42 CORRELATION OF PHYSICAL FORCES. 



sity in the physical character of bodies, and for every change 

 in the structure and arrangement of particles of the same body, 

 a change is apparent in the thermal effects. Thus gold con- 

 ducts heat, or transmits the motion called heat, more readily 

 than copper, copper than iron, iron than lead, lead than 

 porcelain, &c. 



So when the structure of a substance is not homogeneous, 

 we have a change in the conduction of heat by different parts 

 which is dependent upon the structure. This is beautifully 

 shown with bodies whose structure is symmetrically arranged, 

 as in crystals. Senarmont has shown that crystals conduct heat 

 differently 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 inserted 

 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 

 of the plate of crystal are covered with wax. When the 

 platinum is heated, the direction 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 con- 

 ductibility. From experiments of this character the inference is 

 drawn, that ' in media constituted like crystals of the rhombo- 

 hedral 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 absorbed 



