MOLECULAR ENERGY 289 



which is equal to an odd multiple of an erg, namely, 

 4.19X10 7 , or 4.19 joules. The calorie is a relic from 

 the days of the phlogiston theory, which has become 

 firmly fixed in the literature. 



It was adopted at a time when heat appeared or dis- 

 appeared from the view of experimenters in a most 

 mysterious manner, and was a measure of that " im- 

 ponderable fluid." It was and is, however, a very 

 convenient unit in many ways. For example, it was 

 found that if two masses of water, mi and 7^2, at tem- 

 peratures of h and iz respectively, are mixed the mix- 

 ture comes to a temperature, t, given by the relation 1 



m 1 (t l -t)=m 2 (t-t^) (2) 



It is convenient to take unit heat as that lost or gained 

 when 1 gram of water changes 1 C. This is the calorie. 

 In calories, then, the " specific heat" of water is unity. 

 We now see that what Joule (cf. page 112) really meas- 

 ured was the specific heat of water. Up to his time 

 its absolute value was unknown. The calorie is merely 

 an arbitrarily chosen unit for measuring energy. It 

 is therefore usual to write 1 calorie = J ergs, where J 

 is sometimes incorrectly called the " mechanical equiv- 

 alent of heat." 



This " method of mixture" which led to the adoption 

 of the calorie as a unit of "heat" was easily extended. 



1 This equation means that the molecules of the mass mi, which 

 have a kinetic energy of translation corresponding to ti, arrive at 

 an equilibrium with the molecules of mass ra 2 , which are at the 

 lower temperature t?, when both groups of molecules have the same 

 average kinetic energy of translation and hence a common tem- 

 perature t. The energy transferred from the molecules of the first 

 mass is m\(tit) and in a conservative system equals that gained 

 by the other molecules, namely 



