The enormous Wien of lightning which accompany volcanic erup tions, issuing from the ascending column of steam and solid ejections, and which are equally characteristic of eruptions originally subabrial, when they rise into the atmosphere, have been celebrated from the time of Pliny ; but they had not been Recounted for in any satisfactory manner, having only been vaguely referred to an evolution of electricity connected with the intense actions of various kinds taking place in the eruption. After Mr. (now Sir William 0.) Armstrong had observed the electricity of effluent steam, Mr. Brayley pointed out the precise analogy of the sparks given by it. to this volcanic lightning. (' Phil. Mag., Series 8, vol. xviii. p. 94.) Dr. Faraday subsequently ascertained that the electricity of effluent steam is produced, in reality, by the friction of the globules of water, resulting from its partial condensation, and of particles of foreign matter, both merely driven onward by the steam as a mechanical agent, upon the substance of the tube from which it issues. In the eruption of a volcano we have all those elements in perfection, and on an immense scale, the foreign particles being supplied by the volcanic) ashes, and we may therefore conclude, that the enor mous evolution of electricity accompanying it arises from their friction, as in the hydro-electrio machine. It is probably the greatest example in nature of the production of frictional electricity Mass of Volcanic Products.—Now the first thing which arrests the attention in regard to the circumstances which accompany the products of volcanic eruptions, is the enormous mass of materials ejected at particular points. In 48 hours, in 1538, the Monte Nuevo, 440 feet high and 8000 feet in circumference, was thrown up in a place which may be regarded as a new vent of the Neapolitan volcanic region. Iu 1759 a new vent was opened west of Mexico, a new volcanic mountain (Jorullo) was thrown up to the height of 1695 feet, and an area of three or four miles was covered with its lavas. (See coL 670.) Between July and August, in 1831, Graham's Island had been raised from the sea-bed, 100 fathoms deep, to a height of 107 feet above the sea, with a circumference of 3240 feet ; in September its height was 100 to 230 feet, and its circumference 2300. In the winter of 1831.2 the whole vast heap of ashes had been dispersed by the waves, and nothing remained of this abort-lived volcano but a dangerous shoal. Subse quently this has been lowered, and a comparatively alight elevation above the average level of the neighbimring sea-bed is now under deep water. The lava currents from many volcanoes are of the same gigan tic proportions. In 1737 Vesuvius poured forth 33,587,058 cubic feet ; in 1794, 46,098,766 cubic feet ; and /Etna, in 1669, gave forth 93,838,950 cubic feet. which would make a considerable hill ; for it would cover a space of ground one-quarter of a mile across with a conical mound 130 feet high. The accumulated effects of two years' eruptions of Skiptaa Jokul, in Iceland, appear to have filled valleys and lakes and broad plains with floods of melted rock. The lava is said to have flowed in one direction 50, and in another 40, miles, with breadths of 15 and 7 miles respectively, and with a depth averaging about 100 feet, but in places reaching 600 feet. If these data have any claim to be regarded as fair approximations (they are so regarded by Lyell and other writers), the mass of lava poured out in two years by this modern volcano exceeds a hundredfold that of the Plutonic rocks which appear in the chain of the Malvern Hills. It would cover all the coal-fields of the British Islands with a plateau of basaltic rock 20 feet thick, or bury London under a mountain rivalling the cone of Teneriffe. In the eruptions of Tomboro, in Sumbawa, in 1815, ashes and scoria) were thrown out sufficient to form three mountains equal to Mont Blanc, or to cover the whole of Germany two feet deep, The volume of muddy and watery eruptions from volcanoes can seldom be accurately mea sured. Humboldt speaks of mud eruptions, called " Moya," as frequent in the volcanic system of the Andes, and they are abundant enough to fill valleyti and atop the channels of rivers.
From such data as can be collected there appears no sign of any general decay in the magnitude of the volcanic eruptions taken gene rally, though in respect to any particular volcano the contrary may be inferred.
Eruptive Purees.—If the quantity of matter ejected by volcanoes be taken as a measure of the amount of unbalanced pressure which required and obtained relief, the force with which it was ejected may be regarded as a measure of the intensity of this pressure. Accurate observations on this point are needed. If, as recorded by Sir W. Hamilton, stones were thrown so high above Vesuvius as to occupy 11 seconds of time in falling to the level of the crater, this gives an upward velocity of 350 feet in a second at the level of the crater, and a height of about 2000 feet ; but the mountain being above 3000 feet high, we must estimate the pressure at the level of the sea as competent to sustain a column of matter of the ordinary weight of lava (say twice and a half that of water) nearly a mile in height, This would equal the pressure of between 300 and 400 atmospheres.
Lava which had flowed in 1798, was traced by Humboldt to the summit of the Peak of Teneriffe, and must therefore have been sus tained (unless the lava were, as is probable, of a lighter kind) by double the pressure. These pressures appear great, but in no degree improbable if judged by the well-known effects of steam. A tempera ture of 800° Fahr. would give the steam pressure for a height 2000 feet above the cone of Vesuvius ; and so rapidly does this power augment with additional heat, that less than 100u° Fahr. may be sufficient to give steam a force equal to balance the whole column of lava in the Peak of Teneriffe. Now these are temperatures which appear to fall within the observed heats of some of the lava currents, for these have been found to melt silver and to perform heating effects greater than those of red-hot iron. Steam-power, generated by the admission of water to the hot interior parts of the earth, appears entirely adequate to the "eruptive forces" actually witnessed in volcanoes. It is much in favour of this being really the agency employed, that we find in explosive eruptions such considerable bodies of aqueous vapour erupted during most parts of the paroxysm ; that some eruptions have yielded little else than steam, and others chiefly hot water, Moreover, on considering attentively the distribution of volcanoes over the globe, we find the active volcanoes most frequently by the side of the sea, or by other considerable bodies of water ; and the extinct volcanoes in the vicinity of ancient lakes, or desiccated branches of the ancient ocean. Why they should be in the immediate proximity of the ocean or of lakes, will appear in the sequel. But while such proximity undoubtedly facilitates the operation of water in volcanic phenomena, that operation appears not to depend upon it ; volcanoes may receive water on account of their vicinity to it, but that vicinity is not occasioned by the necessary agency of water in their eruptions.
The general type of a volcanic eruption appears to be as follows : The ground is rocked by frequent earthquakes; special movements and noises happen in and about the volcanic mountain ; clouds of steam rise from the crater, followed and mixed with showers of ashes and scoriae driven up by the exploding and expanding vapour ; the tube of the crater becomes filled by melted, or at least flowing matter, which undulates upward and downward with the irregular pressure of the vaporous or gaseous matter ; these burst in large bubbles through it, scattering it into granular dust and ashes, till the lava overtops or breaks through the loose conical walls of the crater, and flows abun dantly. so as partially or wholly to relieve for a time the unbalanced internal pressure.
Volcanic Products.—The substances thrown out during volcanic eruptions, whether stony, liquid, or gaseous. disclose more or less com pletely the nature and condition of the interior masses of the globe, at depths greatly exceeding the dimensions of the greatest volcano or mountain known, but still very small in comparison to the earth's radius, and belonging to the mere outer crust of the globe. The lava or melted rock is generally referrible to a very small number of aggregations, in which felspar, augite (or hornblende), and oxide of iron are the most important ingredients, the mass being modified by additional minerals, as leucite, idocraae, olivine, garnet, epidote, atilbite, heulandite, and many others. Combinations of sulphur, and of uranium, copper, lead, arsenic, and manganese, also occur in various proportions ; but these metallic bodice do not play an important part in volcanic) phenomena. The so-called ashes and seorio3 consist of the same substances as the lava, the most prolific, repositories of the rarer minerals being always in cavities of the lava or scoriform aggregations. [The characters of these mineral substances have been described under their respective names in NAT. HIST. Div.] In these particulars modem lava will bear comparison with ancient Plutonic rocks, for they are composed of similar mineral aggregates, modified by many of the same rarer crystallisations, which .mostly occur in the cavities of their mass. The difference of most importauce between Plutonic rocks (granite, &o.) and volcanic) rocks (trachyte, dm.) is in the degree of their consolidation ; and this difference appears quite intelligible by a comparison of the various appearance and character of lava which has cooled and become solid under different circumstances. Lava cooled in air under slight pressure is often cellular; cooled under the pressure of water (as in the case of the current which passed through Torre del Greco into the sea), it is more compact ; when vitreous, and much distended by steam, it becomes vesicular pumice. We may therefore believe that lavas which remain and grow solid under great pressure about the internal base of the volcano are of a more dense nature than those which come to the surface, and may thus closely resemble, or be even identical with, some of the older Plutonic rocks, which thus regarded, and from other evidence, appear to be in fact unerupted lavas.