THE MECHANISM OF LIFE 549 



satisfactory manner by the calorimetric investigations of very 

 recent times, especially by the very exact work of Rubner ('94). 

 If upon the basis of calorimetric combustions the chemical energy 

 of food be expressed in terms of heat, the food will yield just as 

 many calories as an animal affords when all its production of 

 energy is expressed in output of heat. The differences between 

 the quantity of heat that is evolved by the combustion of food to 

 substances lacking chemical energy, and the quantity of heat 

 which the animal produces with like food during rest, are so small 

 in the extremely delicate experiments of Rubner, that they fall 

 wholly within the unavoidable technical limits of error. If it 

 were at all necessary at the present time to prove the validity of 

 the law of the conservation of energy for living nature, the best 

 evidence would be given by Rubner's new calorimetric researches. 

 The passage of energy through the organic world ends with the 

 output of heat or mechanical work by the animal body. The 

 animal body gives off to the outside no chemical energy that is 

 capable of being used further, with the exception of that adhering 

 to the egg-cell in reproduction. The substances that leave the 

 animal body, such as water, carbonic acid, etc., are compounds 

 that possess in their existing form no more chemical potential, and 

 the introduction of light into the green plant-cell is necessary to 

 enable the latter to create available chemical energy out of these 

 substances. Thus, the circle of the changes of energy between 

 living and lifeless nature is a closed one. Light makes avail- 

 able chemical energy in the plant-cell. Out of this chemical 

 energy are derived all the chemical, mechanical, and thermal activi- 

 ties of the plant in a complex series. The herbivore takes into its 

 body with its food the chemical energy that is stored up in the 

 organic compounds of the plant, and with the materials of its own 

 body-substance becomes to the carnivore the indispensable source 

 of chemical energy; from the latter is derived all the thermal, 

 mechanical, and in special cases also the photic and electrical energy 

 which the animal body gives off to the outside as heat, as mechanical 

 energy of muscular movement, and as light and electricity. Out 

 of the substances that leave the animal body, poor in mechanical 

 energy, carbonic acid and water, the plant-cell under the influence 

 of the light-rays creates anew chemical energy, and thus the endless 

 circulation begins again. 



I. The Principle of the Transformation of Chemical Energy in the Cell 



However clear the main outlines of the organic change of energy 

 appear, its details are obscure. This is true partly because of our 

 lack of knowledge of the metabolism of living substance, but largely 

 because of the extremely slight development of the general theory 

 of energy in physics and chemistry. The transformations of energy 



