ON MOLECULAR PHENOMENA IN MAGNETISED IRON. 151 
even in some specimens a sudden darkening occurs. At this moment 
the anomalous contraction takes place. Heating now continues regularly 
till the wire reaches the melting-point. On cooling the luminosity de- 
creases until the moment when the anomalous expansion takes place, then 
a sudden flash runs through the wire, first beginning at the cooler parts 
and suffusing the whole with a bright glow. This phenomenon of 
Recalescence is most beautifully observed by heating to whiteness with a 
blow-pipe flame the centre of a thin steel plate; a concentric ring of 
darkening will be seen to spread outward and in like manner a beautiful 
incandescent circle runs inward during cooling.! It is needless to refer 
to our experiments made long since, which showed that recalescence was 
not a mere surface effect but a rise of temperature throughout the wire, 
and that it occurred equally when the steel was enclosed in tubes con- 
taining pure nitrogen as well as in other gases. In some specimens of 
iron recalescence could not be seen, but it was present in all specimens 
of steel and was found whenever the jerk occurred in cooling. Numerous 
diagrams were also made of the duration of the after-glow in steel wires 
of various thicknesses. 
It was noticed by one-of us? soon after the discovery of recalescence 
that a faint second glow could be seen; the first and far stronger after- 
glow being exactly coincident with the sudden elongation of the steel 
wire during cooling. Thusin a No. 17 B.W.G. soft steel wire, cooling 
from a white heat unprotected in the air, five seconds elapsed before the 
first after-glow was seen and thirteen seconds before the second glow ; no 
jerk or anomalous expansion being noticed with the second glow, but an 
expansion of 0°2 mm. in a wire 20 centims. long being noticed at the first 
glow. The same result was found with different tensions. In thicker rods 
of soft steel, three glows were noticed, but it is difficult to discriminate 
the subjective and misleading effects produced by expectant attention in 
the faint glows, thermometric methods alone being reliable. This has 
since been accomplished by Osmond, who was the first to determine the 
exact temperatures of recalescence in iron and steel. Continuing the 
early experiments, an attempt was made in 1875 by Barrett to measure 
the temperature of recalescence by observing the amount of expansion 
that occurred in the steel raised from the temperature of the air to the 
critical point. Assuming that the known rate of increase of the coefficient 
of expansion in steel with rise of temperature continued regularly, it was 
found that 830° C. was approximately the temperature of the critical 
point. The uncertainty of the data on which this estimate was founded 
and the difficulty of measuring these high temperatures then with any 
‘approach to accuracy, prevented the publication of a result which turns 
mt now to be not very wide of the mark. M. Le Chatelier has lately 
‘ound the coefficient of expansion for iron at 1,000° C. to be 0:0000145 for 
1°C.; but measurements exactly at the critical point appear to be wanting. 
We now come to M. Osmond’s valuable investigations, which com- 
_menced in 1886. By means of a pyrometer similar to that used by 
? Newall: Camb. Phil. Soc., January 1888. 
? Barrett, unpublished laboratory notes, 1875. 
§ «Transformations du Fer et du Carbone dans les Fers, les Aciers et les Fontes 
Blanches,’ par F. Osmond, Afémoires de UArtillerie de la Marine. A summary of 
M. Osmond’s work is given in his paper read before the Iron and Steel Institute of 
Great Britain in the early part of the present year. M. Osmond has kindly lent us 
nM nens he has employed, and we hope to repeat some of his determinations 
shortly. 
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