The river water has no longer the power which it once possessed of depositing the travertine which we find lying in thick beds upon the slopes of some of the hills of Rome, and from which the larger ruins are all built. This travertine is formed from carbonate of lime which the waters take up as they pass through the soil containing it. In order to give the water the power of holding this carbonate of lime in solution, a certain quantity of carbonic acid gas must be present in it. When by means of the rapid movement of the water or from other causes this gas becomes disengaged, it leaves the carbonate of lime behind in the shape of a hard stony deposit. This natural process of petrifaction is familiar to all who have seen the Falls of the Anio at Tivoli, and the way in which the artificial canals of running water in that neighbourhood are choked by limestone concretions, and it may be seen in all vessels made use of to boil water which is impregnated with lime. The more violent the agitation of the water the more rapid is the disengagement of the carbonic acid gas, and the consequent settlement of the lime. This process is accompanied, in most places where it can be seen, by the presence of sulphuretted hydrogen, which produces a white colour in the water by depositing the sediment called gesso by the Italians. Hence an explanation of the ancient name of Albula given to the Tiber is easy. In the period when the Tiber had the power of depositing travertine, its waters were much more strongly impregnated not only with carbonate of lime, but also with gesso, which gave a white tinge to the water as it now does to the sulphureous waters near Tivoli. The same colour was characteristic of “the white Nar, with its sulphureous stream,” Virgil’s description of the chief stream of the central Apennines. [Sidenote: Climate.] The subject of the climate of Rome is naturally connected with that of the nature of the soil and configuration of the hills and valleys. It is not difficult to see why the peculiar geological formation of the Campagna proves, without careful drainage, extremely deleterious to health. We have there a district containing numerous closed valleys and depressions in the soil without outlet for the waters which naturally accumulate. The tufa which composes the surface seems commonly to take the shape of isolated hills with irregular hollows between them, so as to impede the formation of natural watercourses. Under this tufa is a quantity of marl and stiff clay, which retains the water after it has filtered through the tufa, and sends it oozing out into the lower parts of the country, where it accumulates, and, mixed with putrescent vegetable matter, taints the surrounding atmosphere. A want of movement in the air caused by the mountainous barriers by which the Campagna is enclosed is another source of malaria.