THE ANIMAL AS A MOTOR. 



43 



oxygen absorbed and exhaled through the skin ; to the 

 conversion of stored energies as yet undetected in the 

 system ; or to the combination of other elements, as 

 carbon and hydrogen, through processes resulting in 

 the production of heat.* It is to the latter cause that 

 Hirn would ascribe this excess of exhaled energy. 

 This subject remains still to be investigated. 



The same individuals being set at hard work in a 

 treadmill constructed for the purpose, in such manner 

 as give the figures for a normal condition of labor, 

 unforced but steady, and at a maximum for a day's 

 work, gave out but about one half as much heat per 

 unit of air breathed and of oxygen consumed, showing 

 clearly the transformation of heat into work. The 

 efficiencies of the human system, considered as a heat- 

 motor or -engine, ranged, according to the condition 

 and temperament of the subject of the test, from 17 

 per cent to 25 per cent when raising the load or them- 

 selves ascending, and rose to 30 and 40 per cent in 

 descent. A lymphatic youth of 18 gave the lowest 

 figures ; a strong man of 47 the highest. These results 

 all exceed those obtainable as yet from the most eco- 

 nomical forms of existing gas- or steam-engine, in 

 which 20 per cent may be taken as about the con- 

 temporary limit of their efficiency as heat-engines. 



To fully secure this efficiency in the human body as a 

 heat-engine subjected to the accepted laws of thermo- 

 dynamics would demand a temperature within its 

 working parts not far from 140 Cent. (284° Fahr.), or 

 far above the boiling-point of water. This fact is 

 crucial as a proof that the transformations of energy 



* See Sarason : Revue Scientifiqite, 1887, page 306 et seq. 



