522 Prof. Riicker. On the Magnetic Permeability of 



basic rocks is relatively very high. The average of all the speci- 

 mens from the west of Scotland and from Ireland is O00245. If we 

 exclude the Giant's Causeway and Staffa, it is 0'00271, which is thus 

 the average of all the specimens tested from a district nearly 70 

 miles in length. There would, therefore, be nothing absurd in the 

 supposition that equally large values obtained over equally large 

 areas elsewhere ; but in the calculations the assumed susceptibility 

 is the much smaller value given by the Mull specimens, viz., O0016. 



An experiment was made on the effects of temperature on the 

 permeability of magnetite. It was only of a rough preliminary kind, 

 but the result was quite clear, and further and more elaborate ex- 

 periments on the same point are about to be undertaken in the 

 laboratory at South Kensington. 



The interior of one of the cups of the Hughes' induction balance 

 was lined with asbestos cloth, and a fragment of non-magnetic granite 

 which had been heated to incipient redness in the flame of a Bunsen 

 burner was introduced into it. The balance which had been pre- 

 viously obtained was quite undisturbed. The same experiment was 

 then repeated with a piece of magnetite. The introduction of the 

 rock at oiice caused the telephone to " ppeak," but silence was 

 quickly obtained by turning the screw by which the parallelism of 

 the primary and secondary coils is secured. The compensator used 

 in the previous experiments had not sufficient range, and readings 

 were taken by a paper scale of degrees attached to the screw head. 

 As the magnetite cooled the zero altered, and, in order to maintain 

 silence, it was necessary to keep turning the screw in the direction 

 which indicated that the permeability of the specimen was decreasing. 

 The stone was allowed to cool for half an hour, and the total altera- 

 tion of the zero measured. It was then removed altogether, and the 

 new position of silence found. In one experiment the following values 

 were obtained. The figures in the second column indicate the number 

 of degrees through .which the screw was turned from the first zero 

 obtained after the introduction of the hot magnetite : 



Beading. 



Magnetite, hot ! 



cold j 320 



removed \ 790 



This result proves that, as in the case of iron, the permeability of 

 magnetite increases as the temperature rises, the increase in the ex- 

 periment just described being about 70 per cent. A second experi- 



