550 GENERAL PHYSIOLOGY 



involved in events the material basis of which is known in fullest 

 detail, are still wholly unknown. Thus, concerning the work 

 performed in many chemical transformations, we do not know 

 at all whether the mechanical energy thus set free is derived 

 directly from the transformation of chemical energy, or after passing 

 through other forms, such as heat, or electricity. The direct 

 transformation of chemical energy into heat and electricity has 

 been investigated exactly and in detail, but that of chemical into 

 mechanical energy has thus far scarcely been studied. This cir- 

 cumstance, indeed, has frequently led to the mistaken belief that 

 chemical energy can pass over into mechanical energy never directly, 

 but only through the mediation of heat, an idea that is wholly 

 without foundation. To make the subject still more difficult to 

 understand there is the added fact that the conceptions of the in- 

 dividual forms of energy are not at all fixed, that, e.g., the expres- 

 sions, molecular energy, mechanical energy, etc., are employed in 

 very different senses, which results from the fact that the relations 

 obtaining between the various forms are thus far not at all cleared 

 up. Notwithstanding, it must be assumed that such relations, and 

 even very close genetic ones, exist. It is, accordingly, evident that 

 the more special energetics of living substance is at present one of 

 the most obscure fields of physiology, only isolated and disconnected 

 facts being known. 



The general fact must be regarded as established, that all the 

 work of the organism is based finally upon chemical energy. So 

 far as is known at present, most of it is directly dependent upon 

 this source at the moment of its occurrence. But the energy of 

 many actions comes in a roundabout way. Pfeffer ('93) has 

 recently made this fact especially clear for plants. Thus, it 

 happens very frequently that in metabolism chemical energy first 

 passes over into potential mechanical energy and is stored up as 

 tension, to be transferred at the proper opportunity into the kinetic 

 energy of mechanical work. Jumping-fruits and seeds of certain 

 plants furnish examples of this. Here the chemical energy of 

 growth first accumulates in the form of mechanical tension, and, 

 when the fruit is touched, this passes over into vital motion, the 

 fruit bursts open and scatters its seeds with great force. Analogous 

 cases of the indirect derivation of work from chemical energy occur 

 frequently among both plants and animals. 



The chief forms in which the energy evolved by the cell is ex- 

 pressed, are mechanical energy and heat. The evolution of light 

 and electricity is much more limited. 



In order to obtain an insight into the energetics of the cell, the 

 chief principle that controls the transformation of energy in chemical 

 changes must be recalled. This was formulated as follows : if 

 in a chemical process affinities become united rather than 

 separated, energy is liberated ; if affinities become separated 



