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admirable sagacity with which they had been determined by Berzelius. Mineralogists and chemists had long been occupied with researches on the relation between chemical composition and crystalline form; they had discovered a number of important facts bearing upon the subject, but no one had discovered the basis upon which the phenomena rested. Fuchs had already observed that some of the constituents of a mineral might be replaced by others without any change of form, and had called these constituents vicarious, but by adducing the sesquioxide of iron and lime as vicarious constituents in Gehlenite, he showed that the true explanation had eluded his grasp. Fuchs had moreover remarked the close resemblance of the mineral sulphates to one another, as well as that of the rhombohedral carbonates. He also showed that strontianite was not rhombohedral as Hauy supposed, but prismatic, and that it resembled Aragonite in form. The small percentage of strontian detected in Aragonite by Stromeyer was regarded by Fuchs as the cause of the resemblance of the forms of the two minerals, as the very small quantity of carbonate of lime in chalybite had been supposed the cause of its resemblance to calcite. The only conclusion which Fuchs drew from the resemblances of these minerals was, that certain substances possess such an overpowering force of crystallization, that, even when present in small quantity, they constrain other substances to assume their form.

In November 1821 Mitscherlich returned to Berlin, was elected a Member of the Academy of Sciences and appointed Professor extraordinary in the University, and remained in that position till 1825, when he became Professor in ordinary. In the summer of 1822 he gave his first lecture on Chemistry to a large audience. He also continued his researches on isomorphism, and those which he had commenced in Stockholm, especially those which bore upon the artificial formation of minerals. He exhibited to the Academy a collection of about forty crystallized substances, which he had found in the slag-heaps surrounding the copper-smelting furnaces of Fahlun during a visit he paid to that place in 1820, in the company of Berzelius. Of these, however, he described only two, a silicate of protoxide of iron isomorphous with olivine, and a mica, the composition of which approximates closely to that of a black mica of Siberia. He resumed these researches along with Berthier in the winter of 1823 and 1824, which he passed in Paris, and by fusing the mineral constituents together in proper proportions, succeeded in producing diopside, idocrase, and garnet.

In the course of his examination of the phosphates and arseniates he had observed that the acid phosphate of soda crystallizes in two totally different forms, both of which belong to the prismatic system, but cannot be referred to the same parameters. From this he inferred that the ultimate atoms of crystallized bodies by change of circumstances may admit of a change in their arrangement, and hazarded the opinion that, as Aragonite resembles strontianite and cerussite in form, and calcite re

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sembles dolomite, chalybite, and diallogite, it is possible for the substances isomorphous with Aragonite to crystallize in the form of calcite, and the substances isomorphous with calcite to crystallize in the form of Aragonite, and so greatly enlarge each group of isomorphous bodies. This opinion was looked upon with great distrust by chemists and mineralogists. All the examples he had brought forward were taken from compound bodies, which possibly might have contained admixtures which analysis had failed to detect, and the substances assumed to have the same composition might after all be different. These doubts were suggested by the analyses of Aragonite, which had been pronounced by some of the most eminent chemists of the time to be pure carbonate of lime; then Stromeyer detected strontia in it, which, notwithstanding that its amount was very small, and different in Aragonite from different localities, was immediately regarded as the cause of the difference of its form from that of calcite; lastly, Buchholtz proved the existence of a variety of Aragonite absolutely free from any admixture of strontia, to which, therefore, the difference of form could not by any possibility be due. At this conjuncture Mitscherlich made the remarkable discovery that sulphur also takes different forms under different circumstances. The crystals obtained from solutions belong to the prismatic system, and are identical in form with those which occur in nature; but when sulphur is fused and allowed to cool, with proper management distinct crystals are obtained, but they are entirely different from the former, inasmuch as they belong to the oblique system. This observation was of great importance, because sulphur being a simple substance crystallizable at pleasure in either of its two forms, the difference of form could not be attributed to a difference of composition. He had already proved that the acid phosphate of soda and carbonate of lime possessed the same property of crystallization in two different forms, which he now considered as appertaining to all simple substances and their chemical combinations, and to which he gave the name of dimorphism. He regarded it, moreover, as affording an explanation of the fact that bodies possessing analɔgous chemical constitutions are not always isomorphous. The memoir cn the dimorphism of sulphur was presented to the Academy on the 26th of July 1826.

It was found that the forms of isomorphous substances are not absolutely identical, except, of course, when they belong to the cubic system, but exhibit some differences, showing that the chemical nature of the substance is not altogether without influence on the form. In order to determine the difference between the angles of isomorphous bodies with greater accuracy than was attainable by the use of the ordinary Wollaston's goniometer, he caused a goniometer to be constructed by Pistor, provided with four verniers, each reading to 10", and with a telescope magnifying twenty times for viewing the reflexions of the signal in the faces of the crystal. With this instrument, in the summer of 1823 he began to measure the angles of calcite from Iceland, and was surprised to find differences in the

which, though small, was too large to be attributed to errors of pointing or reading. The observations were made in the morning and in the afternoon in a room facing the south. The morning observations differed from those made in the afternoon, but the observations made at the same period of the day agreed well with one another; also the temperature of the room in the afternoon was nearly 4° C. higher than in the morning. He therefore concluded that the variation of the angle could only be due to the unequal expansion of the crystal in different directions. He increased the difference of temperature by immersing the crystal in a bath of heated mercury, and found that the cleavages became more nearly at right angles to one another, by 8' 34", for an increase of temperature of 100° C. In dolomite from Traversella, Breunnerite from Pfitsch, chaly bite from Ehrenfriedersdorff similar changes occurred amounting to 4' 6", 3' 29", and 2' 22" respectively, for a change of temperature of 100° C. A large number of other crystals examined by him afforded like results. In the winter of 1823-1824, during his stay in Paris, he measured the expansion of calcite in volume by Dulong's method, and found it equal to 0.001961 for 100° C. Hence it appears that by an increase of temperature of 100° C. the crystal expands 0.00288 in the direction of its axis, and contracts 0·00056 in a direction at right angles to its axis. He confirmed the accuracy of this most unexpected result by comparing, at different temperatures, the thicknesses of two plates of calcite of nearly equal thickness, bounded by planes parallel and at right angles to the axis respectively, and the thickness of a plate bounded by planes parallel to the axis with that of a plate of glass of nearly the same thickness, the expansion of which was known. His memoir on this important discovery was presented to the Academy on the 10th of March 1825.

The large goniometer which he employed in these observations being too cumbersome, and also too costly to be used by mineralogists in measuring the angles of crystals, he contrived an instrument more convenient for ordinary use, reading to half a minute, and provided with a telescope having a magnifying power of not more than three. The signal consists of cross wires in the focus of a collimator, as in the goniometers of Rudberg and Babinet. The adjustment of the crystal is effected by a very ingenious contrivance due to M. Oertling, by whom many of these instruments have been constructed. By the invention of this goniometer, which has come into general use under the name of Mitscherlich's goniometer, he conferred a great boon on mineralogists. A minute description of it appeared in the Memoirs of the Berlin Academy for 1843, a considerable time after it was originally contrived, and not till its value had been tested by long use.

Of his observations on the effect of heat on the double refraction of crystals, little is known beyond a notice in Poggendorff's 'Annalen' of the remarkable changes which occur in gypsum when heated. At the ordinary temperature of the atmosphere the optic axes lie in a plane at right angles

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 chromates, 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

observations of his own are embodied in this work, which had never appeared 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, arsenic, 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 time be accounted for, was beset by serious difficulties. These have since been removed by the beautiful experiment made by Daubrée, 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

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