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to the plane of symmetry, and make angles of about 60° with a normal to the plane of symmetry. On warming the crystal the optic axes approach the plane of symmetry, and at about 92° C. they coincide, exhibiting the phenomena of a uniaxal crystal, and on further increasing the temperature they open out in the plane of symmetry.
In 1827 Mitscherlich discovered selenic acid, and the isomorphism of seleniate of potash with sulphate of potash, and afterwards of other seleniates with the corresponding sulphates. In 1830 he observed the isomorphism of manganate of potash with sulphate of potash. This led him to a further examination of manganese, and to the discovery of the isomorphism of the permanganates with the perchlorates, and to the isolation of the hydrate of permanganic acid. At a later period (1860) he repeated, by new and more accurate methods, the analysis of permanganate of potash, which had been called in question, confirming the exactness of the earlier analysis; he succeeded at the same time in isolating the anhydrous permanganic acid.
The crystallographic researches he carried on about the time of the discovery of the new acids were extremely numerous, yet very little has been made known respecting them. He prepared a large number of salts in his laboratory, determined the systems to which they belonged, measured some of the angles, and drew by hand the figures of their principal combinations. But this, though it satisfied his own curiosity, was manifestly insufficient for publication, and the new discoveries that presented themselves were much more attractive than the wearisome and time-consuming task of preparing his researches for the press. He made, however, an attempt to carry out his intention of describing the forms of the most important simple and compound bodies. He commenced with the sulphates, seleniates, and ehromates, because these salts present almost all the phenomena on which the laws of crystalline form and chemical composition are founded. He described the sulphates and seleniates of soda and of oxide of silver; the sulphate, seleniate, and chromate of oxide of silver and ammonia; the sulphate and seleniate of oxide of nickel, and the seleniate of oxide of zinc; the anhydrous and hydrous chloride of sodium; iodide of sodium and bromide of sodium; sulphate, seleniate, and chromate of potash, and sulphate of ammonia. Unfortunately these were his last regular contributions to crystallographic chemistry. Long afterwards he described the forms of the chloride and iodide of mercury, the latter of which is dimorphous, and the forms of phosphorus, iodine, and selenium crystallized from solution in bisulphide of carbon, which proved to be in an isomeric state differing in density from fused selenium.
In 1833 his crystallographic labours were interrupted by the publication of his 'Treatise on Chemistry.' For this work he had been long preparing himself by original researches, by associating with the most eminent chemists of Europe, by visiting their laboratories, and the most important chemical manufactures and smelting-furnaces. A large number of original ippeared in any scientific journal. A fifth edition was commenced in 1855, but left unfinished. In this year he commenced his important labours on the density of the vapour of bromine, sulphur, phosphorus, arseuic, and mercury, nitrous acid, nitric acid, sulphuric acid, &c, and on the relation of the density of vapours to their chemical equivalents. In the same year he commenced his researches on benzoyl, which suggested to him a simple theory of the constitution of those organic combinations in which compound radicals are assumed to exist. His experiments on the formation of ether led him to the doctrine of chemical combinations and decompositions by contact, whereby dormant affinities in mixtures, or compounds held together by feeble affinities, become active by mere contact with a substance chemically inactive. These labours in the domain of organic chemistry wholly occupied him for nearly twelve years. At the conclusion of this period he turned his attention to geology. Indeed, ever since he had engaged in researches on the artificial production of minerals, he used to theorize on the formation of rocks, and on the existence of mineral springs and volcanos. In his earlier travels, while his main object was the examination of chemical manufactures and smeltingfurnaces, his attention was also directed to the geology of the countries through which he passed. He frequently devoted the concluding lectures of each half-year's course to a sketch of the geological structure of the earth, and the changes which its surface had undergone. Year after year he made systematic journeys in the Eifel, with the intention of publishing a complete description of the extinct volcanos of that district, and connecting it with a theory of volcanic action. And, as the study of this region made a comparison with the volcanos of other countries desirable, he visited in succession the principal volcanic districts of Italy, France, and Germany. But, notwithstanding all this preparation, the description of the Eifel was never printed, with the exception of some pages distributed among the hearers of lectures of a popular character given by him in the winter of 1838 and 1839. In these he states the views of the nature of volcanic processes which he then entertained. They appear to have been founded on a very careful study of volcanic phenomena. He supposes the explosive action to be caused by the vapour of water. The only hypothesis, however, by which the presence of water in an active volcano could at that lime be accounted for, was beset by serious difficulties. These have since tan removed by the beautiful experiment made by Daubrtfe, which shows that when one side of a stratum of porous rock is heated, water in contact with the opposite side makes its way through it, in the direction of the heated part, notwithstanding the high pressure of the vapour generated on that side.
During the autumnal vacation of 1861 he made his last geological excursion in the Eifel; in December of that year he began to suffer from disease of the heart, the complaint increased in severity in the summer of 1862, and he had much difficulty in completing his course of lectures. In the autumn of this year he went again to the Rhine, but only to stay in a country-house near Bonn, the home of his son-in-law, Professor Busch. Here his health appeared to revive, and he returned to Berlin feeling so much better that he commenced his winter lectures; a fortnight before Christmas, however, he was obliged to give them up, never again to be resumed. In the spring of 1863 he retired to a country-house at Schoneberg, near Berlin, and here, on the morning of the 28th of August, his valuable life was closed by a painless death. His name will ever be cherished in the annals of that science which he had so greatly enriched. Few philosophers have ever united such a versatility of genius with a mind so severely disciplined, or who, possessing such a talent for observing, were able to deduce such important results from their observations.
He was member of probably every Academy in Europe. He was elected Foreign Member of the Royal Society in 1828; the Royal Medal was awarded to him in 182!), "for his Discoveries relating to the Laws of Crystallization, and the Properties of Crystals."
In 1852 he was elected Foreign Associate of the French Institute, in the place of (Ersted.
The greater part of the preceding notice is extracted from an Address to the German Geological Society by Professor G. Rose, Mitscherluh's successor as President of the Society.
Carl Ludwig Christian Rumker was born on the 28th of May 1788, at Neubraudenburg in Mecklenburg-Strelitz, in the service of which State his father held an important position. After a careful preparatory education at home, he was sent to the Graue Kloster at Berlin, and later to the Engineering Academy of that place. In 1807 he passed the Government examination for qualification as an engineer and architect.
In consequence of the gloomy aspect of affairs in Prussia after the peace of Tilsit, he endeavoured to establish himself in Hamburg; but here also, finding no prospect of occupation in the profession he had adopted, he resolved in 1808 to go to England with the intention of devoting himself to a seafaring life. Accordingly, in the 21st year of his age, he began the world anew, under the most unfavourable circumstances, iu a strange country, without friends, and entirely cut off from his home by the continental blockade. With an energy and strength of character peculiarly his own, be overcame the difficulties of his situation, and obtained an appointment as Midshipman in the Navy of the East India Company. Feeling dissatisfied with this service, he entered into that of the German house of Riicken in London, and visited many parts of the world in their ships. In 181 1 or 1812 he obtained admission into the Royal Navy, and served during the latter part of the war on board various ships of the Mediterranean fleet. He was first appointed to the 'Benbow'; afterwards he became Naval Instructor on board the 'Montague,' Captain Peter Hey wood (formerly of the 1 Bounty'), of whom he used to speak in terms of the greatest regard, as the most kind-hearted and excellent man he had ever known. He was then transferred to the 'Albion,' and on his passage out from England to join Sir Charles Penrose, fought at the battle of Algiers.
During a visit to Genoa Riimker became acquainted with the Baron v. Zach, to whom he submitted the results of various astronomical observations in order to obtain his opinion of their value. The Baron soon discovered his talent for astronomy, encouraged him to cultivate that science, and aided him with his advice and the use of his astronomical library. Riimker's first observations, occultations, and the determination of the latitude and longitude of Malta, where he was stationed for a considerable time, were published in v. Zach's 'Correspondance Astronomique.'
In 1817, when the Fleet returned to England, he quitted the Naval Service and went to Hamburg, carrying with him the friendship and esteem of his comrades of all ranks, which he had won by his ability and energy> combined with a peculiar suavity of manner. Here he was appointed Principal of the School of Navigation. In the society of Schumacher, the Director of the Observatory of Altona, Repsold, and Woltmann, his taste for astronomy was strengthened, and he found many opportunities of extending his knowledge of the subject. As at that time Hamburg did not possess an Observatory, he built one at his own expense on the Stintfang.
In 1821 he resigned his post at the School of Navigation in order to accompany Sir Thomas Brisbane, Governor of New South Wales, to whom he had been introduced by Captain Hey wood, and to take charge of the Observatory which Sir Thomas purposed founding in that colony.
Riimker's labours in the Observatory of Paramatta are well known to astronomers. In 1822 he observed the first calculated reappearance of Encke's comet, which was invisible in Europe, and thereby first confirmed the shortness of its periodic time. He afterwards observed and discovered many other comets, some of which were not seen in Europe. He observed the son in the solstices, made many observations with Kater's pendulum, and determined the magnetic declination and inclination. These and other observations were published in a separate volume of the Transactions of the Royal Society for 1830. His observations of the stars of the southern hemisphere are in part contained in the 'Brisbane Catalogue,' and in the 'Preliminary Catalogue of Fixed Stars in the Southern Hemisphere/ published by himself at Hamburg in 1832. In after years, however, he was never able to find leisure for continuing the work, and the greater part of 'he observations remain still unpublished.
In 1829 he returned to Europe to resume his post as Principal of the School of Navigation, and to undertake the Direction of the New Observatory built by the Senate of Hamburg. He devoted himself with the m°st unwearied diligence to the duties of these two offices. After nights passed hi observing, he made his appearance at the School of Navigation at
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eight both in summer and winter, and remained there teaching for five and even seven hours, regardless of his failing health, which was unable to sustain so severe a trial.
Under his care the school attained an unexpected prosperity. It produced the most distinguished sailors of the German merchant navy, and the teachers of almost all the Schools of Navigation on the coast of the North Sea have been his pupils. The number of students, which at the time of his appointment to the school was only 60, amounted to 250 in 1857. He possessed in an unusual degree the art of teaching. By the clearness of his methods, and a singular patience and mildness which encouraged the self-respect of his pupils and gained their confidence, and especially by his power of adapting his teaching to the comprehension of each individual, he succeeded in preparing the most uncultivated sailor for the examination in navigation often in a surprisingly short time, so as to enable him to pass it with credit.
His 'Handbook of Navigation,' which appeared in 1843, and has gone through three large editions, is used as a text-book in most of the Schools of Navigation on the shores of the North Sea, in Austria, and in Russia.
He devoted himself with equal or even still greater energy to his duties in the Observatory. The principal instruments consisted of an equatorially mounted refractor of 5-feet focal length by Fraunhofer, and a meridian circle constructed by the brothers Repsold, which was mounted in 1836. The observations made with the refractor are published in Schumacher's 'Astronomische Nachrichten,' and in the Monthly Notices of the Astronomical Society. With the meridian-circle he undertook the determination of the places of all the fixed stars visible through its telescope,—a work of many years' duration, the results of which he published in the years 1843-59 under the title of a Catalogue of 12,000 fixed stars, but in reality containing upwards of 15,000.
In speaking of the observations made with the refractor, at the Anniversary Meeting of the Astronomical Society in 1854, when the medal of the Society was awarded to M. Riimker, the Astronomer Royal, President of the Society, expressed himself in the following terms:—
"For a very long time M. Riimker has been known as furnishing extrameridional observations of comets and newly discovered planets, possessing the highest degree of accuracy, and extending to times when the objects which he could successfully observe were lost to other astronomers furnished apparently with much more powerful means. I have myself visited the observatory and inspected the instruments which have been devoted to these observations, and I have inquired, How is it that with instruments so insignificant you have been able to see so much more than others could see who are so much better equipped? The answer was very simple. Energy, care, patience,—in these, I believe, is contained the whole secret. M. Riimker perhaps possesses in perfection the sensibility of eye and the acuteness of ear which are required for the most delicate observations; but