288 
MR. J. LARMOR ON A DYNAMICAL THEORY OF 
between tliese temperatures. Let A -|- Bh be the thermal energy per unit volume 
which it must receive from without at the higher temperature, and h that which it 
must return at the lower, in order to perform the amount of work SW, equal.to 
i H~fZKyiT.ST, in the cycle; then, by Carnot’s principle, SW/ST = A T; and 
h= — ^KW as above; so that c?k/cZT = — k/T, leading to /c = A/T which is 
Curie’s law. Conversely, assuming Curie’s law we can deduce that in para¬ 
magnetic bodies magnetization consists in orientation of the molecules without 
sensible change in their internal energies. In an analytical form the argument will 
then run as follows : dh = M(il + NcZT, and dE = dh — ; whence by the 
thermodynamic formula M/T = so that M/I = Tc//<“^/c?T, = by 
Curie’s law; hence dh = HcAI + N(iT, so that at constant temperature h = A^HI, 
that is the heat that the material developes during magnetization is the equivalent of 
the magnetic energy that is not used up in mechanical work. This is precisely what 
we should expect if the material is a gas ; for there is then no internal work by 
which this energy could be used up, and the magnetization arises from the effort of 
the magnetic field to orientate the molecules which are spinning about as the result 
of the gaseous encounters. The law of Curie thus indicates that the same is 
sensibly true for all paramagnetic media at high temperatures : at lower tempera¬ 
tures they gradually pass into the lerromagnetic condition. It is the magnetization, 
so to speak, of an ideal perfect ferromagnetic, in which the controlling force that 
resists the orientating action of the field is practically wholly derived from the 
magnetic interaction of the neighbouring molecules, wdrich for this purpose form 
elastic systems, that is illustrated by Ewing’s well-knowm model, which so clearly 
represents the hysteresis accompanying ferromagnetic excitation. In ordinary 
paramagnetic substances this mutual magnetic control of the molecules is insensible 
compared with the control due to other molecular causes ; and our conclusion is that 
these causes are such that the magnetic energy expended in w^orking against them is 
transformed into heat energy, not into internal energy of any regular elastic tyjie. 
But w^e have not taken account of the fact that the molecules of every siibstaiice 
are subject to both paramagnetic and diamagnetic intiuence, of which one or the 
other preponderates. The theoretical law should thus be k = — B + AT”^ or 
xT = A — BT; so that in a diagram of the relation between kT and T each 
substance would be represented by a straight line. In paramagnetics the line should 
slope slightly down towards the axis : for diamagnetics it should pass not through 
the zero of teiuperature but on the positive side of it. According to CuRuq his 
exjierimental results are equally w'ell represented by this formula, on account of the 
preponderant infiuence of tlie paramagnetism. 
Similarly, should it turn out that for W'eakly electric media such as gases, (K-l)/p 
is independent of the temperature, it A\ ould follo^v that the electric polarization is 
mainly an affair of change of internal constitution of the molecules : while were the 
})olarizatioii maiidy an affair of molecular orientation, (K — l)/p would vary inversely 
