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 prowlers. They sleep and rest in the daytime, and hunt their prey at night. The house cat hunts mice at night. So a cat needs eyes that are veiled from strong light by day, and that can expand to admit all the few rays of light that are abroad at night.

When mama sends you for something in a hurry, she says: "Run, quick!" The real old meaning of quick is to be alive. When you run quick you have to be very much alive. So when men found a silvery white metal that ran about in the liveliest kind of way, they called it quick, or live silver. They called it mercury, too. That means much the same thing. Mercury was the young messenger god of the Greek people. He had wings on his heels, and when he was sent on an errand, he fairly flew.

Quicksilver isn’t really alive. It just happens to be melted, or in a liquid state. Gold, silver, iron, copper, lead and all metals can be melted, if they are made hot enough. The difference is that quicksilver is a sort of polar bear, or Esquimo metal. Temperatures at which other metals are solid, just melts quicksilver. If heated to a temperature that melts gold, away Mercury goes in vapor or gas, like water. Cold shrinks quicksilver, heat expands it. Indeed, it is the most sensitive thing we know that is not alive. That is why it shrinks and swells, running up and down the little tube in the thermometer, and tells us the slightest change in the temperature of the weather.

Quicksilver behaves as it does because it is a liquid. You cannot pick it up for the same reason that you cannot pick up water. But it does not wet paper. The reason it doesn’t soak into things is because it is denser than any solid thing we know. It is so much heavier, for the space it fills, that gold, silver and iron float on it as wood floats on water. Besides, it likes its own company best. No matter how much of it you pour out on a table it will not spread. Every particle clings together in one ball. If you split that ball up, each lot will roll up into a smaller ball, or bead. And if these beads get near enough to each other they promptly run together again. The particles of quicksilver have a stronger attraction for each other than anything else has for them. They have a strong attraction also for other metals. They seem a to swallow gold and silver and other metal dust, in the most curious ways. So men have learned to send little Mercury ball messenger boys into all the cracks and crannies