250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
referred to earlier. Various silicates and borates of calcium, barium, 
and magnesium show the same kind of phenomenon more readily 
and by quickly cooling when a stage of dense opal appearance is 
reached, sections (or, what is just as good for the purpose, the finely 
ground glassy material mounted in Canada balsam) can be examined 
under the microscope and the opal effect shown to be due to the scat- 
tering of light by numerous small transparent globules. Any glass 
from which, on cooling, part of it segregates out in very small particles 
evenly diffused through the mass, may be called an opal glass whether 
the fine particles are crystalline or amorphous, but the usual opals 
owe their milkiness to globules in which no evidence at least of the 
crystalline state can be found. Under the microscope the globules, 
if sufficiently visible as distinct particles, appear vitreous and trans- 
parent, just as in milk the fat globules are seen to be themselves 
colorless and transparent. Among the many substances which can 
be used to produce opal glasses, the most common are fluorides such 
as calcium fluoride, cryolite (the double fluoride of sodium and alumi- 
nium), and calcium or sodium phosphate, less commonly the arsenates 
of these metals. These substances can be included in the batch mix- 
tures of ordinary soda or potash lime glasses, or lead glasses or zinc 
glasses. In the making, opal glasses are usually clear at a high tem- 
perature and " strike " opal on cooling. To what extent the glass 
has to be cooled before becoming opal depends on the concentration 
of the particular opal-producing compound which is held in solution 
in the very hot glass. Opal glasses produced with phosphates 
" strike," generally speaking, at higher temperatures than those made 
with fluorides, the compounds formed in the glass by the latter being 
more soluble than those due to phosphates, at least in the case of most 
opal glasses made on a conimercial scale. Whatever opal-forming 
material is potential in the molten glass, if its concentration be great, 
the glass " strikes " opal quickly and with relatively little cooling and 
becomes a denser opal as cooling proceeds, until the stage is reached 
when no more material segregates. Just, however, as in the crystal- 
lization of an ingredient in a glass, cooling may occur so quickly 
that a state of viscosity is reached in which crystallization can not 
proceed, so with opal glasses the concentration of the opal-producing 
material may be such that only a little of it comes out of solution 
before the viscosity of the glass gets too great to allow of further 
separation. With still less concentration, moderate-sized pieces of 
the glass may even solidify in a perfectly clear condition, but again, 
just as reheating will often cause a glass which has cooled vitreous 
to become more or less crystalline, so reheating the intended opal 
glass will cause it to " strike." This can be illustrated by blowing 
a bulb from a tube of an opal glass. If the bulb be not too thick. 
