It is necessary also to be skilled in the good and bad properties of waters, for of all things water is of most use in every mode of regimen. It is necessary to know that the best water is wholly devoid of quality, as regards taste and smell, is most pleasant to drink and pure to the sight; and when it passes through the præcordia quickly one cannot find a better drink. But such as remains long there, and proves pungent to the stomach, which it swells up with wind and loads, is to be reckoned pernicious. Such waters are neither soon heated nor cooled, and those things which are boiled in them are slowly and improperly boiled. It is better therefore to prove such water by trial, which he who wishes may do by attending to the following characters: Those which run to the north, and from the sun, pass slowly through the stomach, and are indigestible; they are slowly heated and cooled. Those which are strained through any passage or soil to the east are soon cooled and soon heated, and are to be supposed to be very good. Those also are good which are colder in summer and warmer in winter. Some judge of waters by weight, counting that the best which is lightest. This, if joined to the circumstances already mentioned, may be deserving of consideration, but is not of itself a sufficient criterion of goodness. Rain water, as Hippocrates remarks, is the lightest, the sweetest, the most limpid, and the thinnest, because that which is lightest and thinnest is attracted by the sun, who draws such particles to him, not only from all other waters, but also from the sea, and from bodies. Hence also it is the most prone to putrefaction, as being composed of many different qualities. Let no one suppose that the water which is most prone to putrefaction is the worst, for susceptibility of change is rather a good than a bad property; so that if it has the other characteristics of the best water, and is prone to putrefaction, it is to be reckoned the finest possible. When beginning to turn it proves the cause of hoarseness, coughs, and roughness of speech, to those who drink it. Of rain waters that which falls in summer and during thunder is preferable to that which falls in stormy weather. That from ice and snow is the worst, for during congelation the finest particles of the water are squeezed out. But waters which contain impurities, have a fetid smell, or any bad quality, may be so improved by boiling as to be fit to be drunk; or, by mixing them with wine, adding the astringent to that which is sweeter, and the others to the more austere. Some kinds of water it may be expedient to strain, such as the marshy, saltish, and bituminous. Those which are very cold are to be drunk after food, and not in great quantity. Some also have discovered certain articles of food and drink for correcting the bad properties of waters. Thus some beforehand drink of the decoction of chick-peas, or eat them; others, wild carrots boiled with some small fish, and fennel