32 PRINCIPLES OF GENERAL PHYSIOLOGY 



Further discussion of the application of the doctrine of energy to living 

 organisms will be found in the essay by Zwaardemaker (1906). 



Warburg (1914, pp. 256-259) calls attention to the fact that many cells, such 

 as those of the central nervous system, the fertilised egg-cell, and nucleated 

 red blood corpuscles, use energy in considerable amount, as shown by their 

 consumption of oxygen, although they do no external work. It is evident that 

 energy is required for some cell processes. Warburg suggests that it may be 

 necessary for the maintenance of the " structure " of the cell, in the sense of 

 keeping apart substances, which would mix by diffusion, the preservation of the 

 properties of semi-permeable membranes, and so on, all in microscopic dimensions, 

 or less. 



HEATS OF COMBUSTION 



The complete oxidation of such substances as fats and carbohydrates sets free a 

 large amount of available energy. If this energy is all converted into heat, for the 

 purpose of measurement, it is possible to obtain a number expressing the total 

 energy content of any oxidisable substance. Numbers obtained in this manner 

 are known as "heats of combustion." They play a useful part in comparing the 

 energy changes in various reactions. 



The usual methods of determining heats of combustion will be found in the textbooks 

 of Physical Chemistry (see that by Findlay, 1906, pp. 245-263). The adiabatic calorimeter of 

 Benedict and Higgins (1910) appears to be a convenient and accurate form of apparatus. The 

 name "adiabatic" is used in general for any process in which no heat is allowed to escape or 

 be taken in. A gas, for example, may be compressed under such conditions that the heat 

 produced escapes as fast as it is formed, so that the temperature remains constant ; the process 

 is " isothermal." If the heat produced by compression is prevented from escaping, the process 

 is "adiabatic" and great rise of temperature may result. In the Diesel engine, the heat of 

 compression is great enough to ignite the heavy oil used for combustion, although the process 

 is not absolutely adiabatic, owing to cooling by the walls of the cylinder. 



Heats of combustion, however, do not necessarily give the actual energy 

 values of food-stuffs, as available in the organism. If converted into heat at 

 once, only a comparatively small part can be utilised, even with large rise of 

 temperature. Hence the importance of using the chemical energy of food in 

 the way that will give most free energy. As A. V. Hill remarks (1912, ii. p. 511), 

 " if it is shown that carbohydrate has, calorie for calorie of total energy, a higher 

 proportion of free energy than fat has, this would have an enormous influence on 

 theories of nutrition." This is given, of course, merely as an illustration of the 

 necessity of due consideration of the difference between free and bound eneruv. 

 In fact, Baron and Polsinyi (1913, p. 10), assuming Nernst's theorem (1913, 

 p. 744), find that the free energy of the oxidation of glucose at 37 is 13 per 

 cent, greater than the total energy, calculated from the heat of combustion. Heat 

 must be acquired from surrounding bodies and converted to free energy. 



Boltzmann, in one of his " Populare Schriften " (1905, p. 40), points out how 

 the " struggle for existence " of living beings is not for the fundamental con- 

 stituents of food, which are everywhere present in earth, air and water, nor even 

 for energy, as such, which is contained, in the form of heat, in abundance in all 

 bodies, but for the possession of the free energy obtained, chiefly by means of the 

 green plant, from the transfer of radiant energy from the hot sun to the cold 

 earth. 



Boyle's law tells us that the volume of a gas is inversely proportional to the 

 pressure, if the temperature is constant ; and the law of Gay-Lussac tells us it 

 is proportional to the absolute temperature, if the pressure is constant. In 

 symbols: 



where V is volume, P is pressure, T is absolute temperature, and R is a numerical 



