100 Mr. G. F. Fitzgerald on the Rotation 
(6) Film 80 millim. long. When the black, which appeared 
in ten minutes, had increased to 14 millim. a current was 
passed. C=1°9 m.a. In five minutes the black was reduced 
to 8 millim., and then the film broke. This shows that even 
a feeble current is, at certain stages of a film’s existence, 
sufficient to destroy the black. 
(7) The last case we may mention was one in which two 
films, each 30 millim. long, were formed side by side. The 
current from the battery was divided and passed up one and 
down the other film. The effect was most striking. In twelve 
minutes, when one of the films broke, there were 15 millim. 
of black in the one, while the other was thick and colourless 
and liquid was streaming from the top of it. 
The experiments cited above were made with a solution of 
potash-soap (Brit. Pharm.) without any glycerine. The ad- 
vantage of using such a solution is that the films formed with 
it grow thin with tolerable rapidity, and in the course of two . 
or three hours several films may be observed with each appa- 
ratus. Similar experiments were carried out with liquide 
glycérique (made both with oleate of soda and potash-soap). 
The results were identical in kind with those enumerated 
above, but very different in degree. Films made with this 
solution are very persistent, but thin slowly. Generally three 
or four hours elapsed before any black appeared; and when a 
current was passed through, its effect was not so promptly 
manifested as in the case of the plain soap solution. The 
addition of nitrate of potassium may be partly accountable for 
this sluggishness, for it certainly increases the viscosity of the 
liquid. Nevertheless a great number of films made of the 
glycerine solution were observed, with currents flowing up 
or down. ‘The effect of the downward current was invariably 
to promote, of the upward current to retard, the thinning of 
the film. 
XIII. On the Rotation of the Plane of Polarization of Light 
by Feflection from the Pole of a Magnet. 
To the Editors of the Philosophical Magazine and Journal. 
2 Trinity College, Dublin, 
GENTLEMEN, December 27, 1884. 
| a a A. KUNDT, in his paper translated from the Berlin 
Sitzungsberichte into the October number of the Phi- 
losophical Magazine does me the honour of noticing the 
theoretical explanation I have given (R. 8. Proc. xxv. p. 447) 
of the rotation of the plane of polarization of light by reflection 
