5. _Dubius Sermo_, in eight Books, published A.D. 67, towards the end of Nero's reign, when purely technical subjects alone could be treated without danger to an author. Cf. _N.H._ praef. 28, 'libellos quos de grammatica edidi.' 6. _A fine Aufidii Bassi_, in thirty-one Books. At what point Bassus' history ended and Pliny's began is not known: but the latter certainly dealt with the closing years of Nero's reign (_N.H._ ii. 199, 'anno Neronis principis supremo, sicut in rebus eius exposuimus'), as well as with the times of Vespasian and Titus (_N.H._ praef. 20, 'Vos omnes, patrem te fratremque diximus opere iusto, temporum nostrorum historiam orsi a fine Aufidii Bassi'). The work was completed in A.D. 77, but not published till after the author's death. His nephew says he wrote with scrupulous care: _Ep._ v. 8, 5, 'historias et quidem religiosissime scripsit.' The book was used by Tacitus (_Ann._ xiii. 20; xv. 53; _Hist._ iii. 28). 7. _Naturae Historiae_, in thirty-seven Books, is Pliny's only extant work. As he speaks of Titus as 'sexies consul,' the date of its presentation to him was A.D. 77. Book i. consists of a dedicatory epistle to Titus and a table of contents. The body of the work is arranged as follows: Book ii., the universe and the elements; iii.-vi., geography of Europe, Asia, and Africa; vii., anthropology and human physiology; viii.-xi., zoology; xii.-xix., botany; xx.-xxvii., the use of vegetable substances in medicine; xxviii.-xxxii., the use of animal substances in medicine; xxxiii.-xxxvii., mineralogy applied to medicine and the fine arts. This work, which was meant not for continuous perusal, but for consultation as a book of reference, contained twenty thousand facts; and its preparation involved the reading of about two thousand volumes by one hundred authors (see _N.H._ praef. 17). The extracts he had made from these sources Pliny bequeathed to his nephew in one hundred and sixty volumes. He makes a point of acknowledging his obligations to other writers (praef. 21, 'in his voluminibus auctorum nomina praetexui, est enim benignum ... et plenum ingenui pudoris fateri per quos profeceris'); cf. the lists of authorities, Roman and foreign, prefixed to the work. Such devotion to natural science was unusual in men of Pliny's class, and not generally appreciated; cf. xxii. 15, 'Plerisque ultro etiam irrisui sumus ista commentantes atque frivoli operis arguimur.' As a scientific writer Pliny fails because he is not an original investigator, and because he lacks the critical faculty. For his method of working see Plin. _Ep._ iii. 5. Politically, Pliny recognizes the necessity of the empire, but his heroes are old Romans such as Cincinnatus and Cato. His Roman and Italian feeling is intense: cf. xxxvii. 201, 'In toto orbe ... pulcherrima omnium est in rebusque merito principatum naturae obtinet Italia, rectrix parensque mundi altera.'