THERMODYNAMICS OF RADIATION 
31 
dQi = — dQ, and energy in state 2 receives an addition dQ 2 = dQ. 
Accordingly (25-1) may be written 
dS = 6 1 dQ 1 + 9 2 dQ 2 . 
Combining any number of transfers by summation or integration the 
total change of S will be 
8S = fddQ (25-2), 
where dQ is the infinitesimal addition to energy in the state for which the 
corresponding coefficient is 0. 
In considering a system composed of several bodies A, B, C, ... we 
apportion S among the different bodies so that 
B — £>a + + B c + ... . 
If the transfer above considered is from state 1 in body A to state 2 in 
body B we should write 
dS a = d^dQi, d.Sb = 9 2 dQ 2 . 
It follows that (25-2) applies to each body of the system separately, 3 S 
referring to that body only and dQ on the right being limited to energy 
transferred to or from or within that body. 
In order that S may fulfil the purpose for which it was introduced, 
viz. to measure the progressive degradation of the energy of the universe, 
it must in any natural process satisfy the condition 
23 S > 0 (25-3), 
the summation being taken over all bodies concerned in the process under 
consideration. The coefficients 9 must be determined by applying this 
condition. 
The quantity S is called entropy. It is unique among fundamental 
physical quantities in having a one-sided conservation, that is to say, it 
is indestructible but not uncreatable. Since only the change of S has 
been defined the entropy of each body involves an arbitrary additive 
constant. 
Mechanical work is taken as the standard from which increasing 
disorganisation of energy is measured, so that the coefficient 9 is zero for 
energy dQ added to or taken from mechanical work. We need not exclude 
the possibility of forms of energy with negative entropy referred to this 
standard, although no such form has been recognised. Subatomic energy 
has been converted into heat but the converse process is unknown; it is 
conceivable (but unlikely) that the conversion of heat into subatomic 
energy involves a greater drain of organisation than its conversion into 
work. 
26 . A process involving creation of entropy cannot be reversed, since 
the reversal would involve destruction of entropy, which is impossible.