The best kind of air is that which is perfectly pure, such as that which is not defiled with the exhalation from lakes or marshes, nor from any pit which emits pestilential vapours. That also which is impregnated with the exhalations from a canal conveying the impurities of a city is deleterious, and indeed every kind which is loaded with vapours is not good; as also that which is contained within any hollow place, shut up on all sides by high mountains, and not admitting of ventilation. Those, therefore, which are thus deleterious, prove hurtful to all ages and temperaments, whereas the best kinds agree with all. But their differences as to quality—I mean heat, coldness, dryness, and humidity—have not the same effect upon all. Those of a proper temperament are benefited by temperate air, whereas those labouring under an intemperament derive benefit from the opposite kind. * * * * * COMMENTARY. The first part of this Section is taken from Oribasius. (Synops. i, 24.) The remaining part will be found in the ‘Med. Collect.’ (ix, 1), where the subject is treated of at great length. See, also, Aëtius (iii, 162.) All these authors, however, are much indebted to Galen. (De Sanitate tuendâ, i.) According to Galen, the moon produces changes in the atmosphere, whereby she occasions putrefaction of dead bodies, impairs the colour, and hurts the head of living persons. (De Diebus Criticis, iii.) On the supposed causes why the moon occasions putrefaction, see Plutarch (Synops. iii.)