MOLECULAR MOTIONS AND TEMPERATURE 167 



It is most convenient to use a thermometer as a 

 direct reading instrument instead of by the sub- 

 stitution method. It must therefore be calibrated by 

 adding energy, just as the spring scale described on 

 page 31 was calibrated by added weights. But what 

 is the condition which we are to call zero? Obviously 

 the condition when the average kinetic energy of the 

 molecules of its gas is zero. This condition means 

 zero average velocity of the molecules and from equa- 

 tion (1) we see that it corresponds to the condition 

 when the pressure exerted is zero. 



In the development of science a knowledge of ther- 

 mometry preceded by more than two centuries knowl- 

 edge of any such behavior on the part of the gas 

 molecules, as that which we have been describing. Fur- 

 thermore, even to-day, it is impossible to subtract 

 all the energy from a gas and thus reduce it to an 

 absolute zero temperature. In the earlier years there 

 were then two possibilities, either (1) the experimenter 

 might assume that the lowest temperature he was able 



the partition of the energy added to a molecule between the degrees 

 of freedom of translation and of internal motions the more complex 

 molecule would require the addition of a greater amount of energy 

 to produce an equal increase in its k.e. of translation. In other 

 words, the heat capacity per molecule is higher for B than for A. 



Suppose that B has a heat capacity very much greater than the 

 thermometer. If it is at a higher temperature, it will need to 

 lose but little of its energy in order to bring the average k.e. of the 

 molecules of the thermometer to an equilibrium value. On the 

 other hand, if A has a small heat capacity the amount of energy 

 which it must transfer, to raise the temperature of the thermometer, 

 may well result in a decided decrease in its own temperature. When 

 an equilibrium has been reached between A and the thermometer 

 the indication of the latter represents a temperature below that 

 which A originally had. 



