MAGNETIC SUSCEPTIBILITIES OP AUSTKALIAN BASALTS. 51 



On the MAGNETIC SUSCEPTIBILITIES op SPECIMENS 

 of AUSTRALIAN BASALTS. 



By A. W. Rucker, m.a., f.r.s., 



Professor of Physics in the Royal College of Science, London. 



(Communicated by Professor Threlfall, m.a.) 



[Bead before the Royal Society of N. 8. Wales, June 6, 1894.'] 



In a paper recently published in the Proceedings of the Royal 

 Society of London (Yol. xlviii., pp. 505 - 535, 1890), I described 

 a simple method of measuring the magnetic susceptibility of 

 irregular fragments of bodies which were nonconductors of 

 electricity and of low magnetic permeability. Mixtures of mag- 

 netic oxide and glycerine in various proportions were first made, 

 and their permeabilities were determined by the magnetometric 

 method. Approximately equal volumes of one of these mixtures 

 were introduced in test tubes into the two cups of a Hughes' 

 Induction Balance, and silence was obtained by means of a com- 

 pensator. 



A fragment of the rock under experiment was then placed in 

 one of the test tubes, and liquid was removed until the total 

 volume was the same as before. In general, the balance was 

 disturbed as the permeability of the rock was different from that 

 of the liquid it displaced. Finally two liquids were found, to the 

 permeabilities of which that of the rock was intermediate, and by 

 observing the displacement of the compensator necessary to pro- 

 duce silence in each case when the rock was introduced, the 

 susceptibility of the fragment was calculated from the known 

 susceptibilities of the liquids. 



As a full account of the experiment has been published, it is 

 unnecessary to describe here the precautions taken and the various 

 tests to which the results were subjected. It was found that the 

 method applied to susceptibilities between 0*0001 and 0*008, and 



