Consolations in Travel; or, the Last Days of a Philosopher

by Sir Humphry Davy

DIALOGUE THE SIXTH. POLA, OR TIME.

During our stay in Illyria, I made an excursion by water with the Unknown, my preserver, now become my friend, and Eubathes, to Pola, in Istria. We entered the harbour of Pola in a felucca when the sun was setting; and I know no scene more splendid than the amphitheatre seen from the sea in this light. It appears not as a building in ruins, but like a newly erected work, and the reflection of the colours of its brilliant marble and beautiful forms seen upon the calm surface of the waters gave to it a double effect—that of a glorious production of art and of a magnificent picture. We examined with pleasure the remains of the arch of Augustus and the temple, very perfect monuments of imperial grandeur. But the splendid exterior of the amphitheatre was not in harmony with the bare and naked walls of the interior; there were none of those durable and grand seats of marble, such as adorn the amphitheatre of Verona, from which it is probable that the whole of the arena and conveniences for the spectators had been constructed of wood. Their total disappearance led us to reflect upon the causes of the destruction of so many of the works of the older nations. I said, in our metaphysical abstractions, we refer the changes, the destruction of material forms, to time, but there must be physical laws in Nature by which they are produced; and I begged our new friend to give us some ideas on this subject in his character of chemical philosopher. If human science, I said, has discovered

the principle of the decay of things, it is possible that human art may supply means of conservation, and bestow immortality on some of the works which appear destined by their perfection for future ages.

The Unknown.—I shall willingly communicate to you my views of the operation of time, philosophically considered. A great philosopher has said, man can in no other way command Nature but in obeying her laws; and, in these laws, the principle of change is a principle of life; without decay, there can be no reproduction; and everything belonging to the earth, whether in its primitive state, or modified by human hands, is submitted to certain and immutable laws of destruction, as permanent and universal as those which produce the planetary motions. The property which, as far as our experience extends, universally belongs to matter, gravitation, is the first and most general cause of change in our terrestrial system; and, whilst it preserves the great mass of the globe in a uniform state, its influence is continually producing alterations upon the surface. The water, raised in vapour by the solar heat, is precipitated by the cool air in the atmosphere; it is carried down by gravitation to the surface, and gains its mechanical force from this law. Whatever is elevated above the superfices by the powers of vegetation or animal life, or by the efforts of man, by gravitation constantly tends to the common centre of attraction; and the great reason of the duration of the pyramid above all other forms is, that it is most fitted to resist the force of gravitation. The arch, the pillar, and all perpendicular constructions, are liable to fall when a degradation from chemical or mechanical causes takes place in their inferior parts. The forms upon

the surface of the globe are preserved from the influence of gravitation by the attraction of cohesion, or by chemical attraction; but if their parts had freedom of motion, they would all be levelled by this power, gravitation, and the globe would appear as a plane and smooth oblate spheroid, flattened at the poles. The attraction of cohesion or chemical attraction, in its most energetic state, is not liable to be destroyed by gravitation; this power only assists the agencies of other causes of degradation. Attraction, of whatever kind, tends, as it were, to produce rest—a sort of eternal sleep in Nature. The great antagonist power is heat. By the influence of the sun the globe is exposed to great varieties of temperature; an addition of heat expands bodies, and an abstraction of heat causes them to contract; by variation of heat, certain kinds of matter are rendered fluid, or elastic, and changes from fluids into solids, or from solids or fluids into elastic substances, and vice versâ, are produced; and all these phenomena are connected with alterations tending to the decay or destruction of bodies. It is not probable that the mere contraction or expansion of a solid, from the subtraction or addition of heat, tends to loosen its parts; but if water exists in these parts, then its expansion, either in becoming vapour or ice, tends not only to diminish their cohesion, but to break them into fragments. There is, you know, a very remarkable property of water—its expansion by cooling, and at the time of becoming ice—and this is a great cause of destruction in the northern climates; for where ice forms in the crevices or cavities of stones, or when water which has penetrated into cement freezes, its expansion acts with the force of the lever or the

screw in destroying or separating the parts of bodies. The mechanical powers of water, as rain, hail, or snow, in descending from the atmosphere, are not entirely without effect; for in acting upon the projections of solids, drops of water or particles of snow, and still more of hail, have a power of abrasion, and a very soft substance, from its mass assisting gravitation, may break a much harder one. The glacier, by its motion, grinds into powder the surface of the granite rock; and the Alpine torrents, that have their origin under glaciers, are always turbid, from the destruction of the rocks on which the glacier is formed. The effect of a torrent in deepening its bed will explain the mechanical agency of fluid-water, though this effect is infinitely increased, and sometimes almost entirely dependent, upon the solid matters which are carried down by it. An angular fragment of stone in the course of ages moved in the cavity of a rock makes a deep round excavation, and is worn itself into a spherical form. A torrent of rain flowing down the side of a building carries with it the silicious dust, or sand, or matter which the wind has deposited there, and acts upon a scale infinitely more minute, but according to the same law. The buildings of ancient Rome have not only been liable to the constant operation of the rain-courses, or minute torrents produced by rains, but even the Tiber, swollen with floods of the Sabine mountains and the Apennines, has often entered into the city, and a winter seldom passes away in which the area of the Pantheon has not been filled with water, and the reflection of the cupola seen in a smooth lake below. The monuments of Egypt are perhaps the most ancient and permanent of those

belonging to the earth, and in that country rain is almost unknown. And all the causes of degradation connected with the agency of water act more in the temperate climates than in the hot ones, and most of all in those countries where the inequalities of temperature are greatest. The mechanical effects of air are principally in the action of winds in assisting the operation of gravitation, and in abrading by dust, sand, stones, and atmospheric water. These effects, unless it be in the case of a building blown down by a tempest, are imperceptible in days, or even years; yet a gentle current of air carrying the silicious sand of the desert, or the dust of a road for ages against the face of a structure, must ultimately tend to injure it, for with infinite or unlimited duration, an extremely small cause will produce a very great effect. The mechanical agency of electricity is very limited; the effects of lightning have, however, been witnessed, even in some of the great monuments of antiquity, the Colosæum at Rome, for instance; and only last year, in a violent thunderstorm, some of the marble, I have been informed, was struck from the top of one of the arches in this building, and a perpendicular rent made, of some feet in diameter. But the chemical effects of electricity, though excessively slow and gradual, yet are much more efficient in the great work of destruction. It is to the general chemical doctrines of the changes produced by this powerful agent that I must now direct your especial attention.

Eub.—Would not the consideration of the subject have been more distinct, and your explanations of the phenomena more simple, had you commenced by dividing the causes of change into mechanical and chemical;

if you had first considered them separately, and then their joint effects?

The Unknown.—The order I have adopted is not very remote from this. But I was perhaps wrong in treating first of the agency of gravitation, which owes almost all its powers to the operation of other causes. In consequence of your hint, I shall alter my plan a little, and consider first the chemical agency of water, then that of air, and lastly that of electricity. In every species of chemical change, temperature is concerned. But unless the results of volcanoes and earthquakes be directly referred to this power, it has no chemical effect in relation to the changes ascribed to time simply considered as heat, but its operations, which are the most important belonging to the terrestrial cycle of changes, are blended with, or bring into activity, those of other agents. One of the most distinct and destructive agencies of water depends upon its solvent powers, which are usually greatest when its temperature is highest. Water is capable of dissolving, in larger or smaller proportions, most compound bodies, and the calcareous and alkaline elements of stones are particularly liable to this kind of operation. When water holds in solution carbonic acid, which is always the case when it is precipitated from the atmosphere, its power of dissolving carbonate of lime is very much increased, and in the neighbourhood of great cities, where the atmosphere contains a large proportion of this principle, the solvent powers of rain upon the marble exposed to it must be greatest. Whoever examines the marble statues in the British Museum, which have been removed from the exterior of the Parthenon, will be convinced that they have suffered from