on Magnetic Rotatory Polarization in Gases. 311 



ferent gases has been invariably taken. My wish to operate 

 at the ordinary temperature and pressure very greatly com- 

 plicated the apparatus, which would have been extremely 

 simple if the experiment had been made only under very strong 

 pressure ; and I have been obliged to have recourse, therefore, 

 to means of amplification which I shall describe more closely 

 hereafter. The apparatus employed is composed of two 

 distinct parts : — first, the electromagnetic apparatus ; and, 

 secondly, the optical system, the arrangement of which is of 

 the most delicate character. 



1. Electromagnetic Apparatus. 



The apparatus is composed of a copper tube, 0*122 metre in 

 external diameter and 3*27 metres long, closed by parallel mir- 

 rors, and around which are six large electromagnetic bobbins. 

 The whole of this arrangement is mounted horizontally on a 

 wooden bench formed of a large beam 3*82 metres long. This 

 serves also to sustain the optical system (see Plate VIII. fig. 1). 

 The tube is held by special supports in the middle of the bob- 

 bins so as not to touch them. Each of these is 0*48 metre 

 long, 013 metre internal diameter, and contains 15 kilogr. 

 of copper wire 0'03 millim. in diameter, thoroughly insulated 

 and rolled upon a copper tube which forms the body of the 

 bobbin. The total weight of the copper wire employed was 

 about 90 kilogr., which corresponds to 1380 metres in length 

 for the six bobbins. 



They are associated in series, so as to form a single solenoid 

 round the tube which contains the gas. 



Into these bobbins was passed the electric current produced 

 by 80 elements (nitric acid) of large size. The battery was 

 formed of two groups of 40 elements arranged in series, which 

 were united together in multiple arc. 



Under the influence of an electric current so intense, the 

 conductors and the tube itself soon become considerably 

 heated, and the temperature rises to 30° or 40° C. It is ne- 

 cessary, on the one hand, to allow for this elevated tempera- 

 ture, and, on the other, to guard against the variations in 

 intensity of the electric current. These variations are slow 

 and regular, and are due to the progressive weakening of the 

 acids of the couples, and to the heating of the conductors by 

 the passage of the electric current. In order to allow for 

 them, it is sufficient to measure at convenient intervals of 

 time the intensity of the electric current and to trace the 

 curve of the intensities. For this purpose there was arranged, 

 in connexion wth the circuit, a sine-compass, which measured 

 at every moment theintensitvof the current in the apparatus, 



2B2 



