144 



PHYSIOLOGY 



Every substance in solution possesses, therefore, a certain amount 

 of potential energy in the form of osmotic pressure. This pressure is 

 independent of the nature of the substance dissolved and is deter- 

 mined merely by its molecular concentration. It can be used as a 

 driving force for the movement by diffusion of 

 the molecules themselves, or by the use of 

 appropriate mechanisms or ' machines ' for the 

 performance of mechanical work, or, as will be 

 seen later, for the production of electrical 

 differences of potential. 



In addition to this osmotic or volume 

 energy every molecule in solution can be re- 

 garded as endowed with a chemical energy, 

 which is dependent not only on the number of 

 molecules present, but also on the nature of the 

 molecules. In the case of electrolytes and of 

 substances which are susceptible of ionisation, 

 the potential or intensity of the chemical energy 

 of each molecule is capable of measurement. 

 On the other hand, the chemical energy of a 

 substance such as glucose cannot be definitely 

 expressed apart from consideration of the con- 

 ditions under which it is present. If we take 

 the whole course of transformations undergone 

 by glucose in the body, we may speak of it as 

 having a potential energy, which is measured by 

 the total heat energy given out by this sub- 

 stance on its complete combustion with oxygen 

 to carbon dioxide and water. In the inter- 

 mediate changes which it undergoes during its 

 metabolism in the cells of the body, this energy 

 is probably set free by degrees, but its chemical 

 energy in any given phase cannot be measured 

 unless the conditions and the end results of 

 denl- the chemical changes which it is undergoing 

 tion of freezing-point. are known. This chemical energy may 

 be utilised for the production of heat, 



for the performance of chemical work in the building up of other 

 substances, or, by the multiplication of the number of molecules in a 

 solution, for the production of increased osmotic pressure, which in 

 its turn may be converted into the energy of movement either of masses 

 or of molecules. 



