt on grinding the spongy mass up. The ground ma- 
terial was then fused and gave a stable glass. 
Eeverting to the question. Is glass truly vitreous, or is there evi- 
dence of any crystalline structure in it? and bearing in mind that 
glasses are known which exhibit all states of preparedness to yield 
crystals at some temperature or other, and that the tendency to the 
segregation of some ingredient of a glass is enhanced by the presence 
of small amounts of foreign substances and notably of water, one 
would rather expect to find a good many glasses in which some 
12573°— 21 17 
