XIU 



angle between the same pair of cleavages amounting to 20", a difference 

 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 after- 

 noon in a room facing the south. The morning observations differed from 

 those made in the afternoon, but the observations made at the same jDcriod 

 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 there- 

 fore concluded that the variation of the angle could only be due to the un- 

 equal 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, chalybite from Ehren- 

 friedersdorff 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 expan- 

 sion 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 tempera- 

 ture 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 con- 

 firmed 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 expan- 

 sion 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 colHmator, 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 



