322 THE ANIMAL AS A PRIME MOVER. 



heat; that the efficiency of energy conversion is thus 10 per cent. On 

 the other hand, the fact that external "work alone gives transformation 

 of 20 per cent shows that both internal and external work must find 

 ultimate conversion into heat from some other and antecedent form of 

 energy of food conversion and chemical action. 



Rejected heat energy increases rapidly with increase in the amount 

 of work performed by the machine, and this seems to confirm the idea 

 that the expenditure of internal energy in the accelerated operations 

 of circulation, respiration, and nutrition must find ultimate conversion 

 into heat and rejection in that form. The disappearance of thermal 

 energy observed by Hirn is proof of this action. Hirn also found that 

 the total heat exhaled exceeded by one-third that computed, and thus 

 proved that it must be derived, in part at least, from other processes 

 than combustion. The quantity was five calories when resting, about 

 half as much when at hard work j>er gram of oxygen inhaled. Men- 

 tal effort and work have precisely the effect of manual labor in this 

 increase of the heat waste and conversion of energy. The source of 

 energy as an effect of oxidation would seem to be the food taken into 

 the system and the broken-down tissues of muscle, bone, and nerve, 

 while the office of the food supply is to replace this tissue and to 

 furnish the energy of chemical action as well. 



Dalton 1 and others take the heat waste of the human machine as 

 about 200 British thermal units per hour, 1-28 British thermal units per 

 pound nearly, or a total for the day of 4,800 British thermal units, 

 equivalent to 3,734,400 foot-pounds. Assuming the possibility of com- 

 plete suppression of this as waste, or, what is equivalent to the same 

 thing, its application to internal work of equal value with the energy 

 applied to useful external work, the efficiency of the animal machine 

 becomes 



5,734,400 - 10,000,000 = 0-57 + ; 



over 57 per cent, and exceeds that of any known form of thermody- 

 namic machine in actual use nearly three to one. 



Whether the suppression of this waste, with corresponding gain in 

 useful conversion of energy, can be effected remains uncertain and is 

 perhaps unlikely to be practicable, although animals and human races 

 in the Tropics often live for long x>eriods in temperatures at which no 

 conduction or radiation of heat is possible, and must depend entirely 

 upon evaporation of moisture from the exterior of the body for its dis- 

 tribution. Since it certainly, in part at least, represents the retrans- 

 forined energy of internal work, it would seem probable that only by 

 effecting a balance between internal and external work of that class 

 could this waste be completely suppressed. This is probaly impossible 

 with ihe vital engine, but nothing is known that would indicate similar 

 necessary limitations in any artificial machine in which the essential 



1 Human Physiology. Philadelphia, 1875, page 302. 



