154 REPORT—1890, 
tures he found practically coincident with the sudden loss of magnetic 
susceptibility of the metals. Le Chatelier finds mild and hard steel show 
two changes of curvature in their electric resistance, one at 850° C. and 
the other at 710° C., whereas manganese steel, in which we find re- 
calescence to be absent,! shows no such change, the curve of increased 
resistance with temperature being perfectly regular. Le Chatelier also 
finds that pure nickel undergoes a sudden change in its electric resist- 
ance, the temperature coefficient altering at 340° C., which corresponds 
to the temperature of other changes in its physical properties. 
Fia. 2,—Electric Resistance in ohms of Wires of the Metals named. 1 metre long 
and 1 mm. diam. heated from 0° to 1,000° C., in pure dry hydrogen (Le Chatelier). 
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200° 500° 800° 1000° C 
VII. Some recent experiments made by Dr. E. Ball? appear to indicate 
that there is a still higher critical point than those observed by Osmond 
and ourselves. Dr. H. Ball has measured the tensile strength and 
roughly the magnetic state of iron and steel suddenly cooled down from 
different high temperatures. He finds that there are three critical points 
when a change in the tensile strength and magnetic character of ironand 
steel occur with sudden quenching ; two of these points agree with 
Osmond’s a, and a3, but the third point is higher than either of these ; he 
estimates it approximately as 1,300° C. More exact means of measuring 
the temperature and magnetic susceptibility are, however, necessary. 
VIII. This higher temperature is near that at which M. Pionchon? 
has found a change in the specific heat of iron. Pionchon’s results show 
that the specific heat of iron changes suddenly between 660° and 720° C., 
1 Barrett, Proc. R. Soc., Dublin, Dec. 1886. 
2 Proc. Iron and Steel Institute of Great Britain, 1890. 
3 Pionchon, Comptes Rendus, June 1886. 
