Magnetism of Basalt. 579 



The bar from Sgurr nan Gillean was heated at about 400° C. 

 In a current of hydrogen for various periods, amounting in all 

 to four hours. The total result was to further diminish the 

 magnetic power of the bar, the deflexion caused by remanent 

 magnetism being reduced from 9*5 to 8*7 mm. (see Tables II. 

 and III.). 



The temperature of one of the Dattenberg bars was reduced 

 by means of liquid air, and the deflexions were noted which 

 it produced when a field of 60 O.G.S. was reversed, no readings 

 of temperature being taken. The low temperature diminished 

 the susceptibility of the bar to one half its value at ordinary 

 temperature, the bar slowly regaining its former condition as 

 the temperature rose. This behaviour is different from that 

 observed in other materials, e. g., a bar of ilmenite became 

 temporarily 1*2 times stronger in liquid air ; a bar of mag- 

 netite exhibited a slight increase, also temporary in nature ; 

 and a bar of Heusler alloy became 1"2 times stronger than 

 before, the change this time being permanent. 



Average Composition and Probable Average 

 Susceptibility of Basalt. 



From a series of eight basalts chosen for me by Professor 

 W. W. Watts from Roth's ' Petrography ' as giving average 

 analyses, it is found that in these the average content of iron 

 is 6*1 per cent. Fe, or iron in the form of Fe 2 3 , and 5*9 

 per cent. Fe, or iron in the form FeO, these two forms 

 varying in quantity in the eight basalts, the Fe from 16*4 

 per cent, to zero, and the Fe from 11*6 per cent to zero. 



Now if we assume that basalt contains, on an average, 



12 per cent, by weight of magnetite, and the average weight 



of the basalt bars employed is 65 gms., the weight of mag- 



12 

 netite contained in the bar is -^rx x ^5 or ?*8 g m s. The 



density of magnetite is 5*2, and therefore its volume is 



7*8 



-— - or 1*5 c.cs. Since the average volume of the bar is 



5'2 1>5 ° 



22 c.cs. ^r- of that volume, or 7 per cent., is magnetite. 



Now, if we consider the susceptibility of magnetite to vary 

 from *15 to *75*, the susceptibility of basalt, if taken as 



* Roughly the values found for two bars of magnetite. The former of 

 the two values was erroneously stated in the previous paper as -016 

 instead of 0*16. 



2R2 



