Electric Conductivity of various forms of Carbon. 25 



School of Mines, as having been shown to him several years 

 since by Dr. von Kobell, of Munich. As I have not found any 

 account of it published, I have ventured to bring it before this 

 Society. 



A fragment of the substance to be tested, whether charcoal, 

 coke, anthracite, or other form of carbon, is held between the 

 jaws of a pair of tongs formed by bending a strip of sheet zinc 

 mto a horseshoe form, and immersed in a solution of cupric sul- 

 phate. If the carbon is a non-conductor, the copper salt is de- 

 composed, and deposit of copper only takes place on the immersed 

 surface of the zinc; but when it possesses a high degree of con- 

 ductivity a zinc-carbon couple is formed, and deposit of copper 

 takes place on the surface of the carbon as in ordinary electro- 

 typing. 



Of the different forms of carbon experimented upon, the most 

 rapid results have been obtained with some American anthracites, 

 and coals that have been subjected to the action of intruded 

 igneous rocks. The most remarkable of these is an anthracite 

 from Peru, which contains a large amount of sulphur in organic 

 combination, and is found in a nearly vertical position, interstra- 

 tified in quartzite, in the high plateau of the Andes, about 13,000 

 feet above the sea-level, near Truxillo. It is probably of secondary 

 age, the metamorphism having taken place at the time of the 

 great trachytic outbursts which form the gold- and silver-bearing 

 rocks of the adjacent mining-district. This is coppered by im- 

 mersion almost as readily as graphite. The anthracite of Penn- 

 sylvania possesses the same property, but not in quite such a 

 high degree. The Heathen coal of South Staffordshire, when 

 altered by the intrusion of the "white-rock " trap, is more slowly 

 coppered ; but this is probably due to the resistance interposed 

 by the numerous laminae of ealcite filling the fractures in the 

 mass of the coal, which renders the conductivity less perfect. 

 A specimen of coal from Bengal, altered in the same manner by in- 

 trusion of igneous rock, behaves much in the same way as coke, 

 being coppered directly. This is rather remarkable, as this coal 

 is a very impute one, and contains such a large quantity of water 

 very intimately combined, probably as a hydrated silicate inter- 

 spersed through the mass, as to decrepitate explosively when 

 suddenly heated. 



The ordinary Welsh anthracite does not appear to be a con- 

 ductor by this method; but after having been heated to a full 

 red heat it conducts electricity freely. The lowest temperature 

 at which this change takes place appears to be somewhere be- 

 tween the melting-points of zinc (430° C.) and silver (1000° C), 

 as fragments of anthracite packed in a thin clay crucible and 

 plunged into molten zinc were not foui.d to be altered, but were 



