After sixty-seven years of almost undisturbed health he was overtaken by a painful illness towards the end of the year 1869.[1] He still continued his lectures on physics until February 25, 1870, but during March he was scarcely able to leave his bed, and he died on April 4. [Footnote 1: Carcinoma recti.] Magnus’s was a richly endowed nature, which under happy external circumstances could develop in its own peculiar manner, and was free to choose its activity according to its own mind. But this mind was so governed by reason, and so filled, I might almost say, with artistic harmony, which shunned the immoderate and impure, that he knew how to choose the object of his work wisely, and on this account almost always to attain it. Thus the direction and manner of Magnus’s activity accorded so perfectly with his intellectual nature as is the case only with the happy few among mortals. The harmonious tendency and cultivation of his mind could be recognised in the natural grace of his behaviour, in the cheerfulness and firmness of his disposition, in the warm amiability of his intercourse with others. There was in all this, much more than the mere acquisition of the outer forms of politeness can ever reach, where they are not illuminated by a warm sympathy and by a fine feeling for the beautiful. Accustomed from an early age to the regulated and prudent activity of the commercial house in which he grew up, he retained that skill in business which he had so frequently to exercise in the administration of the affairs of this Academy, of the philosophical faculty, and of the various Government commissions. He retained from thence the love of order, the tendency towards the actual, and towards what is practically attainable, even although the chief aim of his activity was an ideal one. He understood that the pleasant enjoyment of an existence free from care, and intercourse with the most amiable circle of relatives and friends, do not bring a lasting satisfaction; but work only, and unselfish work for an ideal aim. Thus he laboured, not for the increase of riches, but for science; not as a dilettante and capriciously, but according to a fixed aim and indefatigably; not in vanity, catching at striking discoveries, which might at once have made his name celebrated. He was, on the contrary, a master of faithful, patient, and modest work, who tests that work again and again, and never ceases until he knows there is nothing left to be improved. But it is also such work, which by the classical perfection of its methods, by the accuracy and certainty of its results, merits and gains the best and most enduring fame. There are among the labours of Magnus masterpieces of finished perfection, especially those on the expansion of gases by heat, and on the tension of vapours. Another master in this field, and one of the most experienced and distinguished, namely, Regnault of Paris, worked at these subjects at the same time with