494 Professor Silvanus P. Thompson [May 17, 



I came across a new magnetic body unknown hitherto, I believe, to 

 the chemist, namely, a magnetic double oxide of cobalt and iron — a 

 ferroso-cobaltic oxide, I think — a black powder, a sami:)le of which I 

 have here. 



It also occurred to me, as a matter of speculation, that if I could 

 strongly magnetise a crystal of ferrous sulphate ornickelous sulphate, 

 whilst viewing it by convergent polarised light, I might find some in- 

 teresting phenomena, which should, if they existed, show some sort 

 of a relation between the direction of the optic axis and that of the 

 lines of the magnetic field. I thought that a longitudinal magneti- 

 sation might possibly set up a rotatory phenomenon like that in 

 quartz in so far as to disturb the central field between the arras of 

 the black cross ; however, not by the most powerful magnetising 

 could I discover any such effect. Again, I thought that by magnetis- 

 ing transversely to the optic axis I might possibly succeed in turning 

 the uniaxial crystal into a biaxial, or producing by magnetism an 

 efi"ect resembling the action of heat on crystals of selenite. Owing 

 probably to the small depth of any crystals that can be obtained, I 

 have failed so far to obtain any such effect, though I am convinced 

 that it must exist. 



An effect precisely analogous to the magnetic effect which I vainly 

 sought has, however, been lately discovered by Prof. Eontgen. I 

 sought a distortion of the optic axis by transversely magnetising, and 

 I sought it in crystals of sulphate of nickel ; he has found a distortion 

 of the optic axis by transversely electrifying, and he has found it in 

 crystals of quartz. 



Suppose a piece of quartz crystal is cut as a square prism, its long 

 faces being principal planes of section respectively parallel to and at 

 right angles to two of the natural faces of the hexagonal prism. Fig. 

 18 shows the form of the portion cut. The + and - signs in this 



Fig. 19. 



figure refer to the pyro- electric poles of the crystal. Such a piece 

 viewed by convergent light shows the usual rings and black cross 

 with a coloured centre (Fig. 19). If now two opposite faces be covered 

 with tinfoil, and the crystal be electrified transversely, the rings are 

 distorted into lemniscates, the direction of the distortion changing 

 with the sign of the electrification. It is necessary to use a red 



