按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
almost nothing about it can be used to make reliable predictions about the properties of otherliquids and vice versa。 if you knew nothing of water and based your assumptions on thebehavior of pounds most chemically akin to it—hydrogen selenide or hydrogen sulphidenotably—you would expect it to boil at minus 135 degrees fahrenheit and to be a gas at roomtemperature。
most liquids when chilled contract by about 10 percent。 water does too； but only down to apoint。 once it is within whispering distance of freezing； it begins—perversely； beguilingly；extremely improbably—to expand。 by the time it is solid； it is almost a tenth morevoluminous than it was before。 because it expands； ice floats on water—“an utterly bizarreproperty；” according to john gribbin。 if it lacked this splendid waywardness； ice would sink；and lakes and oceans would freeze from the bottom up。 without surface ice to hold heat in；the water’s warmth would radiate away； leaving it even chillier and creating yet more ice。
soon even the oceans would freeze and almost certainly stay that way for a very long time；probably forever—hardly the conditions to nurture life。 thankfully for us； water seemsunaware of the rules of chemistry or laws of physics。
everyone knows that water’s chemical formula is h2o； which means that it consists of onelargish oxygen atom with two smaller hydrogen atoms attached to it。 the hydrogen atomscling fiercely to their oxygen host； but also make casual bonds with other water molecules。
the nature of a water molecule means that it engages in a kind of dance with other watermolecules； briefly pairing and then moving on； like the ever…changing partners in a quadrille；to use robert kunzig’s nice phrase。 a glass of water may not appear terribly lively； but everymolecule in it is changing partners billions of times a second。 that’s why water moleculesstick together to form bodies like puddles and lakes； but not so tightly that they can’t be easily separated as when； for instance； you dive into a pool of them。 at any given moment only 15percent of them are actually touching。
in one sense the bond is very strong—it is why water molecules can flow uphill whensiphoned and why water droplets on a car hood show such a singular determination to beadwith their partners。 it is also why water has surface tension。 the molecules at the surface areattracted more powerfully to the like molecules beneath and beside them than to the airmolecules above。 this creates a sort of membrane strong enough to support insects andskipping stones。 it is what gives the sting to a belly flop。
i hardly need point out that we would be lost without it。 deprived of water； the human bodyrapidly falls apart。 within days； the lips vanish “as if amputated； the gums blacken； the nosewithers to half its length； and the skin so contracts around the eyes as to prevent blinking。”
water is so vital to us that it is easy to overlook that all but the smallest fraction of the wateron earth is poisonous to us—deadly poisonous—because of the salts within it。
we need salt to live； but only in very small amounts； and seawater contains way more—about seventy times more—salt than we can safely metabolize。 a typical liter of seawater willcontain only about 2。5 teaspoons of mon salt—the kind we sprinkle on food—but muchlarger amounts of other elements； pounds； and other dissolved solids； which arecollectively known as salts。 the proportions of these salts and minerals in our tissues isuncannily similar to seawater—we sweat and cry seawater； as margulis and sagan have putit—but curiously we cannot tolerate them as an input。 take a lot of salt into your body andyour metabolism very quickly goes into crisis。 from every cell； water molecules rush off likeso many volunteer firemen to try to dilute and carry off the sudden intake of salt。 this leavesthe cells dangerously short of the water they need to carry out their normal functions。 theybee； in a word； dehydrated。 in extreme situations； dehydration will lead to seizures；unconsciousness； and brain damage。 meanwhile； the overworked blood cells carry the salt tothe kidneys； which eventually bee overwhelmed and shut down。 without functioningkidneys you die。 that is why we don’t drink seawater。
there are 320 million cubic miles of water on earth and that is all we’re ever going to get。
the system is closed： practically speaking； nothing can be added or subtracted。 the water youdrink has been around doing its job since the earth was young。 by 3。8 billion years ago； theoceans had （at least more or less） achieved their present volumes。
the water realm is known as the hydrosphere and it is overwhelmingly oceanic。 ninety…seven percent of all the water on earth is in the seas； the greater part of it in the pacific； whichcovers half the planet and is bigger than all the landmasses put together。 altogether thepacific holds just over half of all the ocean water （51。6 percent to be precise）； the atlantic has23。6 percent and the indian ocean 21。2 percent； leaving just 3。6 percent to be accounted forby all the other seas。 the average depth of the ocean is 2。4 miles； with the pacific on averageabout a thousand feet deeper than the atlantic and indian oceans。 altogether 60 percent ofthe planet’s surface is ocean more than a mile deep。 as philip ball notes； we would better callour planet not earth but water。
of the 3 percent of earth’s water that is fresh； most exists as ice sheets。 only the tiniestamount—0。036 percent—is found in lakes； rivers； and reservoirs； and an even smaller part—just 0。001 percent—exists in clouds or as vapor。 nearly 90 percent of the planet’s ice is inantarctica； and most of the rest is in greenland。 go to the south pole and you will bestanding on nearly two miles of ice； at the north pole just fifteen feet of it。 antarctica alone has six million cubic miles of ice—enough to raise the oceans by a height of two hundred feetif it all melted。 but if all the water in the atmosphere fell as rain； evenly everywhere； theoceans would deepen by only an inch。
sea level； incidentally； is an almost entirely notional concept。 seas are not level at all。
tides； winds； the coriolis force； and