and Pyroelectric Properties of Matter. 25 



In carbonate of iron (a paramagnetic crystal) the difference of 

 inductive capacities in different directions was reduced to one 

 third when the temperature was raised from 70° to 289° Fahr., 

 and was tripled when the temperature was again brought down 

 to 70° (Exp. Res. 3400 and 3411). 



(2) Thermodynamic Relations. — The theory of the mutual 

 convertibility of heat and mechanical work in reversible ope- 

 rations when applied to these phenomena proves : — 1. That a 

 piece of soft iron at a moderate or low red heat, when drawn 

 gently away from a magnet experiences a cooling effect, and 

 when allowed to approach a magnet experiences a heating 

 effect; that nickel at ordinary temperatures, and cobalt at 

 high temperatures, within some definite range below that of 

 melting copper, experience the same kind of effects when sub- 

 jected to similar magnetic operations. 2. That cobalt at ordi- 

 nary atmospheric temperatures, and at all temperatures up- 

 wards to its temperature of maximum inductive capacity, 

 experiences a cooling effect when allowed to approach a mag- 

 net slowly, and a heating effect when drawn away. 3. That 

 a crystal in a magnetic field experiences a cooling effect when 

 its axis of greatest paramagnetic or of least diamagnetic in- 

 ductive capacity is turned round from a position along to a 

 position across the lines of force, and a heating effect when 

 such a motion is reversed. 



[27. Let there be three rectangular axes fixed relatively to 

 the movable body, whether soft iron, or copper, or a crystal 

 in a magnetic field, and, considering the whole magnetic mo- 

 tive * on the body, reduce it, after the manner of Poinsot, to 

 three component forces along the magnetic axes and three 

 couples round these axes. Let P, Q, R be the force-compo- 

 nents, and S, T, U the couple-components thus obtained, which 

 we must suppose to be known functions of t, the temperature. 

 Equation (22) of § 20 above gives H, the quantity of heat 

 which must be supplied to prevent the body from becoming 

 cooler when it is moved through infinitesimal spaces x — x w 

 y—y w -—^o i n the directions of the three axes, and turned 

 through infinitesimal angles f— £ , v — V w £ — ? round the 

 same axes. The lowering of temperature which it experiences 



XT 



if heat is neither given to it nor taken from it is equal to ~-, 



where C denotes the whole capacity for heat of the body, or 

 the product of its mass or bulk by its specific heat per unit of 



* [In dynamics the want is keenly felt of an expression for a system of 

 forces acting on a body : adopting a suggestion of my brother, Professor 

 James Thomson, the word " motive " is used in the text to supply this 

 want.] 



