264 On a momentary Molecular Change in Iron Wire. [Jan. 28, 



much less (in one instance g-J-j of the length of the heated part) than 

 when a battery was employed, apparently in consequence of the wire being 

 less intensely heated. 



A large number of experiments were made with wires of palladium, pla- 

 tinum, gold, silver, copper, lead, tin, cadmium, zinc, brass, german- silver, 

 aluminium, and magnesium (wire and ribbon), diminishing the length and 

 thickness of the wire in each case, and adjusting the tension until suitable 

 temperature and strain were obtained ; but in no instance could a similar 

 molecular change to that observed in iron be detected. Palladium and 

 platinum wires of different lengths, thickness, and degrees of strain were 

 examined at various temperatures, up to that of a white heat ; but no irre- 

 gularity of cohesion, except that of gradual softening at the higher tempera- 

 tures, was observed ; they instantly contracted with regular action on stopping 

 the current. Several gold wires were similarly examined at different tem- 

 peratures up to that of a full red heat ; no irregularity occurred either 

 during heating or cooling ; but little tension (about 4 ounces) was applied, 

 on account of the weak cohesion of this metal. Wires of silver similarly 

 examined would only bear a strain of about 2 ounces, and a temperature 

 of feeble red heat visible in daylight ; no irregularity of elongation or con- 

 traction occurred during heating and cooling. By employing exactly the 

 proper temperature and strain, a very interesting phenomenon was ob- 

 served ; the wire melted distinctly on its surface without fusing in its inte- 

 rior, although the surface was most exposed to the cooling influence of the 

 air ; this occurred without the wire breaking, as it would have done if its 

 interior portion had melted ; the phenomenon indicates the passage of the 

 electricity by the surface of the wire in preference to passing by its inte- 

 rior. Wires of copper expanded regularly until they became red-hot ; they 

 then contracted slightly (notwithstanding the strain applied to them), 

 probably in consequence of a cooling effect of increased radiation produced 

 by the oxidized surface, as a similar effect occurred with brass and german- 

 silver*. On stopping the current the wire contracted without manifest 

 irregularity. Wires of lead and tin were difficult to examine by this method, 

 on account of their extremely feeble cohesion and the low temperature at 

 which they softened: wires about 1*63 millimetre diameter, 25*5 centi- 

 metres long (with a strain upon them of about one ounce), were employed ; 

 no irregularity was detected. Wires of cadmium from 1*255 millimetre to 

 1'525 millimetre thick, and 24-2 centimetres long (with a strain of two 

 ounces), exhibited a slight irregularity of expansion at the lower tempera- 

 tures ; they elongated, and also cooled, with extreme slowness, more slowly 

 than those of any other metal. Wires of zinc exhibited a slight irregu- 

 larity of expansion, like those of cadmium ; the most suitable ones were 

 about 25 centimetres long and T2 millimetre in diameter, with a strain 

 of 10 ounces. Wires of brass and german-silver, when heated to redness, 



* This supposition does not agree with the results obtained with iron wire, which also 

 oxidizes freely. 



