818 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion of the electrostatic capacity of the carborundum. The black lines 

 running in a general horizontal direction across the surface of the 

 specimen are outcroppings of conducting dykes of the crystal ; while 

 the white spaces separating the black lines are outcroppings of the in- 

 sulating matrix in which these dykes are imbedded. The lines which 



TABLE IX. 

 Values of C, RC, and R for Specimen II. 



are black and white in the pictures are really merely dlfferenthj colored 

 in the crystal. The white regions are generally a transparent blue in 

 the crystals, and the regions reproduced as black are usually seen as 

 brown or red in the specimen. 



The difference in electrical conductivity of the dark and light strata 

 was discovered by mounting the specimen in the field of the micro- 

 scope, and exploring it with two pin-points serving as electrodes. The 

 pin-point electrodes were connected with a battery and galvanometer, 

 or telephone, and were moved about over the surface of the specimen 

 by means of two independent mechanical microscope stages insulated 

 from each other. By means of the slow motion of the mechanical 

 stages, the two pin-points could be put down upon the same dyke or 

 on two different dykes, or one pin could be placed on a dyke while the 



