Since Hildegard lived on the Nahe or Rhine all her life she must indeed have been absorbed in her visions and monastic life not to have learned in which direction a river flows; and perhaps we should supply the explanation, which she certainly does not expressly give in the _Subtilitates_, that the sea feeds the rivers by evaporation or through subterranean passages. Perhaps a passage in the _Causae et curae_ may be taken as a correction or explanation of the preceding assertions, in which case that work would seem to be of later date than the _Subtilitates_. In it too Hildegard states that “springs and rivers” which “flow from the sea” are better in the east than in the west, but her next sentence straightway adds that they are salt and leave a salt deposit on the sands where they flow which is medicinal.[360] The waters rising from the southern sea are also spoken of by her as salt.[361] Even in the _Causae et curae_ she speaks of the water of the great sea which surrounds the world as forming a sort of flank to the waters above the firmament.[362] [Sidenote: Suggestions concerning drinking-water.] On the subject of whether waters are wholesome to drink or not Hildegard comes a trifle nearer the truth and somewhat reminds us of the discussions of the same subject in Pliny and Vitruvius.[363] She says that swamp water should always be boiled,[364] that well water is better to drink than spring-water and spring-water than river water, which should be boiled and allowed to cool before drinking;[365] that rain-water is inferior to spring-water[366] and that drinking snow-water is dangerous to the health.[367] The salt waters of the west she regards as too turbid, while the fresh waters of the west are not warmed sufficiently by the sun and should be boiled and allowed to cool before using.[368] The salt waters arising from the south sea are venomous from the presence in them of worms and small animals. Southern fresh waters have been purged by the heat, but make the flesh of men fatty and of black color.[369] Hildegard is not the first author to advise the boiling of drinking-water,[370] but she certainly lays great stress on this point. [Sidenote: The devil as the negative principle.]