ELECTRICITY. 97 



ably modify the effects, and might account for the opposite 

 character of the results with these two metals. 



Matteucci has made experiments on the conduction of 

 electricity by bismuth in directions parallel or transverse to 

 the planes of principal cleavage, and he finds that bismuth 

 conducts electricity and heat better in the direction of the 

 cleavage planes than in that transverse to them. 



Many other experiments have been made both on the pro- 

 duction of thermo-electric currents by two portions of the 

 same crystalline metal, but with the planes of crystallization 

 arranged in different directions relatively to each other, and 

 also on the differences in conduction of heat and electricity 

 according to the direction in which they are transmitted with 

 reference to the planes of crystallization. 



It is found, moreover, that the slightest difference in ho- 

 mogeneity in the same metal enables it when heated to pro- 

 duce a thermo-electric current, and that metals in a state of 

 fusion, in which state they may be presumed to be homoge- 

 neous throughout, give no thermo-electric current : thus, hot 

 in contact with cold mercury has been shown by Matteucci 

 to give no thermo-electric current, and the same is the case 

 with portions of fused bismuth unequally heated. 



The fact that the molecular structure or arrangement of a 

 body influences indeed I may say determines its conduct- 

 ing power, is by no means explained by the theory of a fluid ; 

 but if electricity be only a transmission of force or motion, 

 the influence of the molecular state is just what would be 

 expected. Carbon, in a transparent crystalline state, as dia- 

 mond, is as perfect a non-conductor as we know ; while in an 

 opaque amorphous state, as graphite or charcoal, it is one of 

 the best conductors : thus, in the one state, it transmits light 

 and stops electricity, in the other it transmits electricity and 

 stops light. 



It is a circumstance worthy of remark, that the arrange- 

 ment of molecules, which renders a solid body capable of 

 5 



