the Magnetism of Basalt. ae 
about the same temperature as during the heating, reaches 
the temperature of the air with about 1/7 of its initial 
strength. The further heating of the bar develops a maximum 
point, or it may be two maxima (see Curve VIII.) about 480°, 
the minimum being near 540°. 
Curve IV. (fig. 4) was obtained as the result of a test 
made on the bar Rowley Blue No. 1, which had previously 
been used in furnace trials. The curve had retained the 
same character for at least six previous heatings. There is 
no sudden change, and the susceptibility increases gradually 
toa maximum at about 185° ,and then diminishes to a minimum 
in the neighbourhood of 550°. 
The next three Curves (IX., X., and XI. in fig. 6) repre- 
sent the results obtained for two bars cut from a piece of 
Dattenberg basalt. It will be seen at once that their be- 
haviour is very different from that of the Rowley basalt, 
being distinguished by a low temperature of maximum sus- 
ceptibility and a gradual loss of magnetic strength with rise 
of temperature. ‘For four out of five bars from this kind of 
rock the temperature of maximum strength was considerably 
below 100°C. The Dattenberg rock also does not deteriorate 
with heat so rapidly as that from Rowley, presumably because 
the former rock is of more recent formation. The bar Dat- 
tenberg No. 1 showed maximum permeability at 32° C. when 
being heated, and 35° on cooling. The curve for the first 
heating is not given, as it was marred by motion of the 
compensating-coil. During the third heating a maximum 
appeared at about 45° C., and on cooling the maximum had 
not been reached at 24°. ‘The second bar gave a maximum 
at 143°, whilst the cooling curve was very similar to that 
obtained for Rowley Blue No. 1 (fig. 5). The effect of 
heating was to raise the susceptibility of the cold bar to 
nearly 2-4 times its initial value; an increase in strength 
due to heating may also be now in Curves VI., VII., and 
VIII. The two bars were cut from a slice taken transversely 
from a basaltic column, and their temperature-magnetization 
curves are distinguished from the others by the fact that 
whilst an ill-defined minimum is reached at a comparatively 
low temperature, the bar retains considerable magnetic sus- 
ceptibility at that temperature. 
With a more numerous series of experiments it is probable 
that an explanation of the difference between the two Dat- 
tenberg bars Nos. 1 and 2 might have been forthcoming. 
At present it can only be surmised that No. 2, which was the 
only one of its kind to exhibit exceptional behaviour, was 
from a piece of rock which had deteriorated by weathering. 
