THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
263 
the available energy is the same, by transition through any intermediate series of 
these states; and we can jiass continuously fioiu one such complexus to the others in 
which the whole series of possible states are included, by additions of available energy 
to the system. The available energy is thus an analytical function of the physical 
condition of the system, including its temperature; and the trend of spontaneous 
change in an isolated system is in the direction in which this function diminishes, the 
positions of stability, as regards mechanical and thermal and also constitutive disturb¬ 
ance, being those for which it is a minimum. The circumstances of all steady con¬ 
figurations of matter, whether static or kinetic, are determined by this law.^" It is 
more direct to- state the proposition in the form that the unavailable energy tends to 
a maximum, the presumption being that sensible energy is available until it is shown 
to be otherwise. This principle, that energy tends to become mechanically dis¬ 
organized, or that it never spontaneously tends to organize itself, cannot from its 
nature be other than axiomatic : and the formation of the available energy function 
for the different states of matter is then the main business of Thermodynamics. The 
reversible processes which thermodynamic argument employs are ideal types of regular 
change, theoretically realizable by mechanical constraints which do not control the 
individual molecules—the limiting forms, it may be, of imperfectly reversible changes 
which we can actually produce; the states of matter thus derivable from each other 
are shown, from the equality of their available energies, to liave definite mutual 
relations which are independent of the ideal process (or construction, to use a 
geometrical analogy) by which the transitions between these states have been 
imagined. 
The really abstruse abstract problem of the subject is that of the nature of tem¬ 
perature; and the principle most in need of elucidation is that, when a body A is in 
thermal equilibrium with B, and also B with another C, then A would be in thermal 
equilibrium directly with C. The most definite thermal specification of a body is 
the quantity of energy it contains ; twm bodies are in thermal equilibrium when there 
is no tendency for energy to pass from one to the other, independently of change of 
molar configuration or molecular constitution; they are then said to be at the same 
temperature. The rationale of this transfer of energy has been made out for the 
case of gases, where the exchange takes place in encounters between the molecules, 
so that there is no tendency to transfer from one mass of gas to another in contact 
* In so far as our constitutive knowledge of material systems relates merely to comparison of different 
steady states, it can be wholly based in Willard Gibbs’ manner, like ordinary statics, on relations of avail¬ 
able energy of a simply additive character: it is where our knowledge becomes more intimate, and we 
attempt to trace the courses and rates of kinetic phenomena, for instance in material kinetics, electro¬ 
dynamics, optics and vibratory phenomena in general, that the simple relations of energetics become 
insufficient as a mathematical basis for general physics. The principle of available energy suffices for 
tracing the relations of matter in bulk through the various steady phasc.s in which ex jiu&t facto it is 
found to exist; but the genesis of these jihases is expressly excluded ITom its domain. 
