150 REPORT—1890, 
doubtless the case even in thin wires, and this being so the strains in 
the wire will no longer be balanced if the inner and outer portion of the 
wire be at different temperatures. For when the wire is twisted or bent 
permanently, it has been submitted to a greater twisting or bending than 
that which it retains; a strain has been given which is larger than that 
which remains when the stress is removed. Now when the wire is raised . 
to the critical temperature, the existence of this original strain reveals 
itself, owing to the greater plasticity of the molecules of the iron which 
have reached the higher temperature, and hence the additional twisting 
or bending which Mr. Tomlinson has observed. By keeping the wire at 
the critical temperature for some time, it is rendered free from strains 
and the effect disappears. 
It is probable that the anomalous contraction and expansion in iron 
which we have been studying is an effect due to the Jongitudinal strains 
in the wire produced by wire-drawing, and which are destroyed by fre- 
quent heating; the non-existence or rapid subsidence of the effect in 
some specimens of very soft iron follows from this explanation, together 
with the more pronounced and enduring effects noticed in steel. Never- 
theless, some other cause appears to exist in hard steel where the effect 
appears to be more or less permanent. We hope to throw more light on 
this point next year, as our experiments are being continued.! 
During the momentary elongation of the wire at the critical tempera- 
ture in cooling, a singular creaking sound is also to be observed. The 
sound resembles that produced by bending tin; in thick wires it more 
resembles a strip of tin struck on the edge by a piece of wood; it is a 
succession of short sounds or ticking, lasting during the anomalous 
change. This crepitation is evidently similar to that noticed by M. Le 
Chatelier at a lower temperature. It reminds one of the crepitation 
heard on magnetisation first noticed by Page in 1837. 
III. We now come to the reglow or Recalescence of iron and steel at 
the critical temperature first noticed by Barrett (‘ Phil. Mag.,’ 1873), and 
which Mons. Osmond has made the starting-point of his admirable 
investigations. 
What is observed by the eye is as follows: The wire heated either by 
a current or gas flame gradually becomes luminous, then as a certain 
temperature is reached the glowing of the wire ceases to increase, and 
1 In a paper published in the Comptes Rendus for July 8, 1889, M. André Le 
Chatelier has shown that in the heating of iron three most remarkable phases in its 
mechanical properties are to be observed. In this respect it behaves differently from 
all other metals he has examined. From 15° to 80° C. the breaking strain, slowly 
applied, decreases with an increase of temperature like other metals. But from 
100° to 240° C. the breaking strain is sensibly constant, and the elongation at rupture 
is much diminished. From 240° C. to 300° C. the breaking strain suddenly increases 
and the elongation also increases. From this point onwards the breaking strain 
decreases, but at 300° C. iron possesses its maximum strength to a steady strain, 
though it is then weakest as regards a sudden shock. It may here be worth noting 
that the temperature of 285° to 300° C. (as Mr. Tomlinson has recently observed) 
has a special significance in connection with the so-called Villari critical point, that 
is, the value of the magnetising force for which the permeability is not altered by 
alteration of stress on the experimental wire. This point varies both with the value 
of the load and the temperature of the wire. M. Le Chatelier, so far as we are 
aware, has not carried his investigation above 500° C., where still more interesting 
results may be expected, M. Le Chatelier finds that the elongation of iron under 
stress at about 150° C. is accompanied by a crackling sound, the lengthening not 
taking place continuously but in a series of jerks, a fact which he observed in up- 
wards of 200 experiments and in all the alloys of iron. 
ee —— ose 
