Since ozone is not present near the earth's surface. they concluded that it müst exist at high levels in the earth's atmosphere, It behaves like a great black bank of clouds always present even in the binest sky, blanketing out all ultra-violet'radiation beyond a certain limiting wavelength.
By optical methods Fabry and Büisson measured its amount, but they cond pot: determine how many miles above: itjlay. The tity of ozone~ termi Am Sh. opedde barrier was equal to a layer of the pure gas only three millimeters, or one-eighth of one inch thick When reduced to the pressure of the air at the earth's surface. This does not mean that the ozone in the atmosphere is confined to so thin a layer of air.
It seems incredible that so small a quantity of ozone can have such a profound effect, yet this result is confirmed by laboratory measurements. If the amount of ozone in the atmosphere were to be reduced to one third, our skins would be destroyed in a matter of minutes' exposure to the sun. On the other hand, if the one-eighth of one inch of ozone were doubled,"the human race would probably die out for lack of an essential vitamin—even if we could survive the enormous increase in bacterial growth that this thicker ozone layer would permit. Somewhat smaller fluctuations in the thickness of the ozone layer de actually occur; observed amounts vary from a value a little less than two- millimeters to one of a little more than three millimeters. In general, ozone is greater in the spring than inthe fall, and is least in the tropics ¿where the yearly change is small,
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