The Solfatara lake, situated about fourteen miles from Rome, on the road to Tivoli, is supplied with an unfailing stream of tepid water, impregnated with carbonic acid gas and saturated with carbonate of lime. The amount of carbonate of lime which the water is capable of holding in solution depends chiefly on three things: first, on the presence of carbonic acid; secondly, on the high temperature of the water; and thirdly, on its quantity. Now the carbonic acid is ever rising in bubbles to the surface and passing away; the temperature of the water is lowered by contact with the cooler atmosphere; and its quantity is diminished by evaporation. Thus the capacity which the water at first had for holding the carbonate of lime in solution is notably diminished, and a part of the lime is precipitated to the bottom in a solid form, or clings to the vegetable matter with which it comes in contact. A very simple and interesting experiment, made in the early part of the present century by Sir Humphrey Davy, will illustrate the rapidity with which the formation of solid stone is even now taking place. In the month of May he fixed a stick in the bed of the lake, and left it standing until the following April, when he found that it was covered with an incrustation of limestone several inches thick.[46] In precisely the same way new layers of Travertine are annually deposited in the bed of the lake, and incrusted on its rocky margin; and so the lake itself is becoming smaller and smaller from year to year. We are told that in the middle of the seventeenth century it was a mile in circuit, and now it is a little more than a quarter of a mile.[47] Here, therefore, we have an immense mass of compact limestone rock, built up by natural agents within the last two centuries. At Tivoli, about four miles beyond the Solfatara, and two miles from the quarries of Ponte Lucano, phenomena of the same kind are exhibited. The waters of the Anio, which are saturated with carbonate of lime, form incrustations of Travertine on the banks of the river; and at the celebrated falls, where the whole volume of the stream leaps at a bound from a height of three hundred and twenty feet, the most beautiful stalactites are formed by the foam.