smell the woody smoke, and hear it crackle.

木頭的煙燻味以及劈啪作響的聲音。

If you get too close,

如果你靠得太近，

it burns your eyes and stings your nostrils.

會灼傷眼睛而且刺激鼻子。

You could stare at the bright flames forever

你可以一直盯著明亮的火焰，

as they twist and flicker in endless incarnations.

因為它們處於無盡的扭動和閃耀狀態

But what exactly are you looking at?

但你真正看到的是什麼？

The flames are obviously not solid,

火焰很明顯地不是固體，

nor are they liquid.

也不是液體。

Mingling with the air, they're more like a gas,

和空氣混合，火焰更像是氣體，

but more visible--and more fleeting.

但是火焰更可見也更迅速。

And on a scientific level, fire differs from gas

從科學層面上來看，火焰不同於氣體，

because gases can exist in the same state indefinitely

因為氣體可以無限期地處於同一種狀態，

while fires always burn out eventually.

但是火焰終會燃燒殆盡。

One misconception is that fire is a plasma,

有種誤解認為火是等離子體，

the fourth state of matter in which atoms

物質的第四種狀態，

are stripped of their electrons.

也就是原子失去電子的狀態。

Like fire and unlike the other kinds of matter,

和火一樣而與其他物質不同的是，

plasmas don't exist in a stable state on earth.

等離子體不以固定的狀態存在。

They only form when gas is exposed to an electric field or superheated

當氣體暴露於電場之中，

to temperatures of thousands or tens of thousands of degrees.

或者溫度高達數千萬度時，它們才會形成。

By contrast, fuels like wood and paper burn

相較之下，木頭和紙張燃燒的溫度

at a few hundred degrees —far below the

只有幾百度，遠低於

threshold of what's usually considered a plasma.

等離子體的溫度門檻。

So if fire isn't a solid, liquid, gas,

那麼，如果火焰不是固體、液體、氣體

or a plasma, what does that leave?

或等離子體，那還能是什麼？

It turns out fire isn't actually matter at all.

其實，火焰根本不是物質。

Instead, it's our sensory experience of a

它是對於燃燒化學反應的

chemical reaction called combustion.

一種感官體驗。

In a way, fire is like the leaves changing color in fall,

某種程度上，火焰就像秋天變色的樹葉、

the smell of fruit as it ripens,

水果成熟時的味道

or a firefly's blinking light.

或是螢火蟲的閃光。

All of these are sensory clues that a

這些都是發生化學反應時的

chemical reaction is taking place.

感官線索。

What differs about fire is that it engages a lot of

火焰較不同的地方在於，它同時

our senses at the same time, creating the kind of vivid

牽涉到很多感覺，進而形成一種

experience we expect to come from a physical thing.

從實體事物中才能獲得的生動體驗。

Combustion creates that sensory experience

燃燒反應利用燃料、熱度和氧氣

using fuel, heat, and oxygen.

來創造出感官體驗。

In a campfire, when the logs are heated to their ignition temperature,

營火中，當木頭被加熱到燃燒溫度時，

the walls of their cells decompose,

它們的細胞壁會分解，

releasing sugars and other molecules into the air.

釋放糖及其他分子到空氣中。

These molecules then react with airborne oxygen

這些分子和空氣中的氧氣發生作用，

to create carbon dioxide and water.

產生二氧化碳和水。

At the same time, any trapped water in the logs

同時，木頭中的水份會

vaporizes, expands, ruptures the wood around it,

蒸發、膨脹並使木頭裂開，

and escapes with a satisfying crackle.

發出劈啪的聲響。

As the fire heats up, the carbon dioxide and water vapor

當火焰溫度升高，燃燒反應產生的二氧化碳

created by combustion expand.

及水蒸氣會膨脹。

Now that they're less dense, they rise in a thinning column.

密度降低後，它們會呈薄柱狀上升。

Gravity causes this expansion and rising, which gives

重力使得火焰膨脹及上升，

flames their characteristic taper.

進而使其呈現特有的錐型。

Without gravity, molecules don't separate

沒有重力的話，分子不會因為

by density and the flames have a totally different shape.

密度而分離，火焰的形狀也會完全不同。

We can see all of this because combustion

我們可以看到這一切是因為

also generates light.

燃燒反應也會產生光。

Molecules emit light when heated,

分子受熱時會發光，

and the color of the light depends

而光的顏色取決於

on the temperature of the molecules.

分子的溫度。

The hottest flames are white or blue.

溫度最高的火焰是白色或藍色。

The type of molecules in a fire can

火焰中的分子類型

also influence flame color.

也會影響火焰的顏色。

For instance, any unreacted carbon atoms from the logs

例如，木頭上未反應的碳原子會

form little clumps of soot that rise

形成小團的煤灰飄到

into the flames and emit the yellow-orange

火焰中，發出我們會聯想到營火的，

light we associate with a campfire.

橘黃色的光。

Substances like copper, calcium chloride,

像銅、氯化鈣和氯化鉀這些物質，

and potassium chloride can add their

能夠在混和物中

own characteristic hues to the mix.

加入自己獨有的色調。

Besides colorful flames,

除了五顏六色的火焰，

fire also continues to generate heat as it burns.

火焰在燃燒時也會持續產生熱能。

This heat sustains the flames by keeping

熱能靠著保持燃料的溫度

the fuel at or above ignition temperature.

等於或高於燃點，來維持火焰。

Eventually, though, even the hottest fires

但最終，即使是溫度最高的火焰，

run out of fuel or oxygen.

也會耗盡燃料或氧氣。

Then, those twisting flames give a final hiss

然後，這些扭曲的火焰發出最後的嘶嘶聲，

and disappear with a wisp of smoke

隨著一縷煙消失，

as if they were never there at all.

彷彿從未存在過。