The ancients seem to have known of these fulgurite tubes, but we owe the first precise description and the first specimen of these extraordinary vitrifactions to Hermann, a pastor at Massel in Silesia. His fulgurite, found in 1711, is in the Dresden Museum.
Since this discovery, fulgurites have often been sought for and found. The tubes, contracted at one end, and ending in a point, are to be seen in sandy soils.
Their diameter varies from 1 to 90 millimetres, and the thickness of their sides from half to 24 millimetres. As to the length, it sometimes exceeds 6 metres. Vitrified inside, they are covered outside with grains of sand agglutinated and apparently rounded as if they had been subjected to a beginning of fusion. The colour depends on the nature of the sand in which they have been formed. Where the sand is ferruginous the fulgurite takes a yellowish hue, but if the sand is very clean, it is almost colourless or even white. As a rule, the fulgurites penetrate the ground vertically, Nevertheless, they have been found in an oblique position. At times, also, they are sinuous, twisted, or even zigzag if they have met with pebbles of considerable size.
It is not uncommon for the fulgurite tube to divide in two or three branches, each of which gives birth to little lateral branches of 2 or 3 centimetres long, and ending in points.
There are also solid fulgurites and foliated fulgurites. The former, no doubt, had a canal originally, which has been stopped up by matter in fusion. The latter, instead of being stretched out in cylindrical form, are composed of slender layers like the leaves of a book.
The scientific museum at the Observatory of Juvisy possesses a very curious fulgurite which was offered to me some years ago by M. Bernard d'Attanoux, and found by him in Sahara. It is not a tube ending in a point. The lightning penetrating the sand, vitrified it on its passage, and branched irregularly in three principal directions. One might say it was slag formed by the juxtaposition, irregular and crumpled, of three blades of vitrified sand, which would be pressed together by leaving a narrow opening to their central vertical axis. This fulgurite, which is extremely light, measures six centimetres in length. It was found in the sand of Grand-Erg, at a depth of several centimetres. It has been found possible to produce miniature fulgurites by means of our electrical machines. By adding ordinary salt to the sand, and directing a strong current into it, complete vitrification of a tube of several millimetres is obtained.
CHAPTER VIII
THE EFFECTS OF LIGHTNING ON METALS, OBJECTS, HOUSES, ETC.
When lightning strikes the earth, it makes straight for metals. Their perfect conducting powers place them in the first rank of conductors, and the innumerable cases of lightning with which they are associated have gained them a certain celebrity in the annals of thunder.