Entropy

and the best of scaling-ladders appears to be the ideal gas, as everybody has used it for the purpose. We therefore receive the definition of the "ideal g a s " : a substance whereof (a) the temperature remains unchanged when the gas is given and seizes the chance of expanding without any hindrance into a vacuous space; and (b) the pressure varies inversely as the volume, so long as the temperature remains unchanged. Now rather than being an aid to enter the citadel, this definition appears to presume t h a t we are in the citadel already, since the word of " t e m p e r a t u r e " is all too prominent in it. B u t the word comes up only in the phrase " . . . temperature remains unchanged . . ."; so nothing more is implied, t h a n t h a t the onlooker knows how to recognize whether temperature is changing or staying the same. This ability can be his, whether or not he knows about the scale called absolute; and so the definition implies nothing about the scale. The definition would be an idle collection of words, were there not actual gases conforming to it so nearly, t h a t at least for a time they may safely be taken to conform to it exactly. All gases in fact approach conformity, as the density lessens; and with helium and hydrogen especially, the approach is already close while the density is still so high t h a t there is no trouble at all in using them as thermometers. Now, to use an ideal gas as a thermometer means simply this: P being the pressure of the gas and V its volume and 11 the quantity of the gas measured in moles, there is the equation, PV = nRT (1)