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' 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 



