英文科學(xué)讀本 第三冊·Lesson 24 Mercury—Its Preparation and Uses

Lesson 24 Mercury—Its Preparation and Uses

What I had already learnt about the liquid metal, mercury, said Will, "made me glad when teacher told us we were to have another lesson on it today. Our lesson has shown us how it is obtained, and some of the uses we make of the metal. Shall I try and tell you about it, Norah?"

Yes, said Norah, "please tell me all you can remember."

Well then, he began, "mercury, like most metals, is usually met with in the form of ore. This ore is known as cinnabar. It is dug out of deep mines in various parts of the world, and looks like a rough, hard, brown stone, with very little metallic lustre. It contains besides the metal, mercury, a large quantity of sulphur. The mercury and the sulphur must, of course, be separated. This is done in a curious way."

The ore is first broken up into small pieces, and mixed with quicklime. It is then put into large retorts and heated. A retort, you know, is a closed vessel, with pipes leading from it.

I think I can tell why this is done, said Norah. "The sulphur would soon melt into a liquid, and boil, and then it would pass away along the pipes in the form of vapor."

That's just it, said Will. "But we have already learned that the mercury too would boil and pass away in vapor. This mercury vapor must not be lost. It is wanted."

Some earthenware pipes are therefore fixed to the retorts, and the vapor is made to pass through them. As these pipes are kept cool, the vapor condenses on the sides in little round, silvery balls, and nothing more need be done than to collect them, and pour off the liquid metal ready for use.

Suppose I tell you now, he continued, "some of the uses to which mercury is put."

One of the most important uses is in separating gold and silver from the ores in which they are found.

You remember that gold is usually found in small grains in the midst of certain rocks. The rock is crushed up very fine, and then mixed with mercury. It is a curious property of mercury that it will at once unite with other metals if it is mixed up with them.

The mercury in this case seizes upon the little grains of gold, but will not unite with the earthy parts of the ore. The earthy parts are carefully washed away in shallow troughs leaving the heavier metallic grains behind.

These are easily collected, and all that remains is to heat them gently. The mercury flies off in vapor with the heat, and is condensed and collected for further use. The grains of gold remain behind.

Mercury is also used for 'silvering' the backs of looking-glasses, and for various other purposes.

SUMMARY

The ore from which mercury is obtained is a rough, brown stone, called cinnabar. It contains, besides the metal, a large quantity of sulphur. It is broken up into small pieces and heated, and the sulphur passes away in vapor, leaving the liquid metal behind.