Editing First law of thermodynamics (section)

===Conceptual revision: the "mechanical approach"===
In 1907, [[George H. Bryan]] wrote about systems between which there is no transfer of matter (closed systems): "'''Definition.''' When energy flows from one system or part of a system to another otherwise than by the performance of mechanical work, the energy so transferred is called ''heat''."<ref>[[George H. Bryan|Bryan, G. H.]] (1907), p. 47. Also Bryan had written about this in the {{lang|de|Enzyklopädie der Mathematischen Wissenschaften}}, volume 3, p. 81. Also in 1906 [[Jean Baptiste Perrin]] wrote about it in {{lang|fr|Bull. de la société français de philosophie}}, volume 6, p. 81.</ref> This definition may be regarded as expressing a conceptual revision, as follows. This reinterpretation was systematically expounded in 1909 by [[Constantin Carathéodory]], whose attention had been drawn to it by [[Max Born]]. Largely through Born's<ref>[[Max Born|Born, M.]] (1921). "Kritische Betrachtungen zur traditionellen Darstellungen der Thermodynamik", ''Physik Z.'' '''22''' '''i''', Supp pp. 218—224.</ref><ref name="Born 1949 {{math|V}}"/> influence, this revised conceptual approach to the definition of heat came to be preferred by many twentieth-century writers. It might be called the "mechanical approach".<ref>Bailyn, M. (1994), pp. 65, 79.</ref>

Energy can also be transferred from one thermodynamic system to another in association with transfer of matter. Born points out that in general such energy transfer is not resolvable uniquely into work and heat moieties. In general, when there is transfer of energy associated with matter transfer, work and heat transfers can be distinguished only when they pass through walls physically separate from those for matter transfer.

The "mechanical" approach postulates the law of conservation of energy. It also postulates that energy can be transferred from one thermodynamic system to another [[Adiabatic process|adiabatically]] as work, and that energy can be held as the internal energy of a thermodynamic system. It also postulates that energy can be transferred from one thermodynamic system to another by a path that is non-adiabatic, and is unaccompanied by matter transfer. Initially, it "cleverly" (according to Martin Bailyn) refrains from labelling as 'heat' such non-adiabatic, unaccompanied transfer of energy. It rests on the primitive notion of [[Adiabatic enclosure|''walls'']], especially adiabatic walls and non-adiabatic walls, defined as follows. Temporarily, only for purpose of this definition, one can prohibit transfer of energy as work across a wall of interest. Then walls of interest fall into two classes, (a) those such that arbitrary systems separated by them remain independently in their own previously established respective states of internal thermodynamic equilibrium; they are defined as adiabatic; and (b) those without such independence; they are defined as non-adiabatic.<ref>Bailyn, (1994), p. 82.</ref>

This approach derives the notions of transfer of energy as heat, and of temperature, as theoretical developments, not taking them as primitives. It regards calorimetry as a derived theory. It has an early origin in the nineteenth century, for example in the work of [[Hermann von Helmholtz]],<ref>Helmholtz, H. (1847).</ref> but also in the work of many others.<ref name="Bailyn 79"/>