28 Messrs. J. Tyndall and H. Knoblauch on the 



During this investigation a great number of crystals have 

 passed through our hands, but it is useless to cumber the reader 

 with a recital of them. The number of natural crystals have 

 amounted to nearly one hundred ; while, through the accus- 

 tomed kindness of Professor Bunsen, the entire collection of 

 artificial crystals, which his laboratory contains, has been 

 placed at our disposal*. 



We now pass over to a brief examination of the basis on 

 which the second law of M. Pliicker rests : — the affirmation 

 that the magnetic attraction decreases in a quicker ratio than 

 the repulsion of the optic axes. The ingenuity of this hypo- 

 thesis, and its apparent sufficiency to account for the phaeno- 

 mena observed by M. Pliicker, are evident. It will be seen, 

 however, that this repulsion arises from quite another cause — 

 a source of error which, unfortunately, has run undetected 

 through the entire series of this philosopher's inquiries. 



The following experiment is a type of those which led M. 

 Pliicker to the above conclusion. A tourmaline crystal 36 

 millimeters long and 4 millimeters across was brought between 

 a pair of pointed moveable poles, so that it could barely swing 

 between them. It set itself axial. On removing the poles to 

 a distance and again exciting the magnet the crystal stood 

 equatorial. The same occurred, if the poles were allowed to 

 remain as in the former case, when the crystal was raised 

 above them or sunk beneath them. According as the crystal 

 was withdrawn from the immediate neighbourhood of the poles, 

 it turned gradually round and finally set itself ' equatorial-^ \ 



A similar action was observed with staurolite, beryl, ido- 

 crase, smaragd, and other crystals. 



We have repeated these experiments in the manner de- 

 scribed, and obtained the same results. A prism of tourma- 

 line three-quarters of an inch long and a quarter of an inch 

 across was hung between a pair of poles with conical points, 

 placed an inch apart. On exciting the magnet the crystal 

 stood axial. When the poles were withdrawn to a distance 

 and the force again evolved, the same crystal stood equatorial. 

 An exceedingly weak current was here used ; a single cell of 

 Bunsen's construction being found more than sufficient to 

 produce the result. 



According to the theory under consideration, the tourma- 

 line, in the first instance, stood from pole to pole because the 

 magnetism was strong enough to overcome the repulsion of 

 the optic axis. This repulsion, decreasing more slowly than 



* We gladly make use of this opportunity to express our obligation to 

 Dr. Debus, the able assistant in the chemical laboratory, 

 t PoggendorfFs Annalen, vol. Ixxii. p. 319. 



