PIERCE AND EVANS. — CAPACITY OF CARBORUNDUM. 819 



other was placed on one of the lighter colored strata separating the 

 dykes. 



It was found that the insulation resistance of a very thin layer of the 

 lighter colored material was so great that the current through it under 

 an impressed e. m, f. of 7 volts did not give a perceptible deflection on 

 the galvanometer, that is, the resistance was more than 7000 million 

 ohms, whereas the resistance of the circuit when both pins were upon 

 the same dark-colored dyke was only a few thousand ohms at this 

 voltage. 



It was not possible with some of the specimens to get into electrical 

 contact with all of the dark-colored outcroppings, because in some 

 cases it was evident that the outcroppings were in depressions or were 

 coated over so that the electrodes could not be brought against them. 

 With other specimens and with the aid of a telephone receiver in the 

 exploring circuit, we could make attachment of one electrode to a 

 solder-bed in which the crystal was mounted and could draw the other 

 electrode across an exposed face of the crystal in the field of the micro- 

 scope and hear a click in the telephone as the moving electrode passed 

 over each of the dark-colored outcroppings. 



Outcrop of Conducting Points. — In addition to the linear outcrop- 

 pings of the conducting dykes, there are also with some of the speci- 

 mens minute points of outcrop of conducting material. Some of these 

 points are visible in Micrograph d. They are the globular markings 

 near the left-hand side of the picture. Each globular marking is a 

 minute darker speck surrounded by a whitish circle. In some cases 

 we found an outcrop in the form of a minute speck, which evidently 

 communicated with one or more conducting layers within the crystal, 

 and which showed measurable capacity when one electrode was brought 

 into contact with the speck while the other electrode was in contact 

 with a distant corner of the crystal. In fact, with the specimen of d 

 many of the apparently linear outcroppings are made up of discrete 

 points of conducting material. 



A Visible Condenser in Micrograph h. — As an illustration of a con- 

 denser completely visible, reference is made to Micrograph h. The left 

 half of the picture shows two nearly parallel lines running up through 

 the center and down along the left-hand side of the specimen. These 

 lines, which run out of the field at the bottom and top of the picture, 

 were seen, by moving the specimen in the original examination of this 

 specimen, to be really two closed curves, one inside of the other. They 

 are the outcroppings of two practically flat parallel strata nearly perpen- 

 dicular to the direction of vision. By exploration with the pin-point 

 electrodes these strata were found to be conducting, whereas the whitish 



