In a system that is not at equilibrium, various changes are occurring. For example, there may be a flow of energy from one part of the system to another and, at the same time, a flow of mass (diffusion). Flows of this type are coupled, i.e. each depends on the other. Equations exist of the type J1=L11X1+L12X2J2=L21X1+L22X2 Here, J1 is the flow of energy and J2 the flow of matter. X1 is the ‘force’ producing energy flow and X2 that producing matter flow. L11 is the coefficient for thermal conductance and L22 the coefficient for diffusion. The coefficients L12 and L21 represent coupling of the flows with each other. Equations of this type can be generalized to any number of flows and are known as the phenomenological relations. In Onsager's theory the coupling coefficients are equal, i.e. L12=L21, etc. These are known as reciprocal relations. It follows that:(∂J1/∂X2)X1=(∂J2/∂X1)X2. The theory was developed by the Swedish physicist Lars Onsager in 1931.