Popular Science Monthly/Volume 30/February 1887/Fulgurites, or Lightning-Holes
By GEORGE P. MERRILL.
THE peculiar and often disastrous results attendant upon an electric discharge have been dwelt upon since time immemorial. To even briefly refer to the numerous recorded instances of the destruction of life and property by the discharge of "heaven's artillery" would far exceed the limits of this paper. It is my purpose, therefore, to call attention only to those peculiarly interesting, though usually quite harmless, effects produced by the lightning striking in loose sand; though, before closing, I shall allude to the closely allied phenomena resulting from similar discharges upon solid rock. In the sand, as is well known, the usual result produced is that of fusion whereby a frail, glassy tube of variable diameter and length is produced, the interior of which is a true amorphous glass, quite smooth, while exteriorly it is roughly granular and greatly corrugated. Such are called fulgurites or fulmination-tubes in English, while, to the German and French, they are known as Blitzröhren and tubes fulminaires respectively.
So far as can be learned from available literature, the earliest description to be made of these peculiar objects was that of Pastor David Hermann in 1711. According to Gilbert, this gentleman, as early as 1706 and 1707, dug from a sand-hill in Massel, Silesia, fulgurites some twenty feet in length, which he very fully described in his work on Massel and its curiosities.
Hermann's account is curious and full of interest, as his statements concerning the origin of the tubes were the purest guess-work, and his views regarding them wild in the extreme. He designated them by the name "Osteocolla," and proposed to use them for medicinal purposes, as will be noted later.
The earliest account of an occurrence of this kind, where the fusion was indubitably proved to be due to lightning, is that of Mr. Withering, in the "Transactions of the Philosophical Society of London for 1790." In the narrative as there given, a man, who had taken refuge beneath a tree at Aylesford, England, during a thunder-storm, was instantly killed and his clothing set fire from a flash of lightning, which first struck the upper portion of the tree and thence passed downward toward the ground. A portion of the electric fluid, after leaving the man's body, passed down a walking-stick held in his hand and thence to the ground, where it made a hole some two and a half inches in diameter. The first ten inches of this bole presented nothing worthy of remark; at this point, however, the fluid was found to have followed along for some eight inches the root of a tree which presented itself, and which, aside from a slight superficial blackening, was unharmed. Some closely adjacent quartz-pebbles did not, however, escape so easily, but were found with their corners and angles very considerably rounded by fusion. The hole was traced only to a depth of about eighteen inches, and no real tube is mentioned as having been discovered.
The next account which we have to notice is that given by Dr. Fiedler, who describes in detail the finding of fulgurites by Dr. Hentzen and others in the great sand-wastes of Paderborn, commonly called Senne. He also describes them as having been found in similar situations in Pillau, near Königsberg, in Prussia; at Nietleben, near Halle; and at Drigg, in Cumberland, England. In a subsequent paper, two years later,* he further describes other tubes found at Rheine, in the bishopric of Münster, in Prussia; the sand-hills of Regenstein, near Blankenburg, in the Harz; and near Bahia, in Brazil. Fiedler is followed in his turn by Gilbert, who gives a history of the finding of the fulgurite at Massel, as above noted, and also those of Bahia, in Brazil. An excellent résumé of the matter up to 1821 is given in the "Ann. de Chemie et de Physique" for that year, and also brief descriptions of the fulgurites formed on solid rock, as described by Saussure and Humboldt.
The tubes found in Cumberland were three in number, and were formed in a white and reddish quartz-sand, in which were a few pebbles of "hornstone porphyry." One of the tubes was followed down to a depth of twenty-nine feet, where it came in contact with a fragment of the porphyry, and glanced off at an angle of about forty-five degrees. The surface of the porphyry fragment was somewhat fused, forming an olive-green glass. Beyond the fragment, the tube resumed its vertical direction, but became weaker and easily broken. The caving in of the sand prevented exploration to a greater depth. One of these tubes was two-pronged, and the main branch was again divided into two, while small, lateral branches, two or three inches long, were given off at intervals.
The account of the occurrence and appearance of the tubes found at Massel, as given by Hermann in 1811, and as quoted by Gilbert, is as entertaining as it is inaccurate. He says: "The glass-like tube resembles molten glass or iron. It grows in yellow sand from the depth of the earth at Massel, on the south side of the Töpel Hill, and also in the Ellgutten wood, and on the high sand-hills close to the village of Klein Schweinern. The tube has sometimes the thickness of a finger or a thumb; at other times it is as thick as a quill-pen, and the deeper one goes, the thicker and stronger it is found(?). Its constituent parts are very soft when underground, but soon become hard by exposure to the air, and have the appearance of a gritty, ash, or iron-colored enamel, glisten like crystal at the point of fracture, give a clear, ringing sound, and cut glass. It is hollow, shines like glass, and has a reddish-brown color. It is not found near the surface, but only after digging several ells into the ground. In the months of May or June it naturally pushes upward, and crops out of the sand. This point afterward breaks off of its own accord, or is knocked off by the feet of passing people, cattle, or vehicles, by means of which many a beautiful piece is found"(?).
Passing by, for lack of space, the interesting accounts given by Darwin and Fiedler of fulgurites found at Maldonado, near the mouth of the La Plata, in South America, and in a vineyard on the right bank of the river Elbe, and simply noting the finding of similar tubes at Northfield Farms, Massachusetts, in 1861, we will mention the next recorded occurrence at Macclesfield, England, which, like that of Cumberland, is remarkable on account of the length of the tube found. This is described as three fourths of an inch in diameter at its upper end, and tapering gradually throughout its length to within three or four feet of its lower end, where it assumed a slanting direction, and then divided into several filaments or branches, and became dispersed and obliterated in the soft, spongy soil. This fulgurite was traced to a depth of twenty-two feet in a straight line, and gives us a good illustration of the immense heat and power of penetration of an electric discharge.
The occurrence described by Roemer, of twenty-five sets of fulgurite tubes, within a space of one hundred by two hundred yards in the great sand-flats of Starczynow in Poland, is, I believe, the most remarkable on record. The sand in this case was a very pure quartz-sand with a few pebbles of rolled flint ("Feuerstein").
The tubes were found with their upper ends exposed, owing to the blowing away of the loose sand, and varied in size from the thickness of one's arm to that of a wooden knitting-needle, while the thickness of the tube-wall varied from one to two millimetres, rarely more.
An article by the present writer gives a detailed description of fulgurites from Santa Rosa Island, Florida; Sumter, South Carolina; and Union Grove, Illinois, now in the collections of the National Museum.
Those from Santa Rosa were formed by the lightning striking a small pine-tree, and thence descending to the ground, where, at a distance of about forty feet from the trunk it formed a tube, which occurred as a crooked, irregular line along the surface. This was nearly pure glass, grayish in color, translucent, and very free from corrugations, though in some cases completely collapsed. The small fragments of the tube from Sumter, South Carolina, were found while digging a well at a depth of twenty feet below the surface. The tube-walls were very thick and strong, brownish and opaque. They lacked the corrugations, but had, externally, rather the knotted appearance compared by Gümbel to that of stag's horns. The great amount of material received from Union Grove, Illinois, shows this to be probably the most extensive find of fulgurites yet noted in this country. The locality where they occurred is the top of a sand-hill some fifty feet square. Several sets or pairs of tubes were found here, but a few inches apart, together with several small, irregular masses of fused material, the largest of which weighed several ounces. The largest tube found was about three and a half inches in diameter, but was too frail to remove. The accompanying plate shows the characteristic forms.
Fulgurite Tubes (natural size). (Reproduced from Proceedings of the United States National Museum, vol. ix, 1886.) Fig. 1, portion of tube, common form; Fig. 2, tube with bulb-like enlargement; Fig. 3, cross-section of tube, characteristic form; Fig. 4, holes fused by lightning in sheet-copper, and resembling fulgurite tubes in outline.
One of these was traced into the sand for a distance of about seven feet, and was found to increase in size slightly from above downward. They were frequently branched, and often sent out small, nearly flat horizontal branches or shoots, about one quarter of an inch wide, and half as thick, the greater diameter being horizontal. The hole in these was quite small, and closed abruptly at the end. Fig.2 is peculiarly striking, and seems quite different in form from any yet described. These "bulbs" are said to correspond with the stratification in the sand.
Concerning the microscopic and chemical properties of the fulgurite, little seems to have been done until within a comparatively recent period. In 1875, Harting examined some fulgurites found in acorn-field at Elspeet. He describes these as vitrified tubes, having a form scoriaceous, rough, and very irregular. Exteriorly they were coated with sand and carbonaceous particles, while interiorly they were of a white glass which was sometimes streaked with brown or black, a coloration which he judges to be due to distillation products from the vegetable matter in the sand. A complete chemical analysis of the fulgurite yielded results as follows: SiO2, 90,2 per cent; Al2O3, 0,9 per cent; Fe2O3, 0,7 per cent; CaO, 0,1 per cent; MgO, 0,5 per cent; K2O, 0,5 per cent; NaO, 0,6 per cent; insoluble in HCl, 0,9 per cent; carbonaceous matter, 5,6 per cent.
Discussing the results, the author argues that the alkalies and other bases, belonging to the vegetable matter, have been driven off by the heat of the flash, since the percentages shown are not greater than is contained by the sand itself. He also conceives that the presence of these bases may have aided in the reduction of the refractory silica—an idea which seems to have also been adopted by Gümbel and Wichmann. When, however, we consider the extraordinary brief duration of the flash and heat, this reaction seems scarcely possible.
Gümbel describes fulgurites from the Libyan Desert between Dachel and the Ammon Oasis, in which the tube-wall consists of an amorphous glass enveloping un fused quartz-kernels. A partial chemical analysis led him to conclude the interior lining to be a true quartz glass. To this conclusion Wichmann takes exceptions. This gentleman pulverized portions of the fulgurite tubes from Senner Heide, from Elspeet, and from Starczynow, and by means of a solution of great density succeeded in separating the glass of the fulgurite from the inclosed sand. Chemical analysis of this yielded silica as follows:
Thus proving that the glass was not a pure quartz glass. Owing to the small amount of material at his disposal, the other constituents could not be determined.
As viewed in their sections under the microscope, this glass was completely amorphous, showing only a few partially fused grains on the outer portion. There were no traces of crystallization products from the fused magma, the duration of the heat and the cooling following being too brief and too rapid for their productions. It was, however, filled with cavities of varying sizes, formed by the volatilization of the moisture in the sand at the time of the formation of the tube.
In the paper by the present writer, already referred to, the composition of both the fulgurite glass and the sand in which they formed is given as follows:
Thus proving conclusively that, in this case at least, the glass was not a pure quartz glass, although it showed itself to be richer in silica than the sand from which it originated—a result which, to say the least, was hardly expected.
Discussing the results of the above analyses, the writer concludes that the composition of the fulgurite glass is dependent entirely upon the conducting power of the various mineral constituents of the sand, regardless of their fusibility; that the glass, showing a larger proportion of silica than the sand in which it forms, points to the fusion of the siliceous (i. e., the quartz) grains, in preference to the feldspathic and ferruginous; hence, that the quartz-grains were poorer conductors of the electric fluid than were the others. This may, perhaps, be in part accounted for from the fact that the feldspar-grains were partially kaolinized, and hence held more moisture, which would render them better conductors.
It is not at all strange that at first many opinions prevailed regarding the nature and origin of the fulgurite tubes, and that some of these were peculiar in the extreme. Pastor Hermann seems to have gone farthest astray, for he says: "This growth (i. e., the fulgurite) is undoubtedly the product of a subterranean fire, whereby not only this tube is formed through melting sand, accedente viscoso quodam fucco, but also the two springs at Massel and Ellgutt, between which this tube is found, are warmed by the same fire." As already noted, he designated the fulgurites by the name of vitrified Osteocalla, or Ostecolla Masselsis, and proposed to utilize them for medicinal purposes, placing them in the same category as deer-horns, crab's eyes and corals, which at that time were considered as excellent for all kinds of "fevers, virulent and febrile diseases." The finding of the molten quartz directly in the track of the lightning at Aylesford, England, as described in the "Philosophical Journal" noted, was accepted as proof positive of the electric origin of fulgurites by Fiedler and most others. Nevertheless, Dr. Clark, of Cambridge, in one of his public lectures in the year 1816, took occasion to deny this method of origin regarding the fulgurites of Drigg, and contended that they were but concretionary forms lined interiorly with a mineral resembling hyalite or pearl sinter. Dr. Fiedler fully discusses all possible source of origin, including the probability of their being incrustations on roots, sinters, or other mineral products, or aggregates of ancient seaworms, and finally proves, apparently conclusively, their origin from electric fusion, an origin concerning which there can at present be no doubt.
The cause of the frequent occurrence of fulgurite in sandy plains, which seemingly present but little attractive force to the electric fluid, has been frequently discussed. Fiedler ascribed it to the fact that at certain depths below the surface there are little portions of water, and the tubes are produced by the passage of the fluid from the surface to these portions, where it becomes neutralized, Darwin, writing of the Maldonado fulgurites, thinks it probable that in that particular instance the flash divided into two or more branches before entering the ground, rather than that they were formed by several distinct discharges. To the present writer, the explanation suggested by Roemer, regarding the extraordinary find at Starczynow, seems most probable. Here it will be remembered that some twenty-five tubes were found in a space one hundred by two hundred yards. Roemer calls attention to the fact that these may have been formed at intervals of even hundreds of years; also to the equally important fact that the lightning, although striking with the same frequency in less exposed places, might fail to produce the tubes, owing to the character of the soil, or, if so produced, they would be obscured by leaves, soil, etc., instead of having their upper ends exposed by the drifting away of the sand.
The deeply corrugated, or winged, and otherwise peculiar form of the fulgurite tubes, has been a matter of some speculation. Darwin, as already noted, considered it to be due to the pressure of the sand acting while the tubes were still plastic—a view which has been in many cases adopted by subsequent writers. Fiedler and Harting, on the other hand, considered that the size and form of the tube, or, in other words, the shape of the bore of the lightning, was largely dependent upon the vapor engendered from the water in the sand at the time of the flash. Indeed, these authorities considered the presence of water in the sand as essential to the formation of the tube. Wichmann does not fully acquiesce in the view that the wings are produced by the collapsing of a portion of the tube-walls, but considers them as original formations.
That they are not due simply to collapsing is the opinion of the present writer. In the article quoted he there expresses the opinion that the irregular form of the tube near the surface is due to the exceeding energetic action of the current during this part of its course, and the lack of homogeneity in the conducting material. At depths of a few feet below the surface, where the force of the current had become to some extent reduced, and the sand was more compact and homogeneous, the tube was found more nearly cylindrical. It is very probable that steam may have been instrumental in producing the bulb-like enlargements so commonly found, but it can scarcely be considered as essential to the formation of the tube itself.
We have next to notice the fulgurites produced upon solid rock. These, as can readily be imagined, differ from those produced in the sand, in being of but slight depth, and frequently existing merely as a thin, glassy coating on the surface.
G. Rose describes fulgurites of this nature as occurring in abundance upon the summit of Little Ararat, in Armenia. The rock is an andesite, somewhat soft and porous, and it is stated that blocks a foot long can be obtained, perforated in all directions by the irregular tubes, from three to five centimetres in diameter, which are filled with a bottle-green glass, formed from the fused rock.
A small specimen of this rock, deposited in the National Museum, has much the appearance of a rock bored by the teredo, the holes in which have subsequently been filled by the green glass. It is stated by this writer that, in fulgurites collected by Humboldt from the Punta del Fraile, in Mexico, the fused mass of the walls had overflowed the tubes upon the surrounding surfaces. Saussure describes fulgurites as occurring also in the hornblendic schists of Mont Blanc, and Ramond mentions similar occurrences on Monts Perdu and Pic du Midi in the Pyrenees, as well as upon Mont Auvergne in France.
Wichmann examined the Ararat fulgurite glass with the microscope, and found it to agree closely with that of those formed in loose sand, being completely amorphous without trace of microlites, and in containing numerous steam cavities.
Dr. Gilbert describes the fulgurites found by Humboldt upon the high peaks of the Nevada de Toluca, in Mexico. The rock on which they were found is described as a trap porphyry taken from the summit of the Pic del Fraile at a height of 2,364 "toisen" (about 4,621 metres). Masses some two feet square are said to occur covered with a thin layer (about one half of a millimetre) of a pistacio-green glass. In one instance, where a feldspar crystal intervened, the glass was white. The peculiar shimmer of this glass in the sun led Humboldt to ascend this precipitous peak even at the risk of his life. In many places the rock was completely pierced by small cylindrical tubes, which were lined with the same greenish glass, and this was found to resemble closely the glass from the inner walls of the tubes found at the Senner Heide.
The most interesting and, so far as I am aware, the only investigation of this kind undertaken in America is that of Mr. J. S. Diller on fulgurite from Mount Thielson, in Oregon. The summit of this mountain is described as being composed of a hypersthene basalt, very precipitous and difficult of ascent. The fulgurite occurs both as a superficial coating on the rock, and in the form of tubes. "Although spread over a considerable surface, it is not evenly distributed, but is arranged in patches of drops and bubbles of glass in very much the same way as paint which has been put upon a greasy surface." The glass is described as translucent and of a greenish color. In places the tube penetrated the rock to a depth of a few inches, having a diameter of from 10,5 to 20 millimetres. These have a glassy lining of some two millimetres in thickness. The tubes are not regarded as having been produced by the lightning itself, but to be pre-existing tubes and cavities (the rock is naturally spongy), into which the fluid passed and fused the walls. The chemical composition of this glass was found to be silica, 55,04 per cent; alumina and iron, 28,99 per cent; lime, 7,86 per cent; magnesia, 5,85 per cent; potash and soda not determined; loss by ignition, 1,11 per cent; total, 98,85 per cent. This was also found to be practically the composition of the groundmass, or non-crystalline portion of the rock.
The fulgurite found on the summit of Dom du Gonte, one of the peaks forming a part of the chain of Mont Blanc, has been recently described by Rutley. The rock is represented as a hornblendic gneiss. The fused material formed merely a superficial coating in the form of attached globules or irregularly fused pellets and blotches of brownish, black, or white glass. The white glass resulting from the fusion of the feldspar, while the dark in like manner was of hornblendic origin. This occurrence of the two glasses, unmixed even when in actual contact, well illustrates the extreme rapidity of the whole proceeding, "the fused surface of each mineral having cooled almost exactly in situ." In nearly every other particular the observations made on the glass corresponded with those above noted.
In conclusion, it may be said that a careful examination of high, isolated mountain-peaks and areas of loose drifting sand will doubtless result in the discovery of many more of these interesting phenomena than have yet been reported.