Vol. 4] Thelen.— Thermal Conductivities of Certain Schists. 215 



For the purposes of demonstration, that is, to obtain wax- 

 figures which would stand out prominently, and which could 

 easily be seen and photographed, numerous expedients were tried. 

 Paris white and various Diamond Dyes were dissolved in the 

 wax to give it body. Paraffin and beeswax were used. Nothing 

 more satisfactory than white wax developed, and the pictures 

 show that it was unnecessary to waste further time in experi- 

 menting along this direction. (See plates 26 and 27.) 



THE CONVENTIONS USED IN SPEAKING OP THE SCHISTS. 



For convenience of reference, the three axes will be denoted 

 by A, B, and C, as in crystallography, the rocks being set up 

 according to the following scheme. A plane parallel to that of 

 the schistosity always contains A and B, and will be designated 

 as the top face of the rock. If in this plane there exists also a 

 linear orientation of the crystals, this direction is A, and is placed 

 pointing directly away from the observer. If the schistosity, 

 however, appears to be linear, only the A axis is fixed, and the 

 choice of the top face is arbitrary until measurements on the 

 face perpendicular to A give the direction of the major axis of 

 the ellipse on this face. The direction of this major axis and 

 that of the linear schistosity then determine the basal plane. If 

 the schistosity is of two dimensions, but there is no linear orien- 

 tation of the crystals, only the top face is determined, and. if 

 the conductivity depended upon the structure, we would expect 

 to find equal conductivity in all azimuths in this plane. Hence 

 the directions of A and B become matters of indifference. 



With our schists thus set up, we can imagine each rock cut 

 into the shape of a rectangular parallelopipedon, with the pina- 

 coicls each containing two axes, and then we may use the term- 

 inology of the orthorhombic system, since all four rocks yielded 

 heat conductivity ellipsoids which are analogous in every impor- 

 tant respect to the light conductivity ellipsoids of the ortho- 

 rhombic system, — the axes of elasticity are unequal, are rect- 

 angular and coincide in direction with the three rectangular 

 crystallographic axes of symmetry. 



We have then, in general, three sections to work upon : the 

 top or basal pinacoidal face (containing A and B) shows the 



