Cassius Dio Cocceianus, Roman senator and praetor, when about forty years of age delivered himself of a pamphlet describing the dreams and omens that had led the general Septimius Severus to hope for the imperial office which he actually secured. One evening there came to the author a note of thanks from the prince; and the temporary satisfaction of the recipient was continued in his dreams, wherein his guiding angel seemed to urge him to write a detailed account of the reign of the unworthy Commodus (Book Seventy-two), just ended. Once again did Dio glow beneath the imperial felicitations and those of the public. Inoculated with the bacillus of publication and animated by a strong desire for immortality,--a wish happily realized,--he undertook the prodigious task of giving to the world a complete account of Roman events from the beginning to so late a date as Fortune might vouchsafe. Forthwith he began the accumulation of materials, a task in which ten active years (A.D. 200 to 210) were utilized. The actual labor of composition, continued for twelve years more at intervals of respite from duties of state, brought him in his narrative to the inception of the reign of his original patron, the first Severus.--All the foregoing facts are given us as Dio's own statement, in what is at present the twenty-third chapter of the seventy-second book, by that painter in miniature, Ioannes Xiphilinus. It was now the year A.D. 223, Dio was either consul for the first time (as some assert) or had the consular office behind him, the world was richer by the loss of Elagabalus, and Alexander Severus reigned in his stead. Under this emperor the remaining books (Seventy-three to Eighty, inclusive) must have been composed, for Dio puts the finishing touches on his history in 229. Since by that time he was nearly eighty years of age and since he has written of no reign subsequent to Alexander's, we may conclude that he did not survive his master, who died in 235. The sum total of his efforts, then, as he left it, consisted of eighty books, covering a period from 1064 B.C. to 229 A.D. At present there are extant of that number complete only Books Thirty-six to Sixty inclusive, treating the events of the years 68 B.C. to 47 A.D. The last twenty books, Sixty-one to Eighty, appear in fairly reliable excerpts and epitomes, but for the first thirty-five books we are dependent upon the merest scraps and fragments. How and by what steps this great work disintegrated, and in what form it has been preserved to modern times, this it is to be our next business to trace.