Our atmosphere envelops the cosmic sand grain of Earth just as Priestley’s glass bells enveloped his mice, just as bitumen envelops a grain of bituminous sand, just as our skin envelops our own bodies: relative to what they are covering, each of these insulating layers is gossamer-thin. The vertical distance from sea level, where most humans live, to icy suffocation at Camp 4 on Mount Everest is less than five miles—a mere .06 percent of Earth’s eight-thousand-mile diameter. Put another way, your skin is ten times thicker, relative to your body, than the habitable portion of the atmosphere is
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