396 Prof. A. De la Rive's Researches on the Magnetic 



A second correction which I should have had to make relates 

 to the variations of intensity of the magnetizing current. I 

 should have been obliged to measure that intensity at every ex- 

 periment — which would have been a complicated and tedious 

 process. In order to dispense with it, I adjusted end to end two 

 perfectly similar tubes, one rilled with the liquid to be operated 

 upon, and the other with distilled water, each tube with its glass 

 ends. One of the tubes was so placed in the interpolar space as 

 to avoid the influence of the stoppers ; the other, which was as a 

 prolongation of the first, was placed entirely within the interior of 

 one of the branches of the electromagnet. The first alone was 

 subject to the influence of the magnetism ; the second was quite 

 uninfluenced by it ; so that the rotation of the polarized ray, 

 which successively passed through them both, took place only in 

 the interpolar tube. It was possible with the greatest facility 

 to place first one and then the other tube in the interpolar space, 

 and thus to obtain, by means of alternate observations (each of 

 which occupied only a few moments), the mean angles of rotation 

 of water and of the liquid, which were found to be independent 

 of any variation in the intensity of the current ; besides, the va- 

 riation had scarcely any amount during the time of a single ex- 

 periment. From one experiment to another the intensity might 

 vary without entailing the slightest inconvenience, since, the 

 coefficient sought being the ratio between the rotation of the 

 liquid and that of distilled water, the important point was to de- 

 termine these two rotations in each case under the same intensity. 

 Moreover the variation of intensity of the electric current was so 

 little during the first hours of the experiments that I might 

 almost have dispensed with these precautions, though I never 

 did so*. 



It, however, occurred to me, when the liquid to be experimented 

 on was not very transparent, not to unite the tube containing it 

 to that which contained the distilled water, but, in order that 

 the polarized ray might not have successively to pass through 

 them both, to place them alternately in the interpolar space, 

 which, though less convenient, could be done rapidly and 

 amounted to the same. 



In most of my experiments the liquid column interposed be- 

 tween the polar surfaces had a length of 10 centims. But as 

 I had not always a sufficient quantity of liquid to fill a tube 



* The pile I used was a large Bunsen's, composed of from 40 to 50 

 pairs, charged with dilute nitric and sulphuric acids (9 parts of water to 

 1 part of acid) ; I found no great advantage in using a more powerful pile 

 (6*0 pairs for example), because the wire of the electromagnet became much 

 heated. What contributed to render constant the intensity of the electric 

 current was the shortness of the duration of each experiment, and conse- 

 quently the discontinuity which took place in the passage of the current. 



