70 
Proceedings of the Royal Society of Edinburgh. [Sess. 
classed the crystals in two groups. Those in the first group exhibit little 
hysteresis and do not change when heated. The crystals of the second 
group have great hysteresis, and suffer some deterioration on heating. 
The critical temperature of the members of the first group is 645°, and 
that of the second group 660° C. 
A. Abt’s tests * * were made on bars cut from blocks of massive hsematite 
of great purity, and they relate chiefly to the remanent magnetism of the 
^ mineral after subjection to fields of increas- 
ing value. The coercive force measured was 
150 C.G.S., and the bars were apparently 
unsaturated in a field of 1000 C.G.S. 
So far as is known to the authors, 
no continuous observation of the magnetic 
behaviour of the ordinary pure artificial 
ferric oxide when heated in air or in other 
gases has yet been made, and the present 
paper deals with this phase of the subject. 
o 
MAGNETISING 
COIL. 
SIDE 
COIL. 
|T~] MAGNETOMETER. 
COMPENSATING 
COIL. 
Method and Apparatus. 
The method and apparatus were practi- 
q LW1R cally the same as employed on former 
occasions.^ For the purpose of this paper, 
the method may be shortly recapitulated. 
The bar to be tested is placed in a com- 
bined magnetising coil and electric furnace 
(see figure); a current is passed in series 
through the magnetising and compensating 
coils, and through a small side coil intended 
to correct want of alignment in the other 
two coils. £ 
The specimen when in the furnace can be tested either by passing it 
through a heat cycle, in which the temperature is gradually raised from 
that of the room to 1000° C. and then brought back to room temperature 
again, or by keeping it at a constant high temperature. In both cases 
magnetometer readings are taken at intervals, such readings indicating the 
magnetic state of the bar at the time. The temperature of the bar is 
taken by means of a thermo-couple with its junction placed at the middle 
* Wied. Ann., lxviii. pp. 658-673, 1899. 
t Phil. Mag., Jan. 1904 and April 1909. 
X Ibid., April 1909, p. 574. 
