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He was born at Milan, presumably of the noble family of de Magni. He received the Capuchin habit at Prague. He was also provincial superior there, as in 1626 was appointed Apostolic missionary for Germany, Hungary and Poland. He was greatly respected by Holy Roman Emperors Ferdinand II and Ferdinand III, as well as by King Wladislaw IV of Poland, who employed him on diplomatic missions. In July 1647 he performed a vacuum experiment (so-called Torricelli's experiment) before a distinguished audience at the Royal Castle in Warsaw.
Landgrave Ernst of Hesse, who had been converted at Vienna on 6 January 1652, and who knew Father Valerian, summoned Capuchins to St. Goar on the Rhine, and was present at the religious disputation between Valerian and Haberkorn of Giessen at Burg Rheinfels in 1651. The Jesuit Johann Rosenthal having attacked certain assertions of Valerian's at this debate, the latter was drawn into the sharp literary controversy between Capuchins and Jesuits, which extended even to Rome. On the appearance of his pamphlet Contra imposturas Jesuitarum in 1659, he was cited to appear at Rome. As he did not obey the summons he was arrested at Vienna in 1661 at the instance of the nuncio, but was liberated at the urgent request of Ferdinand III.
He was apparently on his way to Rome when in the same year death overtook him at Salzburg.
- Apologia contra imposturas Jesuitarum, 1661.
- Christiana et catholica defensio adversus Societatem Jesu, 1661.
- Opus philosophicum, 1660
- Commentarius de homine infami personato sub titulis Iocosi Severi Medii, 1654.
- Concussio fundamentorum ecclesiae catholicae, iactata ab Herm. Conringi ..., 1654.
- Conringiana concussio Sanctissimi in Christo papae catholici retorta ..., 1654.
- Echo Absurditatum Ulrici de Neufeld Blesa, 1646.
- Epistola ... de responsione H. Conringii, 1654.
- Epistola Valeriani Magni Fratris Capucini ..., 1654.
- Epistola de quaestione utrum Primatus Rom. Pontificis ..., 1653.
- Principia et specimen philosophiae, 1652.
- Acta disputationis habitae Rheinfelsae apud S. Goarem, 1652.
- Organum theologicum, 1643.
- Methodus convincendi et revocandi haereticos, 1643.
- De luce mentium, 1642.
- Judicium de catholicorum ei acatholicorum regula credendi, 1628, 1641.
- Demonstratio ocularis, loci sine locato: corporis successiuè moti in vacuo..., 1648