ELECTRICITY. 83 



porary diminution in the coefficient of elasticity in wires while 

 they are transmitting the electric current, which is independent 

 of the heating effect of the current. 



M. Dufour has made a considerable number of experiments 

 with the view of ascertaining if any permanent change in 

 metals is effected by electrisation. He arrives at the curious 

 result that in a copper wire through which a feeble voltaic 

 current has passed for several days, a notable diminution in 

 tenacity takes place ; while, in an iron wire, the tenacity is 

 increased ; and that these effects were more perceptible when 

 the wires had been electrised for a long time (nineteen 

 days) than for a short time (four days), The copper wire 

 was, in his experiment, not perfectly pure ; so that the effect 

 or a portion of it, might be due to the state of alloy : in the 

 case of iron, the magnetic character of the metal would pro- 

 bably 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 respectively parallel or 

 transverse to the planes of principal cleavage, and he finds 

 that bismuth conducts electricity and heat better in the direc- 

 tion of the cleavage planes than in that transverse to them. 



Many other experiments have been made both on the 

 production of thermo-electric currents by two portions of the 

 same crystalline metal, but with the planes of crystallisation 

 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 crystallisation ; the results varying 

 with the arrangements. 



It is found, moreover, that the slightest difference in 

 homogeneity 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 



G 2 



