128 PHYSIOLOGY 



and the depression of freezing-point is proportional to the number of molecules present. 

 Thus the freezing-point of a 1 per cent, solution of NaCl is O61 C. The depression 

 of freezing-point is generally represented by the Greek letter A. This method has 

 the advantage that the fluids are in most cases hi no wise altered by the process of freez- 

 ing, and it can be applied to solutions containing coagulable proteins which would be 

 irretrievably altered by any considerable rise of temperature. 



Every substance in solution possesses therefore a certain amount of 

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

 pendent of the nature of the substance dissolved and is determined 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 regarded 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 conditions 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 substance on its complete com- 

 bustion with oxygen to carbon dioxide and water. In the intermediate 

 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 the chemical changes which it is undergoing 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 multiplica- 

 tion 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. 



