ed from the air. If you had noticed
it when you first came in, you would see that the warmth of the room
has caused it to rise in the tube. This is shown by the marks on the
plate to which it is fastened. Now, if you hold it close to the stove,
the quicksilver will rise still higher. Let it stand outside the window
a moment, and it will sink."

By this time the boys were much interested.

"But what makes it do so, grandpa?" they asked.

"Quicksilver is very sensitive to heat and cold. If the weather is
warm, or if the room it is in is warm, it expands--swells out--and so
rises in the glass tube, as you have seen. The least coolness in the
air will cause it to contract, or draw itself into a smaller space;
then, of course, it sinks in the tube.

"The barometer is another instrument in which quicksilver is used. It
is intended to measure the weight of the air, therefore the quicksilver
in it must be exposed to the pressure of the air. Common barometers
have it inclosed in a small leather bag at the back of the instrument.
This we do not see, but only the tube which is connected with it. When
the weather is pleasant, the air, contrary to the general idea, being
heavier, presses against this little bag and the quicksilver rises in
the tube. When the atmosphere is damp, the pressure being less, the
metal sinks."

"Grandpa," said Harry, "when you think of it, isn't quicksilver a funny
word?"

"Yes; it was so named by people who lived many hundreds of years ago.
They called it _living silver_ also. It is the only metal found in a
liquid state; and so many strange changes did it pass through under
their experiments, that it seemed to them really a living thing. If
they tried to pick it up, it would slip out of their fingers. When
thoroughly shaken, it became a fine powder. They boasted that it had
the faculty of swallowing any other metal, while powerful heat caused
it to disappear entirely. It is now known among metals as mercury. Can
you tell me, Fred, some of the metals?"

"Oh yes, sir! There are gold, silver, iron, lead and copper."

"That is right. But, you know, all these are hard; some of them can be
chipped with a knife, but they cannot be dipped up in pails, unless
they have first been melted. Yet mercury can be frozen so hard that it
may be hammered out like lead, and sometimes it takes the form of
square crystals. Yet it can be made to boil, and then sends off a
colorless vapor."

"Grandpa." said Fred, wh