NATURAL PHILOSOPHY. 131 



THE MAGNETIC RELATIONS OF THE POSITIVE AND NEGATIVE 



OPTIC AXES OF CRYSTALS. 



MR. FARADAY'S discovery of diamagnetic phenomena is likely to lead 

 rapidly to some important knowledge of the molecular forces which 

 determine the conditions of the material creation. Pliicker, of Bonn, 

 in a letter to a friend, says: "I replace the declination-needle by 

 certain crystals suspended horizontally by a fibre of cocoon silk.. 

 They take' under the action of the earth's magnetism, a determinate and 

 fixed direction. I can vary at will, and predict this with certainty ; 

 and obtain crystals to act as needles which shall point constantly to- 

 wards the poles of the earth, towards the magnetic poles, or to- 

 wards some azimuthal point." 



In a later letter to Dr. Faraday, Pliicker says, " The first and 

 general law I deduced from my last experiments is the following. 

 There will be either repulsion or attraction of the optic axes by the 

 poles of a magnet, according to the crystalline structure of the crystal. 

 If the crystal is a negative one, there will be repulsion ; if it is a 

 positive one, there will be attraction" After some other remarks, 

 he continues, " Cyanite is by far the most interesting crystal I have 

 examined. It points very well to the north by the magnetic power of 

 the earth only. It is a true compass-needle, and more than that, you 

 may obtain its declination. The crystal does not point according to 

 the magnetism of its substance, but only in obedience to the magnetic 

 action upon its optical axes. If you approach to the north end of the 

 suspended crystal the south pole of a permanent magnetic bar, strong 

 enough to overpower the magnetism of the earth, the axis of the prism 

 will make with the axis of the bar an angle exactly the same as it 

 made before with the meridian plane, the crystal being directed either 

 more towards the east, or more towards the west. The crystal, re- 

 sembling in that also a magnetic needle, showed strong polarity ; the 

 same end being always directed to the north. I think this may be a 

 polarity of the opto-magneiic power. Between the poles of the strong 

 electro-magnet, the permanent polarity disappeared as long as the 

 magnetism was excited." 



Mr. Alger, in communicating to the American Association these 

 discoveries of Pliicker, made some remarks concerning cyanite, which 

 may perhaps render the subject a little clearer. He said, referring to 

 the effect observed, " This is, of course, founded on the recently 

 discovered magnetic property of aluminum, but in the highly oxidized 

 state in which this metal exists in cyanite, it would hardly seem pos- 

 sible that the direction which the crystal assumes should be due to its 

 metallic base ; nor can we suppose the presence of iron in sufficient 

 quantity to cause it ; yet we must place confidence in so high an 

 authority. Pliicker finds, also, that there is some connection between 

 the direction that cyanite assumes and its cleavage planes." The 

 subject deserves further investigation, and Mr. Alger recommended 

 American mineralogists to examine other aluminous minerals of the 

 same class with respect to their magnetism. 



