actually a saturated solution of this gas, which escapes from it in such quantities in some parts of its surface that it has the appearance of being actually in ebullition.  I have found by experiment that the water taken from the most tranquil part of the lake, even after being agitated and exposed to the air, contained in solution more than its own volume of carbonic acid gas with a very small quantity of sulphuretted hydrogen, to the presence of which, I conclude, its ancient use in curing cutaneous disorders may be referred.  Its temperature, I ascertained, was in the winter in the warmest parts above 80 degrees of Fahrenheit, and it appears to be pretty constant, for I have found it differ a few degrees only, in the ascending source, in January, March, May, and the beginning of June; it is therefore supplied with heat from a subterraneous source, being nearly twenty degrees above the mean temperature of the atmosphere.  Kircher has detailed in his “Mundus Subterraneus” various wonders respecting this lake, most of which are unfounded, such as that it is unfathomable, that it has at the bottom the heat of boiling water, and that floating islands rise from the gulf which emits it.  It must certainly be very difficult, or even impossible, to fathom a source which rises with so much violence from a subterraneous excavation, and, at a time when chemistry had made small progress, it was easy to mistake the disengagement of carbonic acid for an actual ebullition.  The floating islands are real, but neither the Jesuit nor any of the writers who have since described this lake had a correct idea of their origin, which is exceedingly curious.  The high temperature of this water, and the quantity of carbonic acid that it contains, render it peculiarly fitted to afford a pabulum or nourishment to vegetable life.  The banks of travertine are everywhere covered with reeds, lichens, confervae, and various kinds of aquatic vegetables, and, at the same time that the process of vegetable life is going on, the crystallisations of the calcareous matter, which is everywhere deposited in consequence of the escape of carbonic acid, likewise proceed, giving a constant milkiness to what, from its tint, would otherwise be a blue fluid.  So rapid is the vegetation, owing to the decomposition of the carbonic acid, that, even in winter, masses of confervae and lichens, mixed with deposited travertine, are constantly detached by the currents of water from the bank and float down the stream, which being a considerable river is never without many of these small islands on its surface; they are sometimes only a few inches in size, and composed merely of dark-green confervae or purple or yellow lichens, but they are sometimes even of some feet in diameter, and contain seeds and various species of common water-plants, which are usually more or less encrusted with marble.  There is, I believe, no place in the world where there is a more striking example of the opposition or contrast