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  • Mercury is a fascinating material.

  • Even I enjoy playing with it.

  • But like most heavy metals, it is poisonous.

  • It's quite surprising to people, if you have this liquid running around in a dish or something, how can it possibly poison you if you're not touching it?

  • The answer is that there is vapour of mercury coming off, which you can breathe.

  • And, remarkably, it's quite easy to show you how this vapour comes off.

  • Before we show you the experiment,

  • I have to explain a little bit about mercury lamps.

  • UV light can be generated by filling a tube with a small amount of mercury

  • and then passing through an electric current.

  • The electric current heats the mercury to a very high temperature

  • and the mercury atoms give out ultraviolet light.

  • This is light we can't see.

  • And the ultraviolet light is at a very precise wavelength,

  • around 254 nanometers.

  • So now, Neil has one of these lamps

  • quite an old one, but it still works.

  • And let's see what happens when he shines it

  • across the surface of a dish of mercury.

  • To make the effect a bit easier to film,

  • he's warmed the mercury a bit, but it's not very warm.

  • Probably less than 100 degrees Centigrade

  • like a cup of tea, or something like that.

  • We, as humans, cannot see UV light.

  • Our eyes don't detect it.

  • In order to see the output of the lamp

  • you have to shine it on a sheet of paper

  • which contains a dye to make it look white.

  • You know, paper is usually a bit brown.

  • And that dye absorbs the UV light

  • and gives out a sort of purplish color of light.

  • So that's why the piece of paper that Neil is holding

  • is glowing purple.

  • In between the lamp and the paper

  • there is vapour coming off the mercury.

  • Not very much, but the mercury atoms in the vapour

  • can absorb precisely the same wavelength of light

  • as the lamp is giving out.

  • So it acts as a very precise and powerful filter

  • for the light. So the paper doesn't glow as brightly.

  • And you can see these clouds of swirling vapour.

  • And those clouds are the mercury vapour

  • which, if you had your nose close to it,

  • you would be breathing in.

  • And that's why liquid mercury is dangerous.

  • Because, all the time, vapour is coming off.

  • And mercury is one of the elements which can accumulate

  • in biological systems like your body.

  • So even if the mercury is coming off slowly

  • if you're exposed to it a long time,

  • you can absorb quite a lot.

  • And there's stories of old lab buildings, where

  • they used to use mercury, where, when they're doing

  • the refurbishment, they lift the floorboards

  • and underneath there're pools of mercury

  • which have fallen through when people have spilled it

  • over the years.

  • In some of these places,

  • there is quite a high level of mercury vapour

  • that is coming up through the floor.

  • Though, usually, it's so dirty under the floor

  • that there's this layer of dust which prevents most of it coming off.

  • But, the take-home message

  • with this is that if you're using mercury,

  • and there are all sorts of experiments where it's useful

  • to use mercury, you must use it in a fume hood

  • that's well ventilated, so you don't breathe the vapour.

  • ...from the vapour pressure in the air

  • and he dropped in the mercury.

  • What's interesting is that, as it goes into the liquid nitrogen,

  • the nitrogen boils, and so the mercury falls

  • almost encased in this sort of bubble of nitrogen gas.

Mercury is a fascinating material.

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Looking at Mercury Vapour - Periodic Table of Videos

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    林宜悉 posted on 2020/03/27
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