442 Prof. Thomson on the Dynamical Theory of Heat. 



guish it from such a rotatory property with reference to light as 

 that which is naturally possessed by many transparent liquids 

 and sohds, and which may be called an isotropic rotatory pro- 

 perty. The axis of thermo-electric rotation, since the agency 

 distinguishing it as a line also distinguishes between the two 

 directions in it, may be called a dipolar axis ; so may the axis of 

 rotation of a rotating rigid body*, or the direction of magnet- 

 ization of a magnetized element of matter ; and its general type 

 is obviously different from that of a principal axis of inertia of a 

 rigid body, or a principal axis of magnetic inductive capacity in 

 a crystal, or a line of mechanical tension in a solid ; any of which 

 may be called an isotropic axis. 



169. The general directional properties expressed by the first 

 terms of the second members of (40) are perfectly symmetrical 

 regarding the three rectangular lines of reference, and are of a type 

 so familiar that they require no explanation here. We conclude 

 that every substance has three principal isotropic axes of maxi- 

 mum and minimum properties regarding thermo-electric power, 

 which are at right angles to one another ; but that it is only for 

 a particular class of conceivable substances that the thermo- 

 electric properties are entirely symmetrical with reference to 

 these axes ^ all substances for which the rotatory power, p, does 

 not vanish, having besides a dipolar axis of thermo-electric rota- 

 tion which may be inclined in any way to them. 



170. These principal isotropic axes lose distinction from all 

 other directions in the solid when the thermo-electric powers 

 along them (the values of the coefficients 6, <^, y^) are equal ; 

 but a rotatory property, distinguishing a certain line as a dipolar 

 axis, may still exist. By § 159, we see how metallic structures 

 possessing any of these properties (for instance, having equal 

 thermo-electric power in all directions, and possessing a given 

 rotatoiy power, p, in a given direction about a given system of 

 parallel lines) may be actually made. 



171. [Added, July 1854.] It is far from improbable that a 

 piece of iron in a state of magnetization, which I have, since 

 § 147 was written, ascertained to possess different thermo-electric 

 properties in different directions, may also possess rotatory 

 thermo-electric powerf, distinguishing its axis of magnetization, 



* [Added, Liverpool, Sept. 27, 1854.] — As is perfectly illustrated by 

 M. Foucault's beautiful experiment of a rotating solid, placing its axis 

 parallel to that of the earth's, and so turned that it may itself be rotating 

 in the same direction as the earth ; which the meeting of the British Asso- 

 ciation just concluded has given me an opportunity of witnessing. 



t [Added, Sejjt. 13, 1854.] — By an experiment made to test its existence, 

 which has given only negative results, I have ascertained that this "rota- 

 tory power," if it exists in inductively magnetized iron at all, must be ver\' 



