54 Prof. Cayley on the Distribution of Electricity 



It would be useless perhaps to attempt to give a formula to 

 the above ; and the entire absence of crystallization deprives it 

 of much of the interest it should otherwise possess. 



The following, perhaps, would give the best idea of its con r 

 stitution : — 



3(FeO,Si0 2 H 2 0) + Fe 2 3 . 



Found. Calculated. 



Ferrous oxide 39*46 40*74 



Ferric oxide 18*51 15*09 



Silicic acid 31*72 33*96 



Water 11*02 10*18 



100*71 99*97 



It may be merely a coincidence ; but it is worthy of remark 

 that the water in the mineral just described and that in the 

 Cronstedite examined by Messrs. Maskelyne and Flight* is 

 much about the same, which can also be said of the ferrous 

 oxide, neither of them varying 1 per cent., while the silicic 

 acid and ferric oxide seem, so to speak, to have changed places. 



Cronstedite. Green mineral. 



Silicic acid 18*546 31*72 



Ferric oxide 32*752 18*51 



51*298 50*23" 



It has already been remarked that, on heating the green 

 mineral, its colour is changed to yellowish browm; and on ex- 

 amination of the residue, no trace of ferrous oxide could be 

 detected. When the water has been drawn off, at the lowest 

 possible temperature, and the mineral further heated, it rapidly 

 gains in weight from absorption of oxygen. 



IX. On the Distribution of Electricity on two Spherical 

 Surfaces. By Prof. Cayley t- 



IN the two memoirs " Sur la Distribution de PElectricite a 

 la Surface des Corps Conducteurs," Mem. deVInst. 1811, 

 Poisson considers the question of the distribution of electri- 

 city upon two spheres : viz. if the radii be a, b, and the dis- 

 tance of the centres be e (where c > a + b, the spheres being 

 exterior to each other), and the potentials within the two 

 spheres respectively have the constant values h and a, then — 



for Poisson's /(— ) writing </>(V), and forhisF/ ^-j writing 



* From the most carefully selected specimen. 

 t Communicated by the Author. 



