Rotation of the Plane of Polarization of Light. 175 



by a Nicol, was now sent through the vapour ; a Nicol at the 

 other end extinguished the pencil. The current of the 64 ele- 

 ments was now sent through the coils ; when the field was 

 distinctly brightened. The brightening was still more consi- 

 derable when, after the closing of the current, the front Nicol 

 was rotated to darkness and the current then reversed by a 

 commutator. 



The rotation of the polarization-plane took place, as was to 

 be expected, in the direction in which the positive current 

 went through the coils. 



In order to find out whether the observed rotation might 

 not be entirely or in part produced- by the glass plates that 

 closed the ends of the tube, the sulphide-of-carbon vapour was 

 expelled, and the empty tube again heated and observed. On 

 closing the current there was in fact shown a very slight rota- 

 tion proceeding from the glass plates; but its amount was 

 essentially less than when the experiment was made with sul- 

 phide of carbon in the tube. In order to get quite rid of this 

 feeble glass-plate rotation, the two outermost coils (those 

 nearest to the plates) were put out of the circuit, and the 

 four through which the current still passed were now removed 

 so far from the glass plates that their influence upon the latter 

 could only be extremely slight. It was in fact now found that 

 the empty tube, heated by steam, showed no trace of rotation. 

 But when the tube was again filled with sulphide-of-carbon 

 vapour, at the closing of the current through the four coils a 

 distinct brightening of the field of vision, previously darkened 

 by crossing the Nicol, was obtained. We could not measure 

 precisely the amount of the rotation : we estimated it in the 

 last experiment at about \°. 



By this it is demonstrated that saturated vapour of bisulphide 

 of carbon, at about 100° C, rotates the plane of polarization of 

 light in the magnetic field. 



When some sulphuric ether was put into the iron tube and 

 heated, on closing the current no rotation could be observed. 



Although, it is true, by our experiments it has at present 

 been shown only that saturated bisulphide of carbon exhibits 

 electromagnetic rotation of the plane of polarization, yet it is 

 now hardly to be doubted that it will be possible to prove the 

 existence of the polarization in unsaturated vapours and gases 

 also. We are engaged in the construction of an apparatus 

 which shall permit us to examine permanent gases at very 

 high pressures in the magnetic field, in order to demonstrate 

 the rotation for these, and, if possible, to follow up the phe- 

 nomenon with measurements. 



It will be especially interesting to ascertain whether oxygen 



