The poor little boy had often seen salt, and especially sugar, absorbed in water, but never his precious solid silver mug, and the bright tears rolled down his cheeks freely.

But his father thought of two things:  First, that the blue tint told him that the jeweler had sold for silver to the grandfather a mug that was part copper; and secondly, that he would put some common salt into the nitric acid—­which it liked so much better than silver that it dropped the silver, just as a boy might drop bread when he sought to fill his hands with cake.

So the father recovered the invisible silver and made it into a precious mug again.

NATURAL AFFECTION OP METAL AND GAS

A man was waked up one night in a strange house by a noise he could not understand.  He wanted a light, and wanted it very much, but he had no matches that would take fire by the heat of friction.  He knew of many other ways of starting a fire.  If water gets to the cargo of lime in a vessel it sets the ship on fire.  It is of no use to try to put it out by water, for it only makes more heat.  He knew that dried alum and sugar suitably mixed would burst into flame if exposed to the air; that nitric acid and oil of turpentine would take fire if mixed; that flint struck by steel would start fire enough to explode a powder magazine; and that Elijah called down from heaven a kind of fire that burned twelve “barrels” of water as easily as ordinary water puts out ordinary fire.  But he had none of these ways of lighting his candle at hand—­not even the last.

So he took a bit of potassium metal, bright as silver, out of a bottle of naphtha, put it in the candle wick, touched it with a bit of dripping ice, and so lighted his candle.

The potassium was so avaricious of oxygen that it decomposed the water to get it.  Indeed, it was a case of mutual affection.  The oxygen preferred the company of potassium to that of the hydrogen in the water, and went to it even at the risk of being burned.

I was so interested in seeing a bit of silver-like metal and water take fire as they touched that I forgot all about the occasion of the noise.

HINT HELP

Benjamin C. B. Tilghman, of Philadelphia, once went into the lighthouse at Cape May, and, observing that the window glass was translucent rather than transparent, asked the keeper why he put ground glass in the windows.  “We do not,” said the keeper.  “We put in the clear glass, and the wind blows the sand against it and roughens the outer surface like ground glass.”  The answer was to him like the falling apple to Newton.  He put on his thinking cap and went out.  It was better than the cap of Fortunatus to him.  He thought, “If nature does this, why cannot I make a fiercer blast, let sand trickle into it, and so hurl a million little hammers at the glass, and grind it more swiftly than we do on stones with a stream of wet sand added?”