46 PRINCIPLES OF GENERAL PHYSIOLOGY 



The second law deals with the conditions under which these changes take 

 place and the proportion of one kind that can be transformed into another. 



Although the total energy cannot be altered, the amount of it available for 

 conversion into other forms and capable of doing work, i.e., the free energy, is 

 not constant, and indeed, in the present state of the universe, so far as we are 

 able to investigate it, free energy always tends to diminish. This fact, a matter 

 of invariable experience, is known as the " Principle of Carnot and Clausius," and 

 is of great importance in the interpretation of many physiological problems. 



There are two factors which, multiplied together, give energy. One of these, 

 of the nature of a "strength," is called the "intensity" factor; the other, of the 

 nature of a space or mass, is called the "capacity" factor. As regards the latter 

 factor, energy can be added algebraically, but not as regards the former. 



In the animal body, energy is derived from chemical combination. This form 

 of energy is readily converted into various other forms, without the necessity of 

 passing through the form of heat. 



In the vegetable organism, energy is derived ultimately from the sun's rays. 

 It follows, therefore, that animal energy has the same origin. 



The maximal work of a chemical process can be calculated by means of a 

 formula due to Nernst ; it depends on the position of equilibrium in the 

 reaction considered as reversible, and is greater the nearer this position is to 

 that of complete change in one direction. 



That manifestation of molecular forces known as surface energy plays an 

 important part in cell phenomena, owing to the large variations of which it is 

 capable in a small space. This is due to the changes in its capacity factor, 

 surface area, chiefly by aggregation of colloidal particles. 



The phenomena peculiarly characteristic of vital changes are those associated 

 with the actual process of transfer or transformation of energy. Many non- 

 vital phenomena show also a special degree of activity in such states. 



The total energy obtained from a food-stuff by complete oxidation, the 

 " heat of combustion," does not of necessity imply that stuffs of the same 

 heat of combustion are of equal value as sources of available energy. The 

 distinction between free and bound energy must be taken into consideration. 

 The " struggle for existence " is for the possession of free energy. 



The formula for the work done in compressing a gas from a volume v 2 to Vj, 

 or from pressure p^ to p y viz. 



RT log, -~ or RT log,^- 1 , 

 v \ Pz 



is also applicable to that done in concentrating a solution from one osmotic 

 pressure to another, to the potential of metallic electrodes, and to the case of 

 certain solutes confined by a membrane permeable to one ion only, to mention 

 cases of physiological interest. 



The properties of the carbon atom make it of especial value in the trans- 

 formation of chemical energy, so that the body of doctrine known as organic 

 chemistry is of fundamental importance in physiology. 



The effect of a rise of temperature on the rate of chemical reaction must be 

 carefully distinguished from that on the position of equilibrium. The former 

 is always increased, while the latter is controlled by van't HofFs "Principle of 

 Mobile Equilibrium." Whether it is changed in the direction of further progress 

 of a reaction, or the reverse, depends on whether the reaction is accompanied by 

 evolution of heat or the contrary. In the former case, a rise of temperature 

 throws the reaction back, while the opposite is the case in the latter. If the 

 reaction is thermo-neutral, no change is produced by alteration of temperature. 



