ISOTHERMAL AND ADIABATIC CHANGES. 297 



Joule could not detect any alteration of temperature on the whole. But 

 the heat capacity of the gas was so small compared with that of the 

 calorimeter and its contents that a very small change in the temperature 

 of the gas would not have affected the temperature of the calorimeter 

 sensibly. The result of the experiment, therefore, only showed that 

 Mayer's assumption was not far from the truth. 



Lord Kelvin afterwards proposed a method of experiment giving a 

 much more delicate test for the presence or absence of internal work in 

 gaseous expansion. This method was put into practice by himself and 

 Dr. Joule.* Their researches showed that in Joule's original experiment 

 there must have been a slight cooling effect, i.e. that some work must 

 have been done against internal forces. The method of experiment also 

 gave a means of comparing the absolute scale of temperature with the 

 indications of the air thermometer. 



In order to understand the principle of the method, let us suppose 

 that a long pipe AB (Fig. 168) is obstructed in the middle by a narrow 

 passage 0, and let air be gently driven towards by a piston at A, on 

 which the pressure is P. On the other side of let the air expand 

 against a less presssure p, applied by a piston at B. Let the volume of 

 unit mass be V on the A side and v on the B side. Then the work done 



A C B 



:L_ 



FIG. 168. 



on the gas on the A side is PV per unit mass, and the work done by the 

 gas on the B side is pv for the same mass. Let the walls of the enclosure 

 be either non-conducting or everywhere at the temperature of the gas in 

 contact with them. 



It may appear at first sight that this is an adiabatic transaction, since 

 no heat is communicated from the outside as heat ; but if, as we suppose, 

 the passage at C is very small, there will be a " rapid " there, the air 

 passing through it acquiring a considerable amount of kinetic energy. 

 But this kinetic energy is quickly lost on the B side through the viscosity 

 of the surrounding air, i.e. some of the work done on the gas gives rise 

 to kinetic energy and this kinetic energy is transformed to heat, so that 

 there is, on the whole, a gain of entropy. 



Let us first suppose that the temperature is found to be the same on 

 the two sides of at a sufficient distance away from the " rapid." Let 

 us also suppose that Boyle's law is true.. Then 



or the work done on the gas on the A side is equal to that done by the 

 gas on the B side. On the whole, then, no energy is given to the gas 

 from the outside, and since the temperature is unaltered, we conclude 

 that, while the volume has increased from V to v, the gas has not trans- 

 formed any of its own heat into energy of molecular separation. In other 



* Joule's Scientific Papers, vol. ii. p. 217, or Thomson's (Lord Kelvin's) Papers, 

 vol. i. p. 333. 



