FOODS AND THEIR PHYSIOLOGIC AL VALUE. 309 



be changed to motion, this in a motor to electricity, the 

 electricity in a lamp to light, in a coil of wire to magnetism, 

 and in the motor of the street-car back again to motion. 

 While such a change from one form to another is possible 

 and readily accomplished, the change so made is always in 

 definite and fixed proportions. An always invariable and 

 fixed amount of heat is changed into a corresponding cer- 

 tain amount of motion, or vice versa. If in an engine run- 

 ning a factory, lighting and heating it as well, every bit of 

 energy could finally again be collected it would be found to 

 be identical in amount with the original energy producing 

 all, even though in the meantime it might have passed 

 through a half dozen other forms. The forms of energy 

 just mentioned have been forms which might be readily rec- 

 ognized "as energy. There is something moving or dyna- 

 mic about the current of electricity, about the light of the 

 lamp, or the heat in the furnace. Such evident energies 

 are spoken of as dynamic or kinetic energies. But energy 

 may appear in a latent form. A barrel of gunpowder or a 

 cartridge of dynamite, although neither electrified, nor 

 heated nor in motion, possesses a large amount of inherent 

 energy. But a small spark is necessary to transform what 

 is latent into energy of the most dynamic kind. Such latent 

 or resting energy is spoken of as potential energy. The 

 energy which comes from the burning of wood or coal is of 

 this potential kind. 



The form of energy most suited to the body is this kind 

 of latent energy. It can be easily demonstrated that the 

 food constituting an ordinary meal, if dried, can easily be 

 made to burn and yield considerable quantities of dynamic 

 energy. Sugar, fats, meats, breads, all these may be made 

 to burn and give up the latent energy stored in them. It is 

 this energy which is the source of supply to the body. As 

 far as our knowledge goes now it has been impossible to get 

 energy apart from matter. In fact, it is impossible to think 

 of energy except in terms of matter, and energy has some- 

 times been defined as matter in motion, in which the motion 



