400 RECENT PROGRESS IN PHYSICS. 



tive capacity of crystalline bodies in dilFerent directions. According 

 to his idea of specific inductive capacity, it is evidently possible that 

 crystalline bodies have not equal inductive capacity in all directions ; 

 that, for instance, rock crystal, or calc spar, might have a greater or 

 less inductive capacity in the direction of their optical axes than 

 perpendicular to them. Faraday's experiments leave this question 

 wholly uikdecided ; in most cases exceedingly small differences being 

 indicated, while in others, where greater diiferences appeared, colored 

 seams in the crystal, cracks and the like, may have had an injurious 

 effect. I do not consider it necessary here to enter more into detail. 



Faraday's views on electrical induction must necessarily have forced 

 upon him the question, whether magnetic attraction and repulsion_, as 

 heretofore supposed, are to be ascribed to action at a distance, or 

 whether magnetism in a similar manner acts at a distance through 

 the medium of intervening particles, analagous to induction in static 

 electricity according to his view. 



The experiments which he made for the solution of this question 

 gave invariably negative results, whether he used plates of shellac, 

 sulphur, or copper, as intervening bodies. No sign of the influence 

 of intermediate particles could be obtained. 



Even if the first experiment did not succeed in showing that mag- 

 netism acts at a distance through the medium of intervening particles, 

 it is still conceivable that a magnet might affect all the particles of 

 the non-magnetic bodies surrounding it, and place them in a peculiar 

 state of tension, similar to that of the dielectric, through Avhich in- 

 duction takes place from one conductor to another ; and it is certain 

 that Faraday, in his endeavors to discover proofs for such a state, 

 was led to the discovery of the rotation of the plane of polarization by 

 the magnetic poles and galvanic currents as well as to dia-magnelism ; 

 discoveries which alone would be sufficient to make his name immortal 

 in the history of science. 



§ 25. MuNCK AF RosENSCHoLD ON INDUCTION. In the 69th volume of 

 Poggendorf's Annalen, is a memoir by Munck of Rosenschold, in 

 which induction is treated of. In his somewhat extended considera- 

 tion of the subject, into which he naturally introduces much that is 

 known, he starts with the correct view of induction, which is also 

 defended by Riess and Fechner. 



The following constitutes the most important parts of Bosenchold' 8 

 •memoir : 



Let the plate A, Fig. 41, be electrified and act inductively on B 



and C. Let E be the quantity of electricity on A, and it will induce 



Fiff . 41 . upon B, which communicates with the ground through 



X a fine wire, the quantity of electricity — mE. If the 



.- ffiiiii,iiiiiiiMm.mi,i,flniTii|ini|iiim plate C be now brought within the " electrical sha- 



dow" of B^ and connected with the ground, both A 



- miiiiiiiiiiiiiiiiiimiiiiiiimiiiiiiiiim) and B will act upon this plate. If we indicate by m! 

 tlie co-efficient of induction which belongs to the dis- 



i^iniii|iiiiiiniiiimiiMiiiMp] ^ tance between B and C, then m! mE will be disguised 

 •C on C by A, if m" represents the co-efficient of induc- 



tion correspoi.ding to the distance between A C ; then there will be 

 disguised on C, 



c = TO m' E — in!' E, 



