634 Coefficient of Expansion of Pure Nickel. 
The anomalous part of the curve extends from 340° to 
370°, which is very approximately the range over which 
changes occur in the thermoelectric force and resistance of 
the same specimen of nickel. Moreover, it has recently been 
shown by the author, in some experiments not yet published, 
that this interval is also that over which the magnetic per- 
meability of the same specimen changes—the actual critical 
temperature, at which the susceptibility vanishes, being just 
over 370°. 
The present experiments may thus be summarized as 
follows :— 
(1) For pure nickel there exists a sudden change in the 
coefiicient of expansion, between the temperatures of 365° 
and 370° C. 
(2) The change is of the same magnitude, and occurs 
at the same temperature, whether the wire is heated or 
cooled. 
(3) No permanent elongation of the nickel is observed after 
heating. 
(4) Successive heatings and coolings entirely fail to ‘‘ wipe 
out ” the effect, or indeed to modify it in the least. 
In view of the permanent elongation which occurred in alli 
Gore’s experiments, the effect he observed was no doubt 
largely a question of change in the elastic constants. It has 
been pointed out in the ‘Report of the British Association 
Committee that the presence of carbon in the iron, and the 
consequent formation of carbides at the critical temperature, 
largely explained these and other peculiarities in iron. ae 
in view of the purity of the present specimen of nickel, 
seems unlikely that such an explanation would account Be 
the facts above described ; it appears more probable that in 
this case the real cause of the phenomenon which occurs at 
the “critical temperature ” is to be sought for in the metal 
itself, although in the case of impure metals the true effect 
might be masked and modified by changes of a chemical 
nature. 
In conclusion I wish to express my best thanks to Professor 
J. J. Thomson for the facilities that he has placed at my 
disposal and for kind advice given during the course of these 
experiments at the Cavendish Laboratory. 
