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

by Sir Humphry Davy

this agency; and an effect distinct in the pure atmosphere and temperate climate of Athens, must be upon a higher scale in the vicinity of other great European cities, where the consumption of fuel produces carbonic acid in large quantities. Metallic substances, such as iron, copper, bronze, brass, tin, and lead, whether they exist in stones, or are used for support or connection in buildings, are liable to be corroded by water holding in solution the principles of the atmosphere; and the rust and corrosion, which are made, poetically, qualities of time, depend upon the oxidating powers of water, which by supplying oxygen in a dissolved or condensed state enables the metals to form new combinations. All the vegetable substances, exposed to water and air, are liable to decay, and even the vapour in the air, attracted by wood, gradually reacts upon its fibres and assists decomposition, or enables its elements to take new arrangements. Hence it is that none of the roofs of ancient buildings more than a thousand years old remain, unless it be such as are constructed of stone, as those of the Pantheon of Rome and the tomb of Theodoric at Ravenna, the cupola of which is composed of a single block of marble. The pictures of the Greek masters, which were painted on the wood of the abies, or pine of the Mediterranean, likewise, as we are informed by Pliny, owed their destruction not to a change in the colours, not to the alteration of the calcareous ground on which they were painted, but to the decay of the tablets of wood on which the intonaco or stucco was laid. Amongst the substances employed in building, wood, iron, tin, and lead, are most liable to decay from the operation of water, then marble, when exposed to its influence in the fluid form; brass,

copper, granite, sienite, and porphyry are more durable. But in stones, much depends upon the peculiar nature of their constituent parts; when the feldspar of the granite rocks contains little alkali or calcareous earth, it is a very permanent stone; but, when in granite, porphyry, or sienite, either the feldspar contains much alkaline matter, or the mica, schorl, or hornblende much protoxide of iron, the action of water containing oxygen and carbonic acid on the ferruginous elements tends to produce the disintegration of the stone. The red granite, black sienite, and red porphyry of Egypt, which are seen at Rome in obelisks, columns, and sarcophagi, are amongst the most durable compound stones; but the grey granites of Corsica and Elba are extremely liable to undergo alteration: the feldspar contains much alkaline matter; and the mica and schorl, much protoxide of iron. A remarkable instance of the decay of granite may be seen in the Hanging Tower of Pisa; whilst the marble pillars in the basement remain scarcely altered, the granite ones have lost a considerable portion of their surface, which falls off continually in scales, and exhibits everywhere stains from the formation of peroxide of iron. The kaolin, or clay, used in most countries for the manufacture of fine porcelain or china, is generally produced from the feldspar of decomposing granite, in which the cause of decay is the dissolution and separation of the alkaline ingredients.

Eub.—I have seen serpentines, basalts, and lavas which internally were dark, and which from their weight, I should suppose, must contain oxide of iron, superficially brown or red, and decomposing. Undoubtedly this was from the action of water impregnated with air upon their ferruginous elements.

The Unknown.—You are perfectly right. There are few compound stones, possessing a considerable specific gravity, which are not liable to change from this cause; and oxide of iron amongst the metallic substances anciently known, is the most generally diffused in nature, and most concerned in the changes which take place on the surface of the globe. The chemical action of carbonic acid is so much connected with that of water, that it is scarcely possible to speak of them separately, as must be evident from what I have before said; but the same action which is exerted by the acid dissolved in water is likewise exerted by it in its elastic state, and in this case the facility with which the quantity is changed makes up for the difference of the degree of condensation. There is no reason to believe that the azote of the atmosphere has any considerable action in producing changes of the nature we are studying on the surface; the aqueous vapour, the oxygen and the carbonic acid gas, are, however, constantly in combined activity, and above all the oxygen. And, whilst water, uniting its effects with those of carbonic acid, tends to disintegrate the parts of stones, the oxygen acts upon vegetable matter. And this great chemical agent is at once necessary, in all the processes of life and in all those of decay, in which Nature, as it were, takes again to herself those instruments, organs, and powers, which had for a while been borrowed and employed for the purpose or the wants of the living principle. Almost everything effected by rapid combinations in combustion may also be effected gradually by the slow absorption of oxygen; and though the productions of the animal and vegetable kingdom are much more submitted to the power of

atmospheric agents than those of the mineral kingdom, yet, as in the instances which have just been mentioned, oxygen gradually destroys the equilibrium of the elements of stones, and tends to reduce into powder, to render fit for soils, even the hardest aggregates belonging to our globe. Electricity, as a chemical agent, may be considered not only as directly producing an infinite variety of changes, but likewise as influencing almost all which take place. There are not two substances on the surface of the globe that are not in different electrical relations to each other; and chemical attraction itself seems to be a peculiar form of the exhibition of electrical attraction; and wherever the atmosphere, or water, or any part of the surface of the earth gains accumulated electricity of a different kind from the contiguous surfaces, the tendency of this electricity is to produce new arrangements of the parts of these surfaces; thus a positively electrified cloud, acting even at a great distance on a moistened stone, tends to attract its oxygenous, or acidiform or acid, ingredients, and a negatively electrified cloud has the same effect upon its earthy, alkaline, or metallic matter. And the silent and slow operation of electricity is much more important in the economy of Nature than its grand and impressive operation in lightning and thunder. The chemical agencies of water and air are assisted by those of electricity; and their joint effects combined with those of gravitation and the mechanical ones I first described are sufficient to account for the results of time. But the physical powers of Nature in producing decay are assisted likewise by certain agencies or energies of organised beings. A polished surface of a building or a statue is no sooner made rough from the

causes that have been mentioned than the seeds of lichens and mosses, which are constantly floating in our atmosphere, make it a place of repose, grow, and increase, and from their death, their decay, and decomposition carbonaceous matter is produced, and at length a soil is formed, in which grass can fix its roots. In the crevices of walls, where this soil is washed down, even the seeds of trees grow, and, gradually as a building becomes more ruined, ivy and other parasitical plants cover it. Even the animal creation lends its aid in the process of destruction when man no longer labours for the conservation of his works. The fox burrows amongst ruins, bats and birds nestle in the cavities in walls, the snake and the lizard likewise make them their habitation. Insects act upon a smaller scale, but by their united energies sometimes produce great effect; the ant, by establishing her colony and forming her magazines, often saps the foundations of the strongest buildings, and the most insignificant creatures triumph, as it were, over the grandest works of man. Add to these sure and slow operations the devastations of war, the effects of the destructive zeal of bigotry, the predatory fury of barbarians seeking for concealed wealth under the foundations of buildings, and tearing from them every metallic substance, and it is rather to be wondered that any of the works of the great nations of antiquity are still in existence.

Phil.—Your view of the causes of devastation really is a melancholy one. Nor do I see any remedy; the most important causes will always operate. Yet, supposing the constant existence of a highly civilised people, the ravages of time might be repaired, and by defending the finest works of art from the external

atmosphere, their changes would be scarcely perceptible.

Eub.—I doubt much whether it is for the interests of a people that its public works should be of a durable kind. One of the great causes of the decline of the Roman Empire was that the people of the Republic and of the first empire left nothing for their posterity to do; aqueducts, temples, forums, everything was supplied, and there were no objects to awaken activity, no necessity to stimulate their inventive faculties, and hardly any wants to call forth their industry.

The Unknown.—At least, you must allow the importance of preserving objects of the fine arts. Almost everything we have worthy of admiration is owing to what has been preserved from the Greek school, and the nations who have not possessed these works or models have made little or no progress towards perfection. Nor does it seem that a mere imitation of Nature is sufficient to produce the beautiful or perfect; but the climate, the manners, customs, and dress of the people, its genius and taste, all co-operate. Such principles of conservation as Philalethes has referred to are obvious. No works of excellence ought to be exposed to the atmosphere, and it is a great object to preserve them in apartments of equable temperature and extremely dry. The roofs of magnificent buildings should be of materials not likely to be dissolved by water or changed by air. Many electrical conductors should be placed so as to prevent the slow or the rapid effects of atmospheric electricity. In painting, lapis lazuli or coloured hard glasses, in which the oxides are not liable to change, should be used, and should be laid on marble or stucco encased in stone, and no animal or