POLARISATION OF LIGHT, 



scherlich of Berlin ; but we regret that 

 it is out of our power to give any thing 

 more than a meagre account of some 

 of his results. 



In imcrvstallised bodies, and in all 

 crystals which have no double refraction, 

 a rise of temperature throughout the 

 whole mass produces an equal expansion 

 in all directions, without any change of 

 figure. With doubly refracting crystals 

 the case is different. When calcareous 

 spar is heated, it dilates in the direction 

 of its axis of double refraction ; but Pro- 

 fessor Mit scherlich found that in all 

 other directions at right angles to this 

 (Lii-s- it contracts, so that there must be 

 a line inclined to the axis in which there 

 is neither dilatation nor contraction. 

 Hence the angles of the crystal are 

 changed by heat, being diminished (by a 

 heat from the freezing to the boiling 

 point) 8' 30" in the dihedral angle at the 

 extremities of the axis. Its form being 

 thus brought nearer to that of the cube, 

 which has no double refraction, its 

 double refraction, as might have been 

 expected, is diminished. M. Fresnel 

 found that heat dilates sulphate of lime 

 less in the direction of its principal axis 

 of double refraction (in the plane of the 

 laminae) than in a direction perpendicular 

 to it, a difference analogous to that of cal- 

 careous spar, but of a contrary charac- 

 ter, as might have been anticipated from 

 the opposite nature of the double refrac- 

 tion of these two minerals. 



These results being obtained by very 

 nice experiments, which but few per- 

 sons are able to repeat, Mr. Herschel 

 has given the following experiment 

 as an ocular demonstration of the 

 truth of the general fact of unequal 

 change of dimension by change of tem- 

 perature. " Let a small quantity of the 

 sulphate of potash and copper (an an- 

 hydrous salt easily formed by crystalli- 

 sing together the sulphates of potash and 

 of copper) be melted in a spoon over a 

 spirit lamp. The fusion takes place at 

 a heat just below redness, and produces 

 a liquid of a dark green colour. The 

 heat being withdrawn, it fixes into a solid 

 of a brilliant emerald green colour, and 

 remains solid and coherent till the tem- 

 perature sinks nearly to that of boiling 

 water, when all at once its cohesion is 

 destroyed; a commotion takes place 

 throughout the whole mass, beginning 

 from the surface, each molecule, as if 

 animated, starting up and separating it- 

 self from the rest, till in a few moments 



the whole is resolved into a heap of in- 

 coherent powder, a result which could 

 evidently not take place had all the mi-- 

 nute and interlaced crystals of which 

 the congealed salt consisted contracted 

 equally in all directions by the cooling 

 process, as in that case their juxtaposi- 

 tion would not be disturbed." 



When Professor Mitscherlich was ex- 

 amining the double refraction of the hy- 

 drous sulphate of magnesia when heated 

 in oil, he observed that it suffered no 

 change till the temperature reached 126 

 of Fahrenheit The crystal then became 

 opaque, and on being broken, it shewed 

 the structure of a pseudo-morphous crys- 

 tal, consisting of a number of individual 

 crystals, beginning at the surface and 

 meeting in the inside of the original 

 crystal. The same effect was pro-" 

 duced at the same temperature on the 

 hydrous sulphate of zinc : hence he in- 

 fers that a movement of the particles 

 of a solid body may take place, by which 

 the particles take a new symmetrical ar- 

 rangement, and form a new mineral 

 species. 



The most extraordinary fact, however, 

 discovered by Professor Mitscherlich 

 relates to the influence of heat on the 

 double refraction of sulphate of lime. 

 In this mineral, which has two resultant 

 axes in the plane of the laminae inclined 

 60, these two axes, P, P, fig. 33, gra- 

 dually approach with heat till they unite 

 at O, and when further heated they again 

 open out on each side of o towards A 

 and B. 



An analogous fact of equal interest has 

 been recently observed by Dr. Brewster, 

 in Glauberite. This crystal, at ordinary 

 temperatures, has one axis of double re- 

 fraction for violet, and two axes for red 

 light. By applying a heat below that of 

 boiling water the weaker axis for red light 

 disappeared altogether in consequence of 

 the two resultant axes P, P, fig. 33, 

 uniting in O. By a slight increase of 

 heat, the resultant axes again opened out 

 in the plane A B, indicating the creation 

 of a new axis for red light. By the ap- 

 plication of artificial cold the single axis 

 for violet light at O opened out towards 

 P and P, producing two resultant axes in 

 the same plane as that of the two axes 

 for red light at ordinary temperatures. 

 At a certain temperature the violet axis 

 also opened up in the plane A B. * 



* Edinburgh Transactions, vol. xi, 



