The various kinds of taste possessed by natural waters are then discussed, and some marvellous illustrations are given of their effects. Allusion is made to medicinal springs, to petrifying waters, to some with extraordinary dyeing properties, and to others with neither taste nor smell, but rapidly fatal to the drinker by immediately hardening and binding the intestines. Reference is also included to certain kinds of springs, of which the volcanic tracts of Italy supply good examples. Such were those which killed visitors who peered down into the caverns where their waters lurk, and suffocated birds that flew over them. Doubtless many tales were told of the effects of such emanations of carbonic acid gas, like that of the Grotto del Cane which, near Naples, still preserves their classic reputation (134, 261). Again, the same volcanic districts furnished instances of warm, sometimes even boiling, springs, and in alluding to them the author quotes the opinion of Empedocles, who was doubtless familiar with them in Sicily. To complete his record of marvels, the author cites some lakes on which islands float to and fro, of which good illustrations, due to a matted growth of vegetation, were then well known in the Vadimonian Lake (Lago di Bassano),[113] and he mentions other lakes in which he had equal faith, with water so heavy that brickbats would float upon it, and nothing, however heavy, not even hard solid stones, would go to the bottom. [113] Pliny, _Hist. Nat._ ii. 96. Pliny the Younger, _Epist._ viii. 20. Seneca is inclined to agree with some philosophers that certain rivers of peculiar and inexplicable character were created along with the world, and he specially cites the Danube and Nile as examples, these vast streams being too remarkable to have had the same origin as other rivers. Accordingly he reserves the Nile for consideration in a later part of his volume (166). There is another kind of water which, with his Stoic brethren, he places at the beginning of the world--the great ocean and every sea that flows from it between the lands. Yet he found no place in any part of the treatise for a discussion of the phenomena of the ocean.