Falling on solid rocks, the electric spark can break them, cut them, or pierce them in one or more places. Often instead of spoiling or cutting off pieces of the stone, it covers the surface instantaneously with a vitreous coat, having blisters of various colours. This vitrification is often to be seen on mountains.
De Saussure found rocks of schistous amphiboles covered with vitreous bubbles, like those seen on tiles where struck by lightning. Humboldt made similar observations on porphyritic rocks at Névada de Toluca, in Mexico, and Ramond, at the Sanadoire rock in Puy-de-Dôme.
In these cases, the spark, on reaching the surface, melts it more or less completely over a varying extent, and this fusion, worked upon by an extraordinary heat, produces a coat having a peculiar appearance, but in which microscopical analysis finds the elements of the body it covers.
Thus the vitreous layer deposited over chalk is of chalky origin; that covering granite is of the nature of granite, etc.
This does not apply to certain deposits found on rocks, and even on trees, which have been struck by lightning, and which are of very different origin.
Whilst the former is only the stone in a fused or vitrified condition, the latter is caused by the presence of foreign bodies, some fragments of which have been detached by the ray and travel with it. This transport of solid substances by lightning has often been observed. Here are two examples of this strange phenomenon:—
On July 28, 1885, at Luchon, on the Bigorre road a passer-by saw lightning fall twenty yards away from him. Recovered from the shock, he went out of curiosity to look at the result, and saw the wall at the edge of the road, the schistous and chalky rocks, even the trees themselves, coated over with layers of brown. It was certainly a case of the lightning having effected a deposit. This latter was very curious. Lines could be traced on it with the finger-nail, it fell to powder under slight pressure, became soft with gentle rubbing, caught fire from a candle, and then gave off a resinous odour with much smoke. What is this resinous matter? That is what no one yet can say.
In the month of July, 1885, on the day following a violent thunderstorm which had struck the telegraph-office in the station of Savigny-sur-Orge, I myself picked up a little black powder off the telegraph poles, which had been left by the lightning, and which had a sulphurous smell.
The production of this ponderable matter has often been attributed to bolides, but direct observation proves beyond a doubt that the electricity carries various solid substances found on earth after a storm.
Lightning is truly the most venerable of glass-makers. Long before the most remote peoples of antiquity appeared, whose glasswares encrusted with marvellous iridescent tones by the passing of the centuries, are unearthed by scientific excavations, and displayed in national collections; long before man could have learnt to make use of the resources of nature, lightning, burrowing in the sand, there fashioned tubes of glass that hold the hues of the opal, and are called fulgurites.