This is an eruption of Vesuvius. You would think it was throwing out smoke like a gigantic locomotive, wouldn't you, if you hadn't read the text? The darker masses, which look so much like mingled smoke and steam, are shadows. It is probably eight to ten miles high--that cloud. ] Lava, before it cools and for some centuries afterward, is the last thing you would think of farming on, perhaps, but leave it to the little chemists of the water and the air and it will decay into the richest land you ever saw. That is why they raise the finest wheat and the best fruit in the world right in the parts of Washington and Oregon that were once covered by the lava flood. Not only do volcanoes help to supply us with food by making rich soil of the eruptions of the past, but all life might disappear from the earth if they didn't go on exploding. [Illustration: HOW VOLCANOES BLOW BUBBLES The surface of lava is apt to bubble like hot mush; and for a similar reason, the expansion of the gases within it. (In the case of the mush it is the mixture of gases we call "air.") When such lava cools you have sponge-like masses such as this. ] Plants must have carbon and they get it from the air, but the amount of it in proportion to their needs is never large. Moreover, every bit of coal that is formed--and coal is being made to-day just as it was in the coal ages, although not in such quantities--takes carbon from the air and locks it up. Every bit of limestone deposited on the floor of the sea locks up more carbon. But, fortunately, immense quantities of carbon are given back to the air through the gases thrown out by volcanoes, thus offsetting these losses. [Illustration: _From a photograph by the American Museum of Natural History_ ROCKS AND BOMBS THROWN BY MOUNT PELÉE Look at these giant rocks thrown out by Mount Pelée in 1902. Compare them with the man and you will realize how big they are. The rounded rocks in the foreground are volcanic "bombs"--masses of lava discharged by successive outbursts of volcanic gases and given their shape by being whirled through the air. ] [Illustration: WHEN IS A VOLCANO DEAD? This is Mount Rainier with its shroud of snow, reflected in Mirror Lake. To all appearances it is as dead as dead can be; but until after a volcano goes off you never can be entirely sure whether it is dead or not; and then, of course, you know it isn't! ] WHEN IS A VOLCANO REALLY DEAD?