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d we can soon cross a threshold。 much ofthis is only imperfectly understood。 no one knows； for example； whether a tiny amount ofarsenic is necessary for our well…being or not。 some authorities say it is； some not。 all that iscertain is that too much of it will kill you。
the properties of the elements can bee more curious still when they are bined。
oxygen and hydrogen； for instance； are two of the most bustion…friendly elements around；but put them together and they make inbustible water。
3odder still in bination aresodium； one of the most unstable of all elements； and chlorine； one of the most toxic。 drop asmall lump of pure sodium into ordinary water and it will explode with enough force to kill。
chlorine is even more notoriously hazardous。 though useful in small concentrations forkilling microorganisms （it鈥檚 chlorine you smell in bleach）； in larger volumes it is lethal。
chlorine was the element of choice for many of the poison gases of the first world war。 and；as many a sore…eyed swimmer will attest； even in exceedingly dilute form the human bodydoesn鈥檛 appreciate it。 yet put these two nasty elements together and what do you get？ sodiumchloride鈥攎on table salt。
by and large； if an element doesn鈥檛 naturally find its way into our systems鈥攊f it isn鈥檛soluble in water； say鈥攚e tend to be intolerant of it。 lead poisons us because we were neverexposed to it until we began to fashion it into food vessels and pipes for plumbing。 （notincidentally； lead鈥檚 symbol is pb； for the latin plumbum； the source word for our modernplumbing。） the romans also flavored their wine with lead； which may be part of the reasonthey are not the force they used to be。 as we have seen elsewhere； our own performance withlead （not to mention mercury； cadmium； and all the other industrial pollutants with which weroutinely dose ourselves） does not leave us a great deal of room for smirking。 when elementsdon鈥檛 occur naturally on earth； we have evolved no tolerance for them； and so they tend to beextremely toxic to us； as with plutonium。 our tolerance for plutonium is zero： there is no levelat which it is not going to make you want to lie down。
i have brought you a long way to make a small point： a big part of the reason that earthseems so miraculously acmodating is that we evolved to suit its conditions。 what wemarvel at is not that it is suitable to life but that it is suitable to our life鈥攁nd hardlysurprising； really。 it may be that many of the things that make it so splendid to us鈥攚ell…proportioned sun； doting moon； sociable carbon； more magma than you can shake a stick at；and all the rest鈥攕eem splendid simply because they are what we were born to count on。 noone can altogether say。
other worlds may harbor beings thankful for their silvery lakes of mercury and driftingclouds of ammonia。 they may be delighted that their planet doesn鈥檛 shake them silly with itsgrinding plates or spew messy gobs of lava over the landscape； but rather exists in apermanent nontectonic tranquility。 any visitors to earth from afar would almost certainly； atthe very least； be bemused to find us living in an atmosphere posed of nitrogen； a gassulkily disinclined to react with anything； and oxygen； which is so partial to bustion thatwe must place fire stations throughout our cities to protect ourselves from its livelier effects。
but even if our visitors were oxygen…breathing bipeds with shopping malls and a fondness for3oxygen itself is not bustible； it merely facilitates the bus tion of other things。 this is just as well； for ifoxygen were corn bustible； each time you lit a match all the air around you would bur into flame。 hydrogen gas；on the other hand； is extremely corn bustible； as the dirigible hindenburg demonstrated on may 6； 193 inlakehurst； new jersey； when its hydrogen fuel burst explosive） into flame； killing thirty…six people。
action movies； it is unlikely that they would find earth ideal。 we couldn鈥檛 even give themlunch because all our foods contain traces of manganese； selenium； zinc； and other elementalparticles at least some of which would be poisonous to them。 to them earth might not seem awondrously congenial place at all。
the physicist richard feynman used to make a joke about a posteriori conclusions； as theyare called。 鈥測ou know； the most amazing thing happened to me tonight；鈥潯e would say。 鈥渋saw a car with the license plate arw 357。 can you imagine？ of all the millions of licenseplates in the state； what was the chance that i would see that particular one tonight？
amazing！鈥潯is point； of course； was that it is easy to make any banal situation seemextraordinary if you treat it as fateful。
so it is possible that the events and conditions that led to the rise of life on earth are notquite as extraordinary as we like to think。 still； they were extraordinary enough； and one thingis certain： they will have to do until we find some better。
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17 INTO THE TROPOSPHERE
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thank goodness for the atmosphere。 it keeps us warm。 without it； earth would be alifeless ball of ice with an average temperature of minus 60 degrees fahrenheit。 in addition；the atmosphere absorbs or deflects ining swarms of cosmic rays； charged particles；ultraviolet rays； and the like。 altogether； the gaseous padding of the atmosphere is equivalentto a fifteen…foot thickness of protective concrete； and without it these invisible visitors fromspace would slice through us like tiny daggers。 even raindrops would pound us senseless if itweren鈥檛 for the atmosphere鈥檚 slowing drag。
the most striking thing about our atmosphere is that there isn鈥檛 very much of it。 it extendsupward for about 120 miles； which might seem reasonably bounteous when viewed fromground level； but if you shrank the earth to the size of a standard desktop globe it would onlybe about the thickness of a couple of coats of varnish。
for scientific convenience； the atmosphere is divided into four unequal layers： troposphere；stratosphere； mesosphere； and ionosphere （now often called the thermosphere）。 thetroposphere is the part that鈥檚 dear to us。 it alone contains enough warmth and oxygen to allowus to function； though even it swiftly bees uncongenial to life as you climb up through it。
from ground level to its highest point； the troposphere （or 鈥渢urning sphere鈥潱s about ten milesthick at the equator and no more than six or seven miles high in the temperate latitudes wheremost of us live。 eighty percent of the atmosphere鈥檚 mass； virtually all the water； and thusvirtually all the weather are contained within this thin and wispy layer。 there really isn鈥檛much between you and oblivion。
beyond the troposphere is the stratosphere。 when you see the top of a storm cloudflattening out into the classic anvil shape； you are looking at the boundary between thetroposphere and stratosphere。 this invisible ceiling is known as the tropopause and wasdiscovered in 1902 by a frenchman in a balloon； l茅on…philippe teisserenc de bort。 pause inthis sense doesn鈥檛 mean to stop momentarily but to cease altogether； it鈥檚 from the same greekroot as menopause。 even at its greatest extent； the tropopause is not very distant。 a fastelevator of the sort used in modern skyscrapers could get you there in about twenty minutes；though you would be well advised not to make the trip。 such a rapid ascent withoutpressurization would； at the very least； result in severe cerebral and pulmonary edemas； adangerous excess of fluids in the body鈥檚 tissues。 when the doors opened at the viewingplatform； anyone inside would almost certainly be dead or dying。 even a more measuredascent would be acpanied by a great deal of disfort。 the temperature six miles up canbe …70 degrees fahrenheit； and you would need； or at least very much appreciate；supplementary oxygen。
after you have left the troposphere the temperature soon warms up again； to about 40degrees fahrenheit； thanks to the absorptive effects of ozone （something else de bortdiscovered on his daring 1902 ascent）。 it then plunges to as low as …130 degrees fahrenheit inthe mesosphere before skyrocketing to 2；700 degrees fahrenheit or more in the aptly namedbut very erratic thermosphere； where temp