up preparatory to
grinding it up into small particles, so that it would intimately mix
with the sand.

Within an hour the Professor returned with several samples of sand,
either of which, he thought, would make a good article. The yaks were
hitched up, and George went with him to get a good supply.

"How much do you think we ought to make up at first?"

"Several gallons of the sand will do for the experiment."

"What kind of article should be made with the first trial?"

"We might make some window glass. It is true it will not be transparent,
but it will be translucent, and so will give us light, as well as though
it should be transparent."

"What is translucent glass?"

"Where the surface of a cast plate is polished the material is such that
you can see through it, but if it is left rough it is impossible to see
through it, although it will permit light to go through. The term
applied to such glass is translucent."

"If light will pass through, why is it the eye cannot see through it?"

"A powerful magnifying glass shows that the surface of unpolished glass
is formed by a layer of crystals, or of sand, with the faces projecting
out in all directions and at all angles. The result is, that a beam of
light from the eye strikes one or more of these faces and is diverted
from a straight line through the glass. As all the rays are thus changed
from a direct course, confusion results, and the eye distinguishes
nothing."

Several bushels of the sand were brought to the laboratory, and the
Professor then directed the preparation of a half dozen slate slabs,
each slab being nearly two feet square. He explained that in practice
iron plates were used, but as they had nothing of that kind available,
slate would answer admirably.

"The slate slabs must be heated, and when the fused material is poured
on the slabs, the heat must be kept up for a short time and gradually
cooled down."

"What is the object in doing that?"

"If cooled too suddenly the plates, will crack, but by heating the
slates and then cooling them down gradually, we anneal the glass, in a
measure. You remember how we annealed the steel by gradually cooling it
down? Glass, however, cannot be annealed so that it will not fracture,
although attempts have been made for years to find a means for doing it.
The man who can discover a process that will enable it to bend without
breaking, can command any price for the discovery."

CHAPTER XV

MYSTER