Yesterday in Weather Eye, you may recall, tektites, those mysterious lumps of crystal from on high whose origins remain obscure, were cited as an example of what people thought might well be "thunderbolts". Another material often thought to be a thunderbolt was fulgurite, but in this case we know exactly where the glassy substance comes from.
Thunderbolts, as we know, were said to be thrown by Zeus, or Jupiter, when the god was displeased with humans. The belief persisted after Greek and Roman times, and long after Zeus himself had been discarded as a god there was a conviction that the destructive power of lightning was due, not to the lightning itself, but to a missile or mass of heated matter that was discharged from thunderclouds.
Thus, even in Shakespeare's time, the philosopher William Fulke tells us that from a thunderstorm "a great stone is blown out, consisting of brimstone and of other metallic substances, whereby it burneth with an evil stench and smoke, and striketh down steeples and high buildings of stone".
Shakespeare himself in Cymbeline has Jupiter descending from a cumulonimbus cloud upon a golden eagle; inevitably, he throws a thunderbolt at the assembled players underneath, and in case we had not noticed, Sicilius point out "He came in thunder, and his celestial breath was sulphurous to smell."
Later, when Jupiter, they think, has mellowed somewhat, the jubilant sons of Cymbeline rejoice that they need "fear no more the lightning flash, nor the dreaded thunderstone."
Whether these thunderbolts allegedly descending from the sky were the missiles of an angry god or not, people tried to find them in the aftermath of thunderstorms, and sometimes succeeded when they came across elongated stubs of glass. We call them fulgurites.
When lightning strikes dry rock or sand, the heat of the discharge sometimes fuses the material into a solid lump of glass; more often, it leaves a hole surrounded by a glassy tube. In loose sand, fulgurites can be several feet in length and as much as 2in in circumference. Compression of the glass by steam from the intense heat of the ground outside often gives to the fulgurite a somewhat "ribbed" exterior.
Fulgurites are found in the Alps and Pyrenees, and anywhere in the desert regions of the low latitudes. They are formed on the ground by a very simple process following the intense heat produced by a single stroke of lightning. But with their sudden appearance after a violent thunderstorm, one can see how our ancestors might have thought they came from heaven.