Forms of Energy 159 



This is a complex, but a fascinating, field of in- 

 quiry; one that is now receiving much attention in our 

 researches after a more intimate understanding of the 

 facts and principles of nutrition. It will be profitable, 

 therefore, for us to gain some conception of the knowl- 

 edge of this kind, which so far seems to have a practical 

 bearing upon our subject. 



It is natural to first ask, What is energy? This is 

 a difficult question to answer in a popular way, and 

 the physicists 7 definition would hardly serve our pur- 

 pose. All we can do, perhaps, is to illustrate it by 

 pointing out some of its manifestations. Let us re- 

 sort to an old illustration. Every farmer's boy has 

 doubtless seen a blacksmith hammer an iron rod un- 

 til it was red hot. The motion of the hammer-head 

 descending with great velocity was suddenly arrested 

 when it came in contact with the rod. This descent 

 of the hammer-head illustrated one form of active 

 energy; viz., motion of a mass of matter. When 

 the hammer met the iron rod on the anvil, the 

 mass motion ceased. Was the energy therefore lost? 

 Not unless our fundamental conception is wrong, 

 and we find that in this case it is not. The physicist 

 teaches us that the motion of the hammer-head, a 

 mass of matter, was communicated to the smallest par- 

 ticles or molecules of the iron rod, and as the vibra- 

 tions of the molecule increased in rapidity, the rod 

 grew hotter and hotter. Here we have another illustra- 

 tion of energy; viz., the motion of the molecule or heat. 



The iron rod might have been heated in another 

 way, by plunging it into burning charcoal. And from 



