36 PHYSICAL SCIENCE 



fully describe what happens by assuming that the 

 quantity which we call the mass of the body keeps 

 constant, and that the sum of the two kinds of 

 energy keeps constant also. If we include in our 

 view the complete physical and chemical aspects 

 of the phenomena, we may greatly extend these 

 results. When the body reaches the earth, it is 

 possible that processes of decay set in, which 

 eventually result in most of its substance dis- 

 appearing in gases or other products. The energy 

 of motion acquired by the body during its fall 

 also seems to disappear, with no corresponding 

 gain of energy of position. Chemistry, however, 

 generalising from many experimental results, tells 

 us that, if we could trace all the forms of matter 

 into which the body is resolved, we should find 

 that there was no loss. Every particle of the 

 original body still exists in one of its products. 

 Physics, on the other hand, teaches us in the 

 same way that the sum of all the forms of energy, 

 heat, sound, etc., which appear as a consequence 

 of the impact on the ground, could they all be 

 taken into account, would be exactly equivalent 

 to the energy of motion possessed by the body at 

 the instant before contact. These great principles 

 of the conservation of mass and the conservation 

 of energy are two of the most important practical 

 generalisations ever reached by Physical Science. 

 While fully recognising the importance of these 

 generalisations from the physical point of view, 

 we must be careful how we give them any meta- 

 physical significance even under the pre-relativity 

 theory of science. Under certain limiting con- 

 ditions, other physical quantities besides mass and 

 energy maybe conserved. Thus in pure mechanics 



