THE ORGANISM AS A MECHANISM 65 



energy-conservation does apply to all the processes 

 observed in animals and plants. Let us consider the 

 " calorimetric experiments." An animal, together with 

 the food and oxygen supplied to it, and the various 

 substances excreted by it, constitutes a physical system. 

 This system can be approximately isolated so that no 

 heat enters it from outside, while the heat that leaves 

 it can be determined quantitatively. The animal is 

 made to perform mechanical work, and this is measured. 

 The energy- value of the food ingested by it, and that 

 of the excreta, can be estimated. All the physical 

 conditions can thus be controlled, and the results of 

 such experiments show that energy is conserved. The 

 energy contained in the food is greatly in excess of 

 the energy contained in the excreta, but the deficit is 

 quantitatively represented by the work done by the 

 animal, and by the heat lost in conduction and radiation 

 from its body. The difference between the observed 

 results and the theoretical ones are within the limits 

 of error of the experiment. The metabolism of the 

 animal as a whole, then, conforms to the law of con- 

 servation, and the general results of physiology all go 

 to show that this is also true of chemico-physical 

 changes considered in detail. 



It cannot be shown that the second law, that of the 

 dissipation of energy, applies to the organism with all 

 the strictness in which it applies to purely physical 

 systems. If we consider only the warm-blooded 

 animal we do indeed find that its general metabolism 

 does proceed in one direction, and that irreversible 

 changes occur. In the mammal and bird we have 

 organisms which present a superficial resemblance to 

 the heat-engine, with respect to their chemico-physical 

 processes, a resemblance, however, which is rather an 

 analogy than an identity of processes. In the heat- 



