"Simply common sand. Sand is the ground up particles of quartz, and may be found almost everywhere. The princ.i.p.al 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 gla.s.s product. Bottle gla.s.s is usually made of soda and quartz; window gla.s.s is also made of quartz, soda and lime; plate gla.s.s of quartz, lime, soda and potash; and flint gla.s.s has only the alkalis, potash and oxide of lead."

"Well, for our purposes, wouldn't it be better to make the gla.s.s 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 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 gla.s.s. 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 gla.s.s?"

"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 gla.s.s is translucent."

"If light will pa.s.s through, why is it the eye cannot see through it?"

"A powerful magnifying gla.s.s shows that the surface of unpolished gla.s.s 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 gla.s.s. 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 gla.s.s, in a measure. You remember how we annealed the steel by gradually cooling it down? Gla.s.s, 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

MYSTERIOUS HAPPENINGS ON THE ISLAND

The Professor supervised every part of the operation with the utmost care. "Before the plates are heated you must put a raised margin around each slate square, so the molten material will not run off."

"How high shall we make the margins?"

"About a quarter inch above the surface of the slate."

"As we are now ready to heat up the crucible, how shall the materials be mixed?"

[Ill.u.s.tration: _Fig. 37. Making Sheet Gla.s.s._]

"For this trial, measure out eight quarts of sand and two quarts of the lime, and after depositing it in the crucible, we are ready for the heat."

In a short time, with the a.s.sistance of the blower, which has already been described, the sand began to melt. It was now stirred so that the elements were thoroughly mingled. During the melting period the dross or impurities which came to the top were skimmed off, and when no more of the impurities collected the Professor stated that they might remove the crucible and pour the melted ma.s.s into small pockets, which they had previously formed with clay.

"Why not pour this on the slate forms we made?"

"Because we must know that we have a good sample of gla.s.s, and for the further reason that the product we are now to make should have some gla.s.s in it that has already been fluxed before, and we now have such a manufactured material."

The material which had been cast in the pockets was broken up with a hammer, as soon as it had cooled, and its appearance noted.

"I see we shall have to use less lime."

"Why?"

"You will note that it is too white or milky-like. This shows too much lime and consequently it is very brittle."

"Suppose, on the other hand, there was too little lime, what would be the result?"

"We should then have a gla.s.s which would not hang together at all. In the one case, as with the present sample, with too much lime in it, we have made a product which is closely allied to the alkaline base; and if we had, on the other hand, too little lime, we should have something which is nearly like quartz, hence not suitable for our purpose."

"It seems, then, we have to do in this case just as Red Angel did, make a test sample?"

This allusion to their pet caused a smile all around.

Since the trip made by our colonists two months before they made no effort to gain any additional knowledge of their island. What they really knew of the country, aside from two of the trips made in the interior, under very unfavorable circ.u.mstances, was of no value as a means of locating the natives.

That the island was inhabited there could no longer be any reason to doubt. The fire plot on the banks of the Cataract River, the lights near the woods beyond the West River, the finding of the arrows, and the mysterious use of the boat which had so strangely disappeared from the falls in South River, to say nothing of the removal of the flag and flagstaff, were evidences which could not be disregarded.

The further investigations which they must make for their own safety was one of the impelling steps which determined them to build another boat.

The discovery of the wrecked portion of the life-boat and the decision to utilize the recovered portion for the new craft had facilitated their preparations somewhat, but there was still a great deal of work to do.