OF CRYSTALS BY THE POLES OF A MAGNET. 371 



two obtuse angles was equatorial. After this experiment, there 

 can be no further doubt as regards my supposition. 



41. The method of observation used in the last paragraph 

 may be altered, by placing the prism of staurolite in such a posi- 

 tion, that instead of raising it in the different horizontal direc- 

 tions of suspension above the line of the apices of the poles, it is 

 made to oscillate around the position of equilibrium in the line 

 of the poles, and the axial eflfect estimated by the different dura- 

 tion of the oscillations. To effect this, I completely removed 

 the apices of the poles ; the prism then became placed axially 

 in the third position of suspension, and equatorial ly in the fourth. 

 I then again inserted the two apices, and moved them forward 

 until even in this position of suspension the equatorial position 

 became axial. As the magnetic force, when the apices are at 

 the same distance apart, remains constant in the different sus- 

 pensions, the position of the prism is in each case determined by 

 a force which is equal to this constant force minus the variable 

 force acting upon the axes. The proportion of this latter force 

 in different suspensions may be ascertained in this way. The 

 more the direction of suspension approximates to the direction 

 above denoted by X, the slower the crystal oscillates. 



This method of observation, where the object is merely to 

 obtain a general view, is less convenient, because it requires 

 greater care ; it possesses however the advantage of a more ex- 

 tensive application ; it may be used even when the crystal con- 

 tains so much iron that the axial action cannot overcome the 

 magnetic attraction, as was found to be the case in a crystal of 

 lepidolite. It is also applicable when the crystal is diamagnetic, 

 and even in consequence of its form assumes an equatorial posi- 

 tion. A crystal of topaz affords an appropriate example. In 

 this case the diamagnetic repulsion and the action upon the axes 

 combine to produce the action observed. 



42. The last paragraphs contain the first example of the man- 

 ner in which the optic axes of a crystal may be determined by 

 means of a magnet ; and, what must appear surprising, the cry- 

 stal may be opake, and every trace of crystalline form have dis- 

 appeared. 



In the same way we can obtain an answer to the question, 

 whether a solid, transparent or opake, uniaxial or binaxial 

 crystalline mass, consists of elementary crystals (if I may be 

 permitted to use this un-mineralogical expression), in which an 



2 c 2 



