A little boy had a silver mug that he prized very highly, as it was the
gift of his grandfather. The boy was not born with a silver spoon in
his mouth, but, what was much better, he had a mug often filled with
what he needed.
One day he dipped it into a glass jar of what seemed to him water, and
letting go of it saw it go to the bottom. He went to find his father
to fish it out for him. When he came back his heavy solid mug looked
as if it were made of the skeleton leaves of the forest when the green
chlorophyll has decayed away in the winter and left only the gauzy
veins and veinlets through which the leaves were made. Soon even this
fretwork was gone, and there was no sign of it to be seen. The liquid
had eaten or drank the solid metal up, particle by particle. The
liquid was nitric acid.
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.