Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/255

194 mixed with chalk, clay, marl, alabaster, quartz, slate, spar, and other rocks. These sulphurets being frequently moistened with water become decomposed, and produce the phenomena; for modern chemistry has taught us, that water contains $85⁄100$ of oxygen gas (pure air), combined with $15⁄100$ of hydrogen (inflammable air). The first of these gases is also combined in air which consists of $27⁄100$ of oxygen, and $73⁄100$ of nitrogen (that is not vital), and $2⁄100$ of carbonic acid gas; but this is not always the proportion, for reasons too complex to mention. Oxygen gas is necessary to combustion, and from what we have said, it is easily understood how it can exist in the interior of volcanoes, and in the depths of the earth, whither air and water can penetrate by the many known abysses on the globe. Alum also, and other salts, contain oxygen in great abundance. Now it is evident, that earthquakes and volcanoes are closely connected, or, in other words, that subterraneous or pre-existent fire, whether kindled by the combinations already mentioned, or by similar ones, may cause the earthquake. For let us suppose a decomposition of substances, not on a small scale, as in the proposed experiment, but to an immense extent, including that of water into oxygen and hydrogen; the necessary consequence is the development of aëriform fluids, which from their nature are incapable of being confined by the pressure of the earth, and in order to escape from imprisonment, exert all their power against the surrounding obstacles. Hence the earthquake.

To this we must add, that electric fluid, of which the earth is a vast reservoir, (in fact, man and the electric machine alone do not furnish us with electric phenomena,) acts in these cases like lightning and thunder in atmospheric regions. Among the ancients, Pliny perceived the resemblance between the tremor of the earth and the atmosphere. Even prior to him it had been observed by Epicurus, by whom fire was enumerated as one of the probable causes of earthquakes, (as we collect from Seneca's valuable book of Natural Questions). A fire, he says, fulmini similis, capable of throwing down and overturning all obstacles, magna strage obstantium. This is confirmed by the fact, that many earthquakes have been felt at very nearly the same instant, in places hundreds and perhaps thousands of miles distant from each