476 Prof. W. F. Barrett on certain remarkable Molecular 



IV. 



On September 12th I was examining the condition of the 

 wire in a darkened room, when a new and unexpected change 

 revealed itself. During the cooling of the wire it was found 

 that just as it reached a very dull red heat, a sudden accession 

 of temperature occurred, so that it glowed once more with a 

 bright red heat. Illuminating the index and scale of the appa- 

 ratus, which was watched by an assistant, it was at ODce found 

 that the reheating of the wire occurred simultaneously with the mo- 

 mentary elongation. Necessarily no change of this kind can be ob- 

 served on heating; but the reglowing of the wire on cooling is 

 most uniform and conspicuous *. The wire must first be heated to 

 whiteness; and then, being allowed to cool, just as it reaches a 

 point of barely visible redness a sudden cherry-red glow takes 

 place, passing as a wave of heat from one end of the wire to the 

 other, or from both ends to the centre. The measured progress 

 of this wave of temperature along the wire is extremely beautiful 

 to observe. On first sending the current through the wire, the 

 heating begins at one extremity and runs along to the other; on 

 breaking contact, this reheating sweeps along the wire in the 

 contrary direction. This peculiar movement, therefore, may be 

 caused by the unequal thickness of the wire ; though I do not 

 think this is the explanation, as the reheating would then move 

 in the same direction as the heating (namely, from the thinner 

 to the thicker parts of the wire), and this is not the case. I hope 

 shortly to investigate this further. 



When the wire is heated by a row of gas-flames, the same re- 

 sults take place, although the heating by the battery is a far 

 neater and more satisfactory way. 



It is a real accession of temperature, a sudden increase in 

 thermal as well as luminous radiation. This is evident from the 

 following experiment. A wide glass tube (fig. 2) was fitted 

 with corks at each end, so that the iron wire could be enclosed 

 air-tight within the tube. At one end the cork was perforated 

 to allow the insertion of a narrow glass tube bent at right 

 angles, the lower end of which dipped into coloured water. On 

 heating the wire to whiteness by the current, some of the en- 

 closed air was expelled, and on breaking contact the liquid rushed 

 up the tube, but midway suddenly stopped in its course, and 

 was depressed some two inches. At this moment the assistant, 

 who was watching the wire, gave notice the wire drooped and 

 glowed again. There is no difficulty in repeating this experi- 



* Nevertheless during heating I thought I detected a momentary pause 

 in the progress of the reddening of the wire, just after incandescence had 

 been reached. 



