IN all the five editions of Baltzer's “Theorie und Anwendung der Determinanten” there stands at the foot of the first page an historical note, in which reference is made to a work entitled, “Demonstratio eliminationis Cramerianae,” by Mollweide (Leipzig, 1811). About a year ago it became necessary to examine this demonstration for the purpose of having it reported upon in an historical work. The University Libraries in Scotland were applied to in succession, but no copy could be heard of. Inquiries made at the more important libraries in Cambridge by friends resident there, or by letter, ended in the same unsatisfactory way. Letters, followed by an actual visit, to several libraries in London, brought no better result; and after every possible biographical scrap about Mollweide had been ferreted out in the British Museum, the suspicion began to form itself that some curious error had crept into Baltzer's footnote. In order to get to the bottom of the matter, the excellent mathematical library of Gottingen University was next applied to, and the library of Giessen University, where Baltzer was Professor; but in both cases in vain. A last effort was then made about a month ago in a letter to the University Library of Leipzig, where the reputed author Mollweide had taught, and where the “Demonstratio” (or Demon, as it had for more than one reason come to be called) had been published. Even here, at first, there was failure. But Prof. Virchl, who most kindly interested himself in the matter, was soon successful in his quest. What he found, however, was not a “Demonstratio” by Mollweide; the title was simply as follows: “Ad memoriam Kregelio-Sternbachianam in auditorio philosophorum die xviii. Julii, MDCCCXI. h. ix. celebrandam invitant ordinum Academiae Lips. Decani seniores caeterique adsessores—‘Demonstratio eliminationis Cramerianae.’ ” Either, therefore, no author should have been mentioned by Baltzer, or an indication should have been given that Mollweide's name was an interpolation in the title. One or other of these courses would likewise have been less hurtful to Baltzer's reputation for accuracy; for, after all, Mollweide was not the author. In the Leipzig Library Catalogue the work is entered under the name of De Prasse, and Prof. Virchl had no doubt whatever, for perfectly conclusive reasons which he gave, that De Prasse was the author. The work extends to only 15 pages quarto, and is considered by the same authority to be very rare.