126 Progress in Science. (January, 
Prof. GurHR1E—Having had the opportunity of seeing Mr. Barrett’s experi- 
ments, I think Mr. Barrett remarked the re-heating passed along’the wire in a 
definite direction; the glow did not appear immediately all over, but_travelled 
along the wire. 
Prof. G. C. Foster—The wire is not likely to rise equally in temperature, 
or to fall equally, through the whole wire. The ends are fastened to masses 
which tend to cool them quicker than the central parts of the wire. Is there 
evidence of anything more than this in the phenomenon described ? 
Prof. BARRETT—It is the fa@, as Prof. Guthrie has mentioned, that the 
‘“‘after-zglow ’—as Prof. Tait has called it—does not appear simultaneously 
throughout the wire. A wave of heat seems to travel along the wire, and the 
sudden rise of temperature and measured progress of this wave are very beau- 
tiful to observe. I do not think the existence of this wave is due—at any rate 
not wholly due—to the unequal distribution of heat caused by the masses of 
metal to which the wire is attached. For, in reply to Professor Foster’s 
question, I may remark that if an iron wire, such as that tying down the 
cork of a soda-water bottle, be coiled round a pencil, and the helix of wire 
thus formed heated in some dark corner in a Bunsen gas flame, on the with- 
drawal of the wire from the flame the after-glow will be readily seen, generally 
traversing the wire in the manner already described. I am inclined to think 
the existence of this wave may be due either to an inequality in the diameter 
of the wire, to a want of homogeneity in its structure, or to the accidental 
passage of air-currents; for the progress of this wave is sometimes in one 
direction and sometimes the opposite. 
Mr. HERBERT SPENCER—Is it the fact that the wire having once undergone 
this transformation, when it passes to a lower temperature, and is again heated, 
it will undergo a similar change ? 
Prof. BARRETT—This is the case. The after-glow can be made to appear 
a dozen or more times upon the same wire. But the phenomenon becomes 
fainter each time. The gradual extinétion of the after-glow which I have thus 
obtained seems to depend very much on the degree of tension to which the 
wire has been subjected during the heating and cooling; but this is a point I 
have reserved for future experiments, and cannot therefore speak positively 
upon it now. 
Prof. EverETT—Is there a want of symmetry between the sudden change 
which occurs during the heating and that which occurs during the cooling ? 
Would Prof. Barrett be good enough to draw the temperature curves for both? 
Prof. BARRETT—I am much obliged to Prof. Everett for his suggestion. 
The following curve will approximately represent the progressive temperatures 
of the iron wire during its heating: where A B, in Fig. 6, represents the time 
Fic. 6. 
oF] 
d"g 
ee 
Lee 
A= as ——f 
Fra. 7. 
ee 
d a 
ag 
phere is eee B 
the wire is exposed to the source of heat, and a c the increment of tempera- 
ture, judged by the eye or an air thermometer. A pause in heating, dd’, 
