38 THE FOUNDATIONS OF SCIENCE 



shadowing of the theory of energy known as the 

 correlation of forces. Energy now emerged as an 

 exact conception the power of doing work, to be 

 measured by the amount of work done. Moreover, 

 in Joule's experiments, if heat be taken as a form of 

 energy, the total energy is constant in amount, what 

 was lost in work being gained in heat. In this case, 

 then, we have definite experimental proof of the con- 

 servation of energy. Although no such exact proof 

 can be given in all cases, the cumulative evidence is 

 very strong that in all physical and chemical changes 

 we may imagine a quantity which is unchanged 

 in amount throughout the changes, a quantity which 

 may be transformed by friction into an equivalent 

 amount of heat, and may be identified with energy 

 as defined above. 



While mass is constant at all moderate velocities, 

 and energy is conserved in all conditions known to 

 us, other quantities are conserved only in limited 

 conditions. Thus in pure Newtonian mechanics we 

 find a conservation of momentum ; and, in those 

 special changes which in thermodynamics are called 

 reversible, we recognise the constancy of another 

 quantity which is termed entropy. But neither 

 momentum nor entropy are conserved in physical 

 and chemical changes generally ; and we are hence 

 led to a cautious attitude in similar cases. While 

 energy is constant in all conditions known to science, 

 we must not be too sure that science has studied 



