332 M. Pouillet on the Recent Researches o/Prof. Faraday. 



to enter successively in the two directions, we shall see, in 

 the first case, the effect to the right, and in the second the 

 effect to the left, which establishes, as Mr. Faraday has pointed 

 out, a difference, at least apparent, between the substances 

 which have the permanent property of turning the planes of 

 polarization and those which acquire it by magnetic action. 



In the second place it results, that on experimenting in this 

 way care must be taken to give to the pieces subjected to the 

 electro-magnet a length greater than the distance of the axes 

 of the two arms ; for the portions which would exceed those 

 axes would receive similar modifications among one another, 

 and opposed to that which the central portion would receive ; 

 it may even be presumed that the compensation might obtain 

 exactly, so that with a connecting piece exceeding the breadth 

 of the magnet, the action might be perfectly null. This re- 

 sult seems to me opposed to that which is pointed out by 

 Mr. Faraday, namely, that the effect is proportional to the 

 length of the piece subjected to the experiments. 



It results, lastly, that in order to obtain a greater effect, two 

 electro-magnets, opposed to one another, may be presented 

 to the piece of flint-glass, so that the poles of the same name 

 face one another. This I have verified, and it is even by the 

 assistance of two electro-magnets thus opposed that I have 

 obtained, the maximum effect of which I have spoken above. 

 By placing thus several similar systems in succession on the 

 same pencil, the effect would be, without doubt, tripled, &c. 



It has appeared to me very important to examine whether 

 the position of the plane of polarization, with relation to the 

 horizontal plane of the electro-magnet, had any influence on 

 the energy of the action ; but whether the plane of polariza- 

 tion be itself horizontal, vertical or intermediate, the results 

 appeared to me to remain perceptibly the same. 



I have hitherto spoken only of flint-glass, but I have sub- 

 jected to experiment all the other solid transparent bodies 

 which I have been able to procure ; for instance, various 

 kinds of flint-glass, and doubtless of different composition, 

 crown-glass, and glass of all kinds, coloured with copper, 

 gold, chromium, &c, and also rock-salt. All these bodies 

 present, although with less intensity, the same phenomena 

 as the flint-glass ; unfortunately the samples of crown-glass 

 are sometimes so annealed as to modify the colours, and which 

 does not allow of their being compared with other bodies ; 

 nevertheless, after the attempts which I have been able to 

 make on some less imperfect specimens, I am led to think 

 that the action of the crown-glass has an intensity comprised 

 between the half and the two- thirds of that of the flint-glass. 



