212 GENERAL PHYSIOLOGY 



researches upon the relation of heat to mechanical energy, a 

 certain quantity of heat as the unit of heat or calorie. A 

 calorie is that quantity of heat that is necessary to warm one 

 kilogram of water from 0° to 1° C. Heat was chosen with good 

 reason as that form of energy which may serve as a unit ^ of 

 measure for all others, for it holds a peculiar position in relation 

 to all others ; it is the sole form into which all others can be 

 transformed completely. When, therefore, it is desired to express 

 in numbers a quantity of any desired form of energy, e.g., 

 mechanical or chemical energy, the latter is expressed in measures of 

 heat, that is, in the number of equivalent calories. Thus, one 

 calorie computed in the form of mechanical work corresponds to 

 the quantity of energy that is needed to raise a weight of 424 

 kilograms one metre high ; in other words, the mechanical 

 equivalent of one calorie is 424 kilogrammetres and, vice versa, 

 one calorie is the heat-equivalent of 424 kilogrammetres. In 

 the same way the quantity of all other forms of energy can be 

 expressed in heat-equivalents. The calorie is the unit of measure 

 for all energy. 



B. THE INTRODUCTION OF ENERGY INTO THE ORGANISM 



Life has often been compared with fire, an idea which plays a 

 rdle in the oldest mythological folk-views of nature and, as is well 

 known, first assumed a fixed form in the philosophy of Heraclitus. 

 In many points the comparison is fitting. To extend it somewhat 

 further, the organism is the burning coal which is being constantly 

 consumed, the breath is the smoke, and the food is the freshly 

 added fuel which constantly replaces the old. Just as the burn- 

 ing mass of coal represents a physical system in which a continual 

 transformation of energy is taking place, potential energy being 

 introduced with the fuel and transformed into two forms of kinetic 

 energy manifest outside, namely, heat and, by proper arrangement, 

 as in the steam engine, mechanical work, so an organism is a 

 physical system in which a similar transformation of energy con- 

 tinually takes place. Just as by heaping new coal upon the fire, 

 energy is added in the potential form, so also, at least in the animal 

 organism, by far the greater part of all the energy introduced is 

 potential energy. The introduction of energy is considerably less 

 evident to the eye than the proditction of energy; the latter 

 results from the transformation of the introduced potential and 

 is expressed in movements and other visible work. 



1. The Introduction of Chemical Energy 



Since confused ideas concerning the transformation of energy 

 in chemical processes are wide-spread, it will be advantageous first 

 to glance at the general facts. 



