56 CORRELATION OF PHYSICAL FOKCES. 



So when the structure of a substance is not homogeneous, 

 we have a change in the conduction of different parts depend- 

 ent upon the structure. This is beautifully shown with 

 bodies whose structure is symmetrically arranged, as in crys- 

 tals. Senarmont has shown that crystals conduct heat differ- 

 ently in different directions with reference to the axis of 

 symmetry, but definitely in definite directions. His mode of 

 experimenting is as follows : A plate of the crystal is cut in 

 a direction, for one set of experiments parallel, and for 

 another at right angles to the axis ; a tube of platinum is in- 

 serted through the centre of the plate, and bent at one 

 extremity, so as to be capable of being heated by a lamp 

 without the heat which radiates from the lamp affecting the 

 crystal ; the surfaces or bases of the plate of crystal are 

 covered with wax. When the platinum is heated, the direc- 

 tion of the heat conducted by the crystal is made known by 

 the melting of the wax, and a curved line is visible at the 

 juncture of the solid and liquid wax. This curve, with 

 homogeneous substances, as glass or zinc, is a circle ; it is 

 also a circle on plates of calc spar cut perpendicular to the. 

 axis of symmetry ; but on plates cut parallel to the axis of 

 symmetry, and having their plane perpendicular to one of the 

 faces of the primitive rhombohedron, the curves are well- 

 defined ellipses, having their longer axes in the direction of 

 the axis of symmetry, showing that this axis is a direction of 

 greater conductibility. From experiments of this character 

 the inference is drawn, that * in media constituted like crys- 

 tals of the rhombohedral system, the conducting power varies 

 in such a manner, that, supposing a centre of heat to exist 

 within them, and the medium to be indefinitely extended in 

 all directions, the isothermal surfaces are concentric ellipsoids 

 of revolution round the axis of symmetry, or at least surfaces 

 differing but little therefrom.' 



Knoblauch has further shown, that radiant heat is absorb- 

 ed in different degrees, according as its direction is parallel 

 or perpendicular to the axis of a crystal. 



