VI. THE ELECTRICITY OF THE SMOKE AND ASHES.

Our ancestors could judge that a great amount of electricity was occasionally evolved in the smoke, from their observation of the lightning flashes that darted through the Vesuvian pine tree; but they had no proper instruments for ascertaining whether this evolution of electricity was constant or accidental, or what laws regulated its manifestations. My apparatus, with movable conductor, by which comparative observations of electric meteorology can be made, and the errors arising from dispersion corrected, supplied me with an easy method of studying the electricity evolved during eruptions.

I must begin by describing the bifilar electrometer, in order to explain the apparatus which I have named as above, "Apparechio a conduttore mobile."

A A (Plate VIa, Fig. 1) is a glass cylinder, the lower edge of which is ground, well varnished with gum lac, and let into a wooden base, B, furnished with three levelling screws. Through a sufficiently wide glass tube, a a, runs a copper rod covered with insulating mastic, having a little plate or cylindrical cavity of gilded brass at the top (Figs. 2 and 3), with two arms d d, d' d. In the plate a disc of aluminium, m, is suspended by means of two silk fibres, and to the disc a very fine aluminium wire is attached, f f', bent a little at the ends, as are the arms, d d, d' d. The disc has about three millimetres less diameter than the plate. The diameter of the plate may vary within certain limits, but I have found it convenient to make it eighteen millimetres. The glass tube, a a (Fig. 1), should descend below the base as much as it rises above it, that is three to four centimetres. The length of the index is about one decimetre.

The upper ends of the two silk fibres, by which the disc and index are suspended, are attached to the top of the glass tube, C, by a contrivance which permits a change in the distance between the two points of suspension, and a screw, p, is provided to raise and lower the disc with the index. At n, at the lower part of the tube, C, there is a kind of torsion micrometer, arranged so as to bring the index to the zero of the scale engraved on the graduated ring, B, which is formed of a strip of good paper pasted on the rim of a glass disc. The index must be placed at the zero of the scale, and must be some distance from the ends of the arms of the plate with which it is parallel. The plate is about three millimetres deep.

Having levelled the instrument, so as to render the disc concentric with the plate, and placed the index at zero, it is obvious that if an electric charge through the wire, h, reach the plate with the arms, it will electrify the disc and index: the disc will have the opposite electricity, and the extremities of the index will take the same electricity as the arms, and consequently the index will describe an arc more or less great. The motion of the index is sufficiently slow to allow the eye conveniently to follow it. Having traversed the first arc, which I call the impulsive one, the index returns, and, after only two oscillations, comes to rest at what I shall call the definite arc.

When the electric charges are of very brief duration, the impulsive arcs are within certain limits proportional to the tensions, and the ratio between the impulsive and definite arcs is expressed by the following equation:

α(β - α) / β = tang. ¹/₂ α

In which β is the impulsive arc and α the definite arc, showing that α comes out nearly equal to ¹/₂ β. In dry weather all goes perfectly within the limits of proportion, and I can tell whether, during the time in which the index traversed the impulsive arc, there were any dispersions and of what nature; for if the definite arc is not close to the limit of the impulsive arc, it is a sign of dispersions having taken place during the motions of the index. Every degree less in the definite arc denotes two degrees of loss for the impulsive arc; but as the index employs double the time traversing the definite as it does the impulsive arc, we may consider the loss of one equal to the loss of the other.

In excessively damp weather the index gives no definite arc, and it is necessary to resort to artificial heat in order to dry the insulators. The most simple means I know of is to hold the instrument over some hollow vessel, which, for the time, is converted into a stove by the introduction of a spirit lamp.

From Gauss's formula for the bifilar system of instruments of this class, we learn that the maximum sensitiveness of such instruments is given when the length of the suspending fibres is greatest, and the distance between them is smallest, with the weight of the movable or rotating member a minimum; and these elements being the same, the sensitiveness of the instruments is invariable.

To some electrometers, in order to avoid errors of parallax, a small telescope, with a micrometer wire, has been added; but, with a little practice, we can read accurately without this refinement. In order to obtain comparative measurements, it is necessary to select some given unit of tension. I have observed that by making a galvanic pile of copper, zinc and distilled water, and insulating it well, each pole has a tension which remains the same for many days, if the conditions of temperature and the moisture of the surrounding atmosphere are not very different. With thirty pairs of this pile, each element having twenty-five square centimetres of surface, I have on the electrometer a definite arc of 15°, with the temperature of the atmosphere at 20° C., and with the difference of 4° to 5° C. between the thermometers of the psychrometer of August's construction. The first observation was made twenty-four hours after mounting the pile. For unit of tension I took that which corresponded to a single pair, that is, the thirtieth part of the total tension. Other electrometers may be compared with one already properly adjusted, without always having recourse to the pile.

This done, let us see the arrangement of all the apparatus:

H H (Plate VIIa, Fig. 1) is the ceiling of a well-situated lofty room, with an opening, o o, at the upper part.

M M, a bracket or table fastened against the wall, about a metre distant from the ceiling, H H.

N N, a wooden platform for the observer.

A, the bifilar electrometer.

B, Bohnenberger's electroscope.

a a, a movable conductor formed of a brass rod 15 to 18 millimetres in diameter, insulated below by means of a glass rod, well varnished with gum lac, having a suspending pulley, c, and a wooden guide-rod underneath it, l, within the guiding tube, k. At the upper part of this conductor, a a, there is a sliding roof, b, which can be adjusted so as to prevent rain entering at the opening, o o. The conductor terminates in a disc made of a sheet of thin brass, d, 24 centimetres in diameter. Upon this disc, or even in place of it, we may use metallic points.

As a support to the conductor at the upper part, I have made use of a triangular ring, x, drawn at its full size in Fig. 2. The conductor passes between three springs, and the triangular ring is held in place by three silk cords, m m m. Their material should not be mixed with any cotton, and it may be advisable to saturate them with an alcoholic solution of gum lac.

f f f is a hempen cord, which is used to raise and lower the conductor.

i is a copper wire covered with silk, by means of which the triangular ring, x, and through that and its springs the conductor communicates with either the electrometer or the electroscope.

Quickly raising the conductor by pulling the cord, f, the index of the electrometer will describe a more or less large impulsive arc, and, after two oscillations, will stop at the definite arc. Having thus measured the electric tension of the air, and having lowered the conductor, I next place the wire, i, in communication with the electroscope, B, and by again raising the conductor, I ascertain whether the electricity be positive or negative. It is scarcely necessary to say that the conductor, when raised, gives electricity of the same nature as that prevailing at the moment in the atmosphere; and when lowered, manifests the opposite. In some conjunctures we must keep the conductor raised and in communication with the electroscope, in order to observe certain phenomena which I shall presently describe: this method I call observation with a fixed conductor.

I have also constructed a similar but portable apparatus for use on eruptive cones, when required.

Having given this description of the apparatus, it remains for me to relate the results obtained, especially on the occasion of the last eruption of Vesuvius.

The Observatory is distant, in a direct line from the central crater of Vesuvius, 2,380 metres, so that, when the smoke is copious, it is properly situated for the study of electricity, particularly when the wind inclines the pine-tree cloud in the direction of the Observatory, as frequently happened on the last occasion.

With smoke alone, without ashes, we obtained strong tensions of positive electricity; with ashes only, which sometimes fell while the smoke turned in the other direction, we had strong negative electricity; when the smoke inclined towards the Observatory, accompanied with ashes and lapilli, we had sometimes one kind of electricity, and sometimes the other, just as the smoke or the ashes predominated; and often with a "fixed conductor" we obtained negative electricity, and with a "movable conductor" positive electricity. In Naples, too, at the Meteorological Observatory attached to the University, my colleague, Professor Eugenio Semmola, observed negative electricity of strong tension whilst ashes were falling there in abundance. The tensions on this occasion were so strong as to equal those obtained at changes of weather or during storms (temporali), and, being beyond measure with a delicate electrometer, we marked them with the symbol ∞: the same phenomena were observed when lightnings flashed.

When there is but little smoke, it is necessary to approach the eruptive mouths with a portable apparatus, in order to observe those phenomena which, in great eruptions, may be studied from the Observatory itself.

The conditions under which (folgori) lightning flashes are seen from the cloud of smoke are, that it is conveying great abundance of ashes. In 1861, there were small flashes even from the line of eccentric mouths above Torre del Greco, although the smoke was not very great; and when these ceased to discharge, and the central crater became somewhat active, with a moderate amount of smoke but a great deal of ashes, small and frequent lightning flashes were observed in the twilight darting through the smoke, which was dark in colour. In 1850 the eruption was more vigorous, the smoke more abundant, and the ashes scarce, but the flashes were very rare. In 1855, 1858, and 1868, with a scanty supply of ashes and at intervals, no flashes were observed, and the electricity remained constantly positive. But having regard to the facts of antecedent eruptions, one sees that the flashes are always derived, from the midst of smoke accompanied with ashes and lapilli, which separate like rain from the rolling volumes of smoke, in the midst of which they were ejected.

But how can we account for the positive electricity of the smoke, and the negative electricity of the falling ashes? Without denying the probability that a part of the positive electricity depends upon the elevation of the smoke, as in the case of every other conductor we raise aloft, or with a jet of water sent from a vessel by compressed air, I think that the greater part of the electricity proceeds from the rapid condensation of vapours, which are changed from the gaseous condition into dense clouds; for even when the smoke issues tranquilly and does not rise, because carried away horizontally by the wind, it gives signs of positive electricity. From all my studies of atmospheric electricity, and from some experiments made specially, it follows that the condensation of vapours is the origin of this development of positive electricity.

The negative electricity of the falling ashes certainly arises from the fact itself of their fall; for if we place a metallic vessel full of ashes upon an elevated and well-situated terrace, while the atmospheric electricity is positive, and cause the ashes from the vessel to fall gradually into an insulated metallic cup, communicating with Bohnenberger's electroscope placed at three or four metres distance from the vessel, the electroscope will manifest negative electricity. If the upper vessel be insulated, and the ashes permitted to fall upon the ground, we shall obtain, from the vessel, positive electricity. The intensity of these electric manifestations depends (other things being equal) upon that predominant at the moment in the air; so that if the experiment be made while negative electricity prevails, the falling ashes will manifest positive electricity, the upper vessel then showing negative electricity. Now, as the ashes separate from the positively electrified smoke in order to approach the ground, which is negatively electrified, it follows that they must manifest negative electricity upon touching the ground, leaving the positive electricity in the smoke above. For this reason, the electric tension of the smoke is increased by the descent of the ashes and lapilli, so that discharges between the upper and lower part of the pine-tree cloud, or the surface of the crater, are rendered possible. Hence it follows that the flashes of lightning of Vesuvius play through the smoke, and with difficulty strike bodies upon the earth; and from this circumstance our ancestors believed the thunderbolts of Vesuvius to be harmless. However, if the smoke were very great, and driven by the force of the wind to some distance from the crater, with an abundant fall of ashes, it would be possible to have lightning flashes proceed from the smoke to the earth. I possess some documents which relate that, in 1631, thunderbolts fell upon the Church of Santa Maria del Arco, and other places on the coast of Sorrento.

After upwards of twenty years' study and observation of meteoric electricity, I am enabled to prove that atmospheric electricity is never manifested without rain, hail or snow, and that manifestations of light are always accompanied by thunder—manifestations of light (lampi), thunder and rain being most closely connected. We may have rain without manifestations of light, but never the latter without rain or hail. I cannot here repeat what I have demonstrated in other memoirs; I can only say that the lightnings of Vesuvius, erroneously believed to be not accompanied by thunder, are really not accompanied by rain, but are induced by the descent of ashes and lapilli.[6]