RECENT PROGRESS IN PHYSICS. 



455 



§ 54. Mechanism of fusion. — Whenever electrical fusion occurs, 

 there is a mechanical separation of the melted mass ; hence, this 

 fusion is only the effect of heat upon finely divided metal. The dif- 

 ference between fusion by fire and electricity, Biess has characterised 

 as follows : 



" When fire acts on a metal, it heats the metal as an entire mass 

 to the melting point ; electricity, on the contrary, heats the metal (as 

 a whole mass) only to a temperature below the welding point, and 

 completes the fusion hy simultaneous dissipation and heating." 



FranMin proposed in 1747, the view, which he afterwards aban- 

 doned, that lightning loosens the cohesion of a metal without the aid 

 of heat, and brings about a cold fusion. This view was taken up 

 again by BerthoUet, who explained the operation of electricity on a 

 substance, by a separation of the particles, and supposed the heat de- 

 veloped to fuse it, as only a secondary phenomenon. 



This opinion is in some respects true, according to the experiments 

 just given, but it leaves entirely out of consideration the heat which, 

 occurs before the mechanical effect ; on the other hand, the view gen- 

 erally held subsequently, that electrical fusion is wholly the result of 

 heating, is just as one sided, for it disregards the mechanical effect. 



§ 55. Changes in the coefficient of retardation of metals with 

 INCREASING MECHANICAL EFFECTS. — We have sccn that between the tem- 

 perature h of a wire, the quantity of electricity q, and the number 

 of jars s, the relation 



h=z n — 



subsists, iu which % is a constant factor during a whole series of ob- 

 servations. This is no longer the case when a wire in the conducting 

 circuit is affected mechanically by the discharges passed through it 

 and is brought to ignition, as appears clearly from the following 

 results: A platinum wire, 17 inches long, with a radius of 0.0209 

 lines, being inserted in addition to the thermometer, the result was : 



Thus the coefiicient of retardation decreases when mechanical 

 effects and incandescence are produced by increased charges, but it 

 increases again by melting. 



• JRiess is of the opinion that the phenomena of heat obtained by a 

 continuous transmission in the wire are produced by the electricity 



