130 PHYSIOLOGY 



water, 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 0-61 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 in no wise altered by the 

 process of freezing, 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 combustion 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 multiplication of the number of molecules in a solution, for the 

 production of increased osmotic pressure, which in its turn may be con- 

 verted into the energy of movement either of masses or of molecules. 



