xiv 



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 know^n 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 smel ting-furnaces. A large number of original 



