216 THE HUMAN MECHANISM 



The same weight of a given fuel when burned will always 

 yield exactly the same amount of heat, as is proved by 

 burning the fuel under conditions which enable us to meas- 

 ure the heat given off. The simplest means of doing this is 

 perhaps with the ice calorimeter a metal box within which 

 the fuel is burned, the box being everywhere surrounded 

 by a thick layer of ice. The heat produced in burning the 

 fuel is measured by the amount of ice melted. 



In this way we may find the relative amounts of work 

 which can be done with two different fuels, for it has been 

 discovered by actual experiment that if one kind of. fuel 

 produces twice as much heat as another, it will also do 

 twice as much work. 



Now food is the fuel for the muscular work of the body 

 and also for the liberation of heat. Consequently, if we 

 determine how much heat is liberated when a certain amount 

 of protein, or fat, or carbohydrate is burned in a calorimeter, 

 we know how much work it may do in the body ; or at least 

 we know that it can do no more than the amount indicated 

 by the calorimetric experiment. 



3. Units of heat and work. In order to measure we must 

 have units of measurement. Common units of length are the 

 inch or centimeter ; units of area are the square yard, the 

 square meter, or the acre ; units of volume, the quart or 

 peck ; units of weight, the pound or kilogram. We express 

 the results of these measurements by saying that a thing is 

 so many inches long or of so many pounds weight. What are 

 the units of heat and work ? 



Like all units, these are arbitrarily chosen. The unit of 

 heat, known as the calorie, is the amount of heat necessary 

 to raise one kilogram of water one degree Centigrade. The 

 unit of work is the amount of work done in lifting a kilogram 

 (2.2 Ib.) to the height of one meter (39.37 in.) from the 

 surface of the earth against the attraction of gravitation. 

 This is known as the kilogrammeter. Thus, when a man 



