212 9 Basic thermodynamics Jiero-dnaniey to deseribe the new science of heat to which he was to make such major contributions whait 9.3 The first law of thermodynamics Thin ly ern in this section is to describe the content of the first law of thermodynami as possible. My approach has been strongly infuenced by Brian Pippard to t 2leemtx o Cluwslcal Thermmodynemies, which can be warmly recommended forthe of its exposition. 9.3.7 The zeroth law and the definition of empirical temperature To bosin with, we need to establish what is meant by the term empiric/ temperatare ncounter one of the distinctive features of thermodynamic arguments In man we derive the appropriate mathematical structures IFs Let us consider first the thermal properties of fluids, meaning liquids or gases, beens e pressure is isotropic at all points in the fluid and al hape of the containing vessel without changing its volume, so that no work is done. At thin stage, we have no definition of temperature, we now state our first axiom: 是灬兴 It is assumed that the fluid is not subjected to other influences, such as an eleetrie or magnetie field. If the system is wholly defined by twe bulk properties. it is known as a wo-doordinate system. Most of the systems we will deal with are two-coordinnte systems, Notice carefully what this assertion means. Suppose a certain vou r f nuid systeme Then, suppos we ta ke another identica t u n ty p ha win pressure io and volume F wwo neuriuionse to that it alss ends up with coordinates p, and i'. we have asserted that these The constet or systen have been allowed to internet thermalt until atter a very long time no further changes in the bulk properti fluids cannot have arbitrary values for p. F and pa. va and bu n factof experience that two thes t