GLASS — JACKSOl^. 251 
and the concentration of the opal-forming material be not too great, 
the glass will go quite clear in the flame and the blown bulb will 
remain quite clear on cooling. If the bulb be then again heated gradu- 
ally in a flame, the whole process from a mere trace of opalescence 
to a very dense opal can be watched. If during the various stages 
of opacity the light transmitted through the glass be observed, it 
will be seen to change from light orange yellow to darker and darker 
orange and orange red, until no more evidence of color is seen, but 
only a general translucence. If thin sections (or ground-up pieces 
mounted in Canada balsam) of the opal glass in the vanous stages 
be examined under the microscope no separate particles in the earlier 
stages will be seen, even with lenses of large angular aperture, though 
their existence can be inferred from the opalescence which is to be 
w^ell seen under the microscope with suitable black ground illumina- 
tion. In the later stages of denser opal, separate particles are visible, 
and are seen to be progressively larger as the density of the opal is 
greater and greater. 
When an opal glass is required for articles, the making of which 
involves working the glass in a muffle or in the flame, it is important 
that the separated globules shall not tend to aggregate or to pass 
into the crystalline state, otherwise the glass is found to have a 
rough surface. To guard against this, too great concentration of the 
opal-forming material must be avoided, and some workers prefer a 
glass which does not reach its full opal until it has been in the an- 
nealing oven. As a general experience with a wide range of all kinds 
of opals, it would appear that fluoride opals are more kindly in 
working than phosphate opals. This is more especially true for the 
denser kinds of opal. For merely opalescent glasses, phosphates 
give quite good results, but with greater concentration of the opal- 
forming substance there is a tendency toward crystallization, which 
is more marked as a rule in the phosphate than in the fluoride opals. 
A dense opal suitable for working in a flame should " strike " opal 
even in thin pieces on removal from the flame, and should stand long- 
continued heating without losing its fine polished surface. Wlien 
such a glass while opal is drawn out into a rod and longitudinal sec- 
tions of the rod are examined under the microscope, the globules are 
to be seen egg-shaped or even elongated into minute rods. If an end 
of the opal rod be heated again to softening point and sections of that 
end be examined, the opal-forming material is seen to have gone 
back to spheres, showing that even when separated out the opal ma- 
terial has about the same softening point as the rest of the glass. 
It is easily to be understood that if it has not, and the globules are of 
appreciable size, a glass containing them can not be worked without 
roughening. One more point may be mentioned before concluding 
