4 IO GENERAL NUTRITION 



would exactly equal the falling force. A heat-unit is therefore said to 

 be equal to a definite number of foot-pounds or kilogrammeters. Calcu- 

 lations have been made showing the conversion of foot-pounds or kilo- 

 grammeters into heat-units, but mechanical difficulties have thus far 

 prevented the actual conversion of heat-units into their equivalents in 

 foot-pounds or kilogrammeters. As a matter of reasoning, however, it 

 is assumed that if a certain number of foot-pounds or kilogrammeters 

 is equal to a certain number of heat-units, the reverse of the equation is 

 true ; but the application of this law to animal physiology is always 

 by a conversion of heat-units into foot-pounds or kilogrammeters. The 

 experiments on which the law rests have been made by converting foot- 

 pounds or kilogrammeters into heat-units. 



In work by machinery, a large proportion of the force-value of fuel 

 is dissipated in the form of heat. This is well illustrated by Landois : 

 If a steam engine burning a certain quantity of coal, but doing no work, 

 is placed in a calorimeter, the heat produced can be measured. If, now, 

 the engine is made to do a certain work, as in raising a weight, the heat, 

 as measured by the calorimeter, will be less, and the work done is found 

 to be very nearly proportional to the decrease in the measured heat 

 (Hirn). It is estimated by Landois, that of the heat produced by the 

 body, one-fifth may be used as work. In the best steam engine, it is 

 possible to use only one-eighth as work, seven-eighths being dissipated 

 as heat. 



Many elaborate and careful estimates have been made of the 

 mechanical work produced by the human body. The basis of such 

 calculations is more or less indefinite, and the reduction of the work 

 to foot-pounds or kilogrammeters is difficult and inexact. Even the 

 general statement, that of the heat-units produced by the body, four- 

 fifths remain as heat and one-fifth is converted into work, must be 

 regarded as approximate. 



In the application of the law of correlation and conservation of forces 

 to animal mechanics, the matters consumed in the production of heat 

 and force are food-stuffs. These are oxidized, either directly or indi- 

 rectly. If oxidized indirectly, it is the substance of the tissues that is 

 thus consumed, the loss being repaired by food. A certain proportion 

 of the heat produced is used, in maintaining the heat of the body ; an- 

 other part is dissipated by radiation from the general surface ; another 

 part is converted into force and is used by the heart and the respiratory 

 muscles ; and still another part is used in the general work of the mus- 

 cular system. All this involves a considerable oxidation of matter. 



In this connection it is interesting to note again the heat-value of 

 different alimentary matters : 



