After mentioning and dismissing a grotesque suggestion of Diogenes of Apollonia, Seneca suddenly drops the discussion of the Nile and passes on to the subject of hail. It is obvious that there is here a serious gap in the text. It is not probable that he meant to leave off his examination into the probable sources of the Nile without stating his own view of a matter which had been so long the subject of wonder and debate. Either, therefore, he never completed this section of his treatise, or a portion of the work has been lost. The remainder of Book IV. is taken up with a desultory discussion of the subjects of hail and snow, written when the author must have been in a somewhat frivolous mood. He begins by telling Lucilius that if he were to assert that hail is produced as ice is with us, a whole cloud being frozen, he would be rather audacious. So he will imitate the chroniclers, who after they have told a great many lies, refuse to be responsible for some one statement, and refer for its truth to the authorities. If, therefore, his friend doubts his word, he will call in Posidonius, who will tell him that hail is formed from a watery cloud just turned into liquid. No teacher is needed to explain why pellets of hail are round, for all drops take that shape. Hail is nothing else than suspended ice, and snow is suspended hoar-frost. In this light vein Seneca thinks he has finished the subject and might dismiss it, but he cannot resist the temptation to continue the persiflage a little further. He quotes in a bantering style some of the opinions of his brother Stoics, and after this long preamble begins an inquiry into the distribution of density and temperature in the atmosphere. It would have been interesting had he seriously and fully stated what was known or surmised on this last topic, but he dismisses it in three short chapters. We learn from these that he regarded the air to be densest next the earth, and that as all things retain heat better the denser and more compact they are, so the air becomes less warm in proportion to its height (184).[116] The opinion of some persons, that the air on mountain summits ought to be warmer because they are nearer the sun, is sagaciously controverted, and the insignificance of all inequalities on the surface of the earth in comparison with the distance from the earth to the sun is forcibly expressed and illustrated. [116] This view hardly agrees with what is expressed in Book II. (60, 61), but it more accurately expresses the fact. The subject of snow and hail is briefly reintroduced at the end of the Book, probably for the purpose of affording a convenient introduction to the invective against luxury which fills the concluding chapter. The preservation of snow in ice-houses, and its use in the reparation of jaded appetites by cooling drinks, calls forth a denunciation of the young rakes of his day, which closes the discussion. BOOK V