in Crystalline and Non-crystalline Substances. 181 



netization. The foUomng conclusionsj being mathematical de- 

 ductions from the laws stated above, are liable to modification, 

 according to the deviations from those laws which actual experi- 

 ments may point out. 



1. The determination of the conditions of magnetic induction 

 in a body of any kind in any circumstances may be made to 

 depend on a knowledge of the state of magnetization induced in 

 a homogeneous sphere of the same substance, placed in a miiforni 

 field of magnetic force. 



2. A homogeneous sphere of any substance placed in a uniform 

 field of force becomes uniformly magnetized in parallel lines with 

 an intensity which is independent of the radius of the sphere. 



3. If the sphere be of non-crystalline substance, the lines of 

 its magnetization are in the same direction as the lines of force 

 in the field into which it is introduced, and the intensity of mag- 

 netization is equal to the product of a constant (which may be 

 called the inductive capacity of the substance) into the intensity of 

 the magnetizing force. 



4. If the sphere be of crystalline substance, the lines of its 

 magnetization may not in general be in the same direction as 

 the lines of force of the field into which it is introduced ; and 

 they are not so if the sphere, when free to turn round its centre, 

 is obseiTcd to be not in equilibrium. 



Definition. — A principal axis of magnetic induction of a sub- 

 stance is a line in it, such that a spherical portion when intro- 

 duced mth that line parallel to the lines of force into a uniform 

 magnetic field, becomes magnetized in the direction of those lines. 



Definition. — A principal inductive capacity of a substance, or 

 the inductive capacity of a substance ivith reference to a principal 

 axis, is the coefficient by which the intensity of the magnetizing 

 force must be multiplied to obtain the intensity of magnetization 

 when a spherical portion is introduced into a uniform magnetic 

 field, with a principal axis parallel to the lines of force. 



5. Any substance has through every point of it, three principal 

 axes at right angles to one another ; and if the inductive capa- 

 cities with reference to three such axes be different, no other line 

 through the same point is a principal axis*. 



6. If the inductive capacities with reference to two principal 

 axes through any point of a homogeneous substance be equal, 

 every line in the plane of these two, or parallel to it, is a prin- 



* Such, it may be expected, will be the magnetic circumstances in the 

 case of any transparent substance which belongs to the ojitical class of 

 " biaxttl crystals;" and its three ))rincipal axes of magnetic induction will 

 be the three rectangular axes deduced by Sir Uavid Brewster from the 

 " optic axes," and known in the undulatory theory as the principal a.xes of 

 elasticity of the medium in which the undulations are propagated. 



