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



centims., the magnetic force remaining constant during the ex- 

 periment on one and the same liquid. 



Angles of Rotation. 



Length of the Water Alcohol Sul P hide Iodide of 



liquid column. ' of carbon, ethyle. 



25 centims. . ll 20 9 30 33 26° 10 



5 „ . 9 15 7 45 27 19 40 



It results from this Table that the ratio between the rotations 

 for the lengths of 25 and 5 centims. is 1*22 for each of the four 

 liquids — that is to say, the same, although the absolute magni- 

 tudes of these rotations are very different ; so that to obtain the 

 magnetic power of a liquid relative to that of water taken as unity, 

 it is only necessary to give to the water column the same length 

 as to the column of liquid operated upon, whatever the absolute 

 length of the two columns. 



It follows also from the numbers contained in the preceding 

 Table, that not much is gained by increasing the length of the 

 path of the polarized ray through the liquid, since the magnetic 

 force, as we have remarked, diminishes in proportion as the dis- 

 tance of the polar surfaces is increased in order to lengthen the 

 liquid column. It is true that the increase and the diminution 

 do not follow the same law, since a less rotation is obtained with 

 short columns than with longer ones \ but the difference is not 

 very considerable. 



The increase of the magnetic force in proportion as the dis- 

 tance between the poles diminishes can be calculated. In fact, 

 knowing that, if the magnetic force were constant, the rotation 

 would be proportional to the length of the liquid column, and 

 consequently, for each of the liquids, the rotation corresponding 

 to a length of 5 centims. would be one-fifth of that which corre- 

 sponds to 25 centims., we find that for water it would be 2° 16' 

 instead of 9° 15', for alcohol 1° 54' instead of 7° 45', for sulphide 

 of carbon 6° 6' instead of 27°, for iodide of ethyle 4° 50' instead 

 of 19° 40'. On the other hand, knowing that the rotation is 

 proportional to the magnetic force, we can easily determine how 

 much this force has increased by the diminution of the distance 

 of the poles from 25 centims. to 5 centims. We have only to 

 take the ratio between the rotations such as they would be if the 

 distance had not been diminished and the actual rotations with 

 the distance of 5 centims. Now, for each of the four liquids, 

 this ratio is 4*1, or rather a little less; about 4*08; which proves 

 that by the reduction of the distance of the surfaces to one-fifth 

 the magnetic force has only been increased to fourfold, at least 

 with the apparatus I used : it is probable that with electromag- 



