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Professor Rudolf Julius Emanuel Clausius (1822-1888)
1889 Obituary 
RUDOLF JULIUS EMANUEL CLAUSIUS, Professor of Physics at the University of Bonn, who was elected an Honorary Member of the Institution on the 11th of May, 1875, died at Bonn on the 24th of August, 1888.
He was born on the 2nd of January 1822, at Coslin, in Pomerania, and was one of the younger of a number of children, fourteen of whom reached adult age. Both parents belonged to families connected with the Church. The father, Carl Ernst Gottlieb Clausius, who, after a youth of hardship, had also attained priestly office, was in 1819 appointed, by Chief-President Sack, Government School Counsellor at Coslin, where he gained distinction in connection with the re-construction of the disordered school system of the Province of Pomerania. Finding that the heavy duties of this office were affecting his health, he retired in 1833 to Ueckermunde in Western Pomerania as Pastor and Superintendent, where he again established a private school in which his sons commenced their education.
At sixteen years of age Rudolf Clausius left his father’s house to join two elder brothers in the Gymnasium at Stettin. There he spent a happy time, although the small allowance which his father, burdened with a numerous family, could make him necessitated many sacrifices. The naturally delicate physique of the boy was strengthened by regular exercises, particularly drill and swimming, and his mind was developed in every way under the care of excellent masters and by his own quiet plodding industry. His extraordinary talent for mathematics and physics exhibited itself, according to the testimony of a schoolfellow, in that he alone was able entirely to follow the lectures of the well-known Justus Qunther Grassman, and could already, without assistance, pursue his studies further. But at the same time he was, as he himself would often relate, in doubt during the earlier years of his University career whether to choose those branches of science or that of secular history as his vocation in life. His certificate on leaving the Gymnasium in 1840, at the age of eighteen, describes him, both as regards character and mind, exactly as he proved to be in manhood, a remarkable instance of the perspicuity of his masters, and of the early maturity of his intellectual and moral powers.
On leaving Stettin he went to the University of Berlin, where he heard, with the exception of the lectures of the celebrated historian, Leopold Ranke, which he attended for a year and a half, only the mathematical and physical discourses of Dirksen, Ohm, Dirichlet, Steiner, Dove and Magnus. In order that he might not too long claim his father’s assistance at the expense of younger members of the family, he undertook so early as the autumn of 1843 the duties of a House Master, which he fulfilled for three years and a half.
In the summer of 1844 he passed the State scientific examination, obtaining thereby the right of teaching at public schools, and then worked until the year 1860 as mathematical, physical and arithmetical master at the Friedrich Werder Gymnasium at Berlin. At the same time he continued his studies in physics with Magnus, and for four years gained experience in scholastic duties at the Royal Seminary for Learned Studies under the celebrated philologist, Boeckh.
In the year 1847 appeared his first literary works. After he had taken the degree of Doctor of Philosophy at Halle, on the 15th of July of the same year - his dissertation "De iis atmosphaerae particulis quibus lumen reflectitur” appeared in 1840 in German in Poggendorffs Annalen” - Clausius was in the year 1850 appointed physical master at the Royal Artillery and Engineer School, Berlin. This post carried sufficient salary to enable him to establish himself at the University as a Privatdocent, which he did on the 28th of December 1850. For five years he performed the duties of both offices, until in the summer of 1855 he obtained that post for which he had worked with ceaseless industry, and to which he was appointed through his scientific talents and long training as a practical master, viz., Professor in ordinary at the Polytechnic School of Zurich. He soon created for himself an extensive sphere of activity, which from 1857 included also the University of that city. With that devotion to duty which he showed to the end he wrote fresh lectures for almost every year, and conducted exercises and examinations in which exact written notes enabled him to form a sure judgment of the performance of each pupil. While in this way he gained the highest recognition from the Swiss authorities, he became at the Same time an extraordinarily popular master through his winning personal manners, and also through the manly and determined manner in which he advocated with the authorities the rights of the students when he thought then prejudiced. His modesty, truthfulness, reliability, and willingness to render assistance, won for the foreigner - for he never acquired the rights of a Swiss citizen - universal respect and consideration.
For his literary work, which there first reached a high point, honours mere not wanting. More than forty learned societies elected him member, Haarlem being the first in 1857, then the Institution of Engineers and Shipbuilders in Scotland in 1859, Erlangen (Germany) in the sanle year, and in 1865 the Institute of France. The year 1859 brought to Clausius that domestic happiness for which his disposition was fitted; on the 13th of November he married Adelheicl Rimpau, a native of Brunswick and an orphan, who lived with her sister at Zurich. He remained at Zurich for twelve years, although Carlsruhe in 1858, Brunswick in 1862, and Vienna in 18GG, sought to obtain him for their Polytechnic Schools ; in 1867 he accepted a call to the University of Wurzburg and returned to Germany. At Wurzburg also his career was happy and successful; he was supported in every way by the Bavarian Government, and soon became completely at home in his new circle. It cost him a hard struggle to exchange this post for that of' Professor at Bonn in the year 1869, but the hope of finding there a larger sphere of activity decided him. Thus Clausius returned to his native country, Prussia. Although .an advocate of the mathematical method in physics, his first care here, as in Wiirzburg, was to bring the physical apparatus up to (late by making comprehensive additions, the means for which were provided by the Government. In the following year, however, the Franco-German war broke out, and his strong ptriotisnl could not allow him to look quietly on. After the victory of Saarbrucken, he undertook the command of a division of the Ambulance Corps formed by the students of Bonn. The corps, and with it Clausius, remained actively engaged until after the battle of Gravelotte. At the great and decisive battles of the 16th and 18th of August he assisted in carrying the wounded from the field and in ministering to their sufferings. His services were rewarded by the Iron Cross. But unfortunately a contusion of the knee, received on the field, caused him for years much suffering and undermined his strength. By medical advice he learned to ride at fifty-six years of age, becoming a good and even a daring horseman, and deriving much benefit from this healthful exercise. In 1875 Clausius met with a far greater misfortune in the sudden death of his wife. Six children, all of whom were young, were now left to his sole care, and with rare conscientiousness and devotion he performed his duties towards them for eleven years, only at first assisted by a governess. More than ever he devoted himself only to the narrow circle of his family, in which he sought . and found his sole recreation from work. His most pleasant holiday trips were the summer visits of the German Association of Natural Philosophers-to Munich in 1877 ; Baden-Baden in 1879 ; Zurich in 1883; also foreign journeys on festive occasions, as those in 1879 and 1882 to London and Southampton, or, officially, as that to the International Electrical Exhibition at Paris in 1881. The more retired the life he led at home, the more was he pleased and refreshed by personal intercourse wihh celebrated men and, particularly, with scientific colleagues. After his two eldest daughters had married, he found, in the year 1886, a second wife in Sophie Sack of Essen-on-the-Ruhr, a relative of that Chief-President who had, more than sixty years previously, appointed his father Government School Counsellor at Cuslin. In 1887 a son was born to him, and in all respects the close of his life was happy. Bonn had held him fast, although he had been greatly tempted in 1871 by an invitation to Strasburg, and still more so in 1883 by the offer of the Chair of Mathematical Physics at Guttingen. His duties became more numerous and complicated : in addition to lectures, numerous examinations and inquiries as to technical matters of all kinds occupied his time. He had also to take his part in the academical training of Prince Wilhelm, the present German Emperor, and was in later years much pleased by continuous proofs of the grateful remembrance of his distinguished pupil. In the year 1887 he was appointed one of the Curators of the recently founded Physikalisch-technische Reichsanstalt at Berlin. His reputation steadily increased. The highest scientific honours were conferred upon him-the Copley Medal of the Royal Society, the Dutch Huygens Medal, the French Poncelet Prize, the Bavarian Order of Maximilian, the Prussian Order “Pour le Merite ” for Science and Arts. His literary work was somewhat hindered by his having to act as Rector, and for six months as Curator, of the University in 1884 and 1886. But during the last years of his life he worked very hard Besides the third edition of his ‘‘Mechanical Theory of Heat,” he was continually occupied with electrical problems, and was often heard to declare that he had so many ideas to work on he would hardly find time for them. He looked forward with certainty to finish the third volume of his great work in the autumn of 1888, but in the spring that insidious disease, from which he was never to recover, made its appearance. He fought strenuously against the waning of his strength, but at the beginning of August he was obliged to take to his bed. His death came unexpectedly upon the scientific world, even upon his nearest relatives, for he had called into play against the disease all the power of his will, and employed the last remnants of his strength in the devoted performance of all his duties. In a fine notice appearing shortly after his death, Clausius was termed a prince in the realm of mathematical physics, and truly the part taken by him in the development of this science extended far beyond the ordinary limits. In view of the great number of his published writings, it is no easy matter, within the limits of this notice, to do justice to the importance of Clausius. More than one hundred separate treatises were published by him, partly in German and partly in foreign journals. The majority are contained in Poggendorffs (now Wiedemann’s) “Annalen der Physik untl Chemie,” but nearly all have been translated into English and French, as well as his chief work “Die mechanische Wiirmetheorie,” which was brought out in two volumes by Friedrich Vieweg and Son. Besides this Clausius wrote the book “Die Potenzialfunction und das Potenzial” (published by Joh. Ambr. Barth, at Leipzig), which in the lifetime of the author reached a fourth edition. With the third edition of “Die mechanische Wirmetheorie,” commenced in 1888 (the two former editions appeared in 1867 and 1870), it was intended to publish the third volume concluding the whole work. The completion of this volume, the working out of which - as he himself remarks in the preface to the first volume - presented especial difficulties and had led him to new investigations, Clausius did not live to see. The manuscript of the third volume, of which about one-fourth is in fair copy, has been entrusted by the family to Privatdocent Dr. C. Pulfrich, the assistant of the deceased, who in conjunction with Professor Dr. Max Planck of Eel, now in Berlin, will probably publish the work in the course of the present year. The editors have decided to issue at once, as a first instalment, the portion of the work now ready for the press. Clausius commenced his literary career in the year 1847 with theoretical investigations on the refraction of light by the atmosphere, and on the intensity of the sunlight reflected by the atmosphere. The above-mentioned treatise of 1849 is of similar character, and much later Clausius repeatedly came back to these his first works; in 18G6, for instance, in a paper on the relative intensity of direct and refracted sunlight. His view was that the bubbles of steam in the atmosphere exerted a preponderating influence over all other constituents on the reflection of light, and especially on the colour of the sky. According to this view, the blue colour of the sky is caused by reflection from these steam bubbles and is termed a blue of the first order. With reference to the allied phenomena of the red glow at sunrise and sunset, and to the other phenomena belonging to the department of meteorological optics, the respective treatises in “Poggendorffs Annalen,” and the “Uebersichtliche Darstellung, &C.,” of Clausius, which is contained in Grunert’s “Meteorologischer Optik,” iv., 1850, may be mentioned. It should, however, be noticed that the “bubble ” theory of Clausius has given rise to the liveliest discussions and even grave objections, and that opinions on the subject are not even now unanimous. Although Clausius had already distinguished himself by the extraordinary acuteness of his logical reasoning, and by the great clearness and intelligibility of his style of writing in these optical and meteorological works, generalttention was only directed to the young man of science when, in the year 1850, he came before the world with a treatise on the motive power of heat and the laws which may be deduced therefrom for the theory of heat itself. From that moment the name of Clausius is found in the closest connection with the most important advances in this science. For forty years of strenuous intellectual life he devoted himself to investigations on the subject of heat and the closely allied one of electricity, and chiefly to these labours Clausins owes his worldwide celebrity; it is universally considered his chief merit that contemporaneously with, and as one of the foremost of, a large number of eminent savants he assisted in the construction and strictly scientific proof of the so-called mechanical theory of heat. The main pillars on which the present views on the nature of heat rest are the two so-called fundamental principles of the mechanical theory of heat, the first of which, the principle of the equivalence of heat and energy, or as Clausius calls it, of heat and work, formed the starting-point for the whole subsequent development. This principle broke with the prevailing theory, which assumed that heat was a substance, which was present in greater or less quantity in a body and thereby determined its temperature. The question first propounded by Mayer in Heilbronn, and afterwards, independently of him, discussed by Joule in Manchester with particular thoroughness, whether a universally applicable relationship existed between the heat produced by the compression of a gas by friction and other processes and the work expended, was answered in the sense that, in all cases where by the expenditure of mechanical work heat is developed, the quantity of work necessary to produce a given amount of heat remains the same and is independent of the particular nature of the process. The substance (emission) theory can supply no explanation here, while the assumption of motion not only sufficiently explains the whole phenomenon, but also appears as a direct consequence of the universal mechanical laws. Imagination must be carried back to those times to appreciate at their true value the merits of Clausius, who, when this tremendous revolution in fundamental conceptions was accomplished, independently, and without waiting for further experiments, commenced the reconstruction of the theory of heat. The germs proved very fruitful in their mathematical development. They led Clausins in the first place to an accurate distinction between the internal and external work, of which the former, or rather, the equivalent of it, together with the heat present in the body determines the energy of the latter. The principle of the equivalence of heat and work was, however, not the only foundation on which the whole theory with all its consequences was to arise. Before this had been formulated, Sacli Carnot had developed, in the year 1824, out of the old theories certain ideas with reference to the transmission of heat from a hot to a cold body in a working machine ; but it appeared as if the new principles, on which the validity of the first fundamental lam rested, were directly opposed to the ideas expressed by Carnot. The credit of having first correctly recognised w-hat was permanent in Carnot’s train of thought unquestionably belongs to Clausins. On the basis of a new principle, that heat cannot of itself (or without compensation) pass from a colder to a hotter body, Clausius first formulated the so-called second principle of the mechanical theory of heat, developed by a modification out of Carnot’s law, and traced it to correct principles. This principle is very often called the law of Carnot and Clausius, or also, briefly, Clausius’ law, and has led to a series of the most important results, obtained by its application. Thus, for instance, the second law has been no less concerned than the first in the great transformation which the steam-engine, the most important of the thermodynamic machines, has undergone. It is scarcely possible to give in a few words an explanation of what is understood by the second law of the mechanical theory of heat. In its briefest shape it has been stated by Clausius as the law of the Equivalence of Transformations, and therewith expressed, that in all cycles occurring in nature, in which a body undergoes a series of transformations of such a character that it eventually reverts to its original state, the algebraic sum of all the transformations must be nil. The separating action of heat, on the individual parts of a body, or its segregation, is thus considered as a positive transformation, the conversion of heat into work as a negative transformation. From the fundamental property of heat, essential to its nature, by which it always equalizes existing differences of temperature and passes without compensation from the hotter to the cooler body, Clausius deduced the important natural law that the world is slowly but surely tending towards a certain limit, the condition of dead equilibrium, in which all causes for a further transference of heat will cease to exist. While the first law, in its widest application, states that the total quantity of energy existing in the world is as constant as the quantity of matter in the world, it contains no confirmation of the view according to which the whole condition of the universe is unalterable and in a condition of everlasting cyclic motion. “One often hears it said,” remarked Clausius, in his peculiar simple and modest way, in the course of a lecture delivered at Frankfort, “everything in the world is cyclic. On the whole the condition of the world remains unchanged; the world therefore may exist for ever in the same way. The second law of the mechanical theory of heat contradicts this most decidedly,” for this reason, namely, that not all the infinitely varied changes going on in the world are complete cycles, bat many processes, such as transmission of heat, development of heat by friction, &C., are not reversible, so that the difference between positive and negative transformations can always only take place in one sense. In the development of the mechanical theory of heat, the deductions derived from the application of the two main principles remained quite unaffected by the conceptions formed as to the nature of the motion which is called heat. But here again he is seen untiringly active in a sphere which he had made peculiarly his own, in conjunction with Kronig, Maxwell, and other scienhific men, in the investigation of these phenomena of motion. The new theory of gases, to which the name of “Kinetic” has been given, has led to definite conceptions as to the size of the molecules and the length of the paths traversed by them. Intimately related to Clausins’ work in the field of the theory of heat are his investigations on electricity. The second volume on the mechanical theory of heat, which also bears the special title of a mechanical treatment of electricity, was termed by Clausius himself an independent treatise on electricity. With these undertakings he commenced simultaneously a new epoch of his creative activity, which was crowned hy the discovery of a new electrodynamic principle. Space does not permit of any mention in detail of the manifold electrical problems which he subjected to a searching mathematical analysis, and which, especially in latter years, occupied him much. It is interesting, however, to notice the views expressed by Clausius in his speech, when installed as Rector, in 1886, on the connection between the great forces of nature, and especially on the position of electricity or magnetism with regard to light and heat, which views were shortly afterwards more closely defined by him in an article appearing in “La Luminre Electrique.” According to this, the part hitherto ascribed to the ether, that substance required by the theory of light, which is supposed to connect the coarser mass-particles of a body, must now be credited to electricity. It can no longer be doubted that electrical forces mast be instrumental in the transmission of light and radiant heat. As an electro-dynamic or electro-magnetic theory of light had already been founded by Maxwell, Clausius considered that to gain a definite idea of the nature of electricity was a problem no longer far from solution. In connection with this point attention is directed to the epoch-making results published a very short time since by the physicist, Professor H. Hertz, of Karlsruhe (now of Bonn), by which the identity of light, radiant heat, and electrodynamic undulations is made clear. The rays of electric force investigated are described as light-rays of very great wave-length, and they allow a repetition of the experiments on the reflection of light, refraction, and so on. In his polemical writings - and he experienced as numerous attacks as any one - Clausins showed true nobility of character. The discussions of such attacks were always appended as a contribution to the mechanical theory of heat to the first and second volumes, and in this may be found a confirmation of what has been previously said of his personal characteristics. “I am accustomed,” so Clausius writes in one part of these discussions, “always to express myself openly, and never think of covertly insinuating what I do not wish actually to say.” He cared only to establish facts, and even the most malicious attacks of a personal character could not move him from this course ; hence his simple and clear explanations have very great effect as criticisms. In an obituary notice of Clausius, which appeared in an English journal, it was said that the students of Bonn had probably lost most by his death. It may be permitted to the writer of these lines, as a former pupil of Clausius for many years, here to recall to mind the great tact which characterized him as a school-man and a teacher, and the amiability which so quickly won the hearts of his pupils. Always simple and clear, almost self-evident to his hearers, were his demonstrations, in which he strove to place himself on the level of his students. Only on working through his lectures at home did they become fully conscious of the ease with which the great teacher and savant handled even the most difficult problems. It has frequently been noticed, as a peculiarity of the scientific work of Clausins, that he never published the results of experimental studies. It is true he never experimented, but his clear vision and practical understanding could not on that account overlook the importance of this branch of physical investigation. His theoretical treatises always go hand-in-hand with their practical application. When the steam-engine and electro-dynamic machines gave an impulse to his investigations, he was occupied, while still a teacher at the engineering school in Berlin, with the flight of projectiles, as is proved by a Paper of his, not yet published, written between the years 1860 and 1855. His participation in the electrical congress at Paris in the year 1881 led him to inquire into the absolute and practical system of electrical and magnetic measurements. A theorist Clausius was, but always in touch with practice, and therein lies to a great extent the extreme importance which his work has had in the development of the physical sciences.