At -269°C, helium gas condenses to become a liquid. Cool it even further and it becomes a state of matter called a superfluid. In this state it has no measurable viscosity and so does some odd things, such as climbing up the walls of a dish, leaking through apparently solid materials and staying motionless while its container is spun. To create the liquid and superfluid states, you cool down helium gas to a few degrees above absolute zero. This is achieved by compressing the gas, and then expelling it through a small nozzle. As the gas expands, it rapidly cools (you’ll have noticed this effect if you’ve ever used an aerosol deodorant). The process is repeated until the gas that rushes out of the nozzle is cold enough to condense to a liquid, then if you repeat the cycle a few more times the helium will become cold enough to turn to a superfluid.
Mercury levels in the oceans have tripled since the Industrial Revolution, thanks to mining and the burning of fossil fuels. All sea creatures absorb some of this heavy metal directly, and, once it’s in the body, there’s no way of getting rid of it. The amount of mercury in fish varies between species. Long-lived predators like tuna and swordfish tend to contain the most, because they also absorb mercury from their prey and they’ve had a long time to accumulate it. The lowest levels are found in short-lived species lower down the food chain, such as oysters and shrimp.
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