The value of the critical density is very small: it corresponds to roughly 6 hydrogen atoms per cubic meter, an astonishingly good vacuum by terrestrial standards!

临界密度的值非常小：它相当于每立方米约6个氢原子，从地球上的标准来看这简直是真空！

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The pure magnesium can then be used as a fuel (its energy density is about ten times that of hydrogen).

到那时纯镁可用作燃料(其能量密度大约是氢的十倍)。

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As well as being very hot, the ionised gas of the corona (called plasma) has a superthin consistency. It is less than a billionth of the density of the hydrogen that makes up the main ball of the Sun.

日冕的电离气体（又叫等离子体）不仅非常炙热，而且异常稀薄，其密度还不到组成太阳主体的氢气密度的十亿分之一。

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Hydrogen has a high energy density and is clean - burning to leave only water vapour.

BBC: Hydrogen's energy promise improves

Hyperpolarisation not only compensates for the low density of the gas (the 3He is 10, 000 times more polarised than the hydrogen in water), it also allows images to be taken rapidly enough to avoid any blurring due to a patient's natural tendency to breathe.

ECONOMIST: Medical imaging

Instead of being stored in batteries, the electricity made by solar panels can be used to split water into oxygen and hydrogen molecules, the latter of which can be used as a fuel with high energy density.

WHITEHOUSE: Engage and Connect