ng it, the making of glass
articles was known at a very early date, certainly fifteen hundred years
before the beginning of the Christian era. In the first stages only
opaque glass was produced, and it was not until eight hundred years
later that the first transparent product was manufactured. Under Pharaoh
it was one of the products extensively made and exported to Phonecia and
other Mediterranean ports. Five hundred years before Christ,
Aristophanes mentions glass or crystal vessels, but as its value at that
time was next to gold it could not have been a common article."

"What is glass made of?"

"Simply common sand. Sand is the ground up particles of quartz, and may
be found almost everywhere. The principal thing is to get the pure
quartz. In connection an alkali of some kind must be used."

"What is an alkali?"

"A substance which is the exact opposite of an acid. Potash, soda and
hartshorn (or ammonia) are the best known. They have most remarkable
chemical activities, and an alkali united with an acid entirely
neutralizes or destroys the activity of both. The compound produced by
the union of an acid and an alkali is termed a salt."

"What is the effect of using an alkali with the quartz sand?"

"Quartz possesses all the qualities of an acid, so that when the alkali
is fused with the quartz a neutral substance, unlike either, is formed."

"What kind of alkali is best to use?"

"That depends on what it is to be used for. Quartz and lime make a fine
window glass product. Bottle glass is usually made of soda and quartz;
window glass is also made of quartz, soda and lime; plate glass of
quartz, lime, soda and potash; and flint glass has only the alkalis,
potash and oxide of lead."

"Well, for our purposes, wouldn't it be better to make the glass out of
quartz and lime if windows can be made out of it?"

"By all means, for several reasons: We have the lime on hand, and also
because it makes a very hard article."

"What can we melt it up in?"

"The clay retort or crucible will just be the thing for the purpose, and
the first thing in the morning I will make a tour to a point close at
hand, where I think we shall be able to get a good quality."

The boys were astir in the morning earlier than usual. They had a new
impulse--something to learn and to do. Harry busied himself with putting
the crucible in order, and in getting the fuel. George, after his usual
morning's work, brought in the lime, and broke it