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 Colosaeum 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?