44 /'AV.vr//'A/:\ OI- GENERAL PHYSIOLOGY 



ANIMAL TEMPERATURE AND ITS REGULATION 



From the preceding paragraph it will be obvious that, for rapidity of 

 adaptation to outside changes, it is of advantage to the reacting organism 

 that its processes be carried on at a raised temperature. Suppose, however, 

 that a chemical reaction, such as an oxidation, is set in progress. Heat pro- 

 duced accelerates the reaction, and it will tend to become faster and faster, 

 verging on an explosion. Some means of regulation of such reactions is 

 clearly necessary. One obvious way of doing this, in the case of oxidation, is 

 to limit the supply of oxygen. Organisms provided with circulation of blood 

 conveying oxygen have the power of cutting down the supply to their various 

 parts by methods to be described later. In warm-blooded animals the chief 

 source by which the temperature is kept up is muscular contraction, controlled 

 by the nervous system. 



Apart from its effect on chemical reactions, a high temperature is also of advantage in its 

 action on physical processes, diminishing the internal friction of liquids such as blood, 

 hastening diffusion, and so on. 



The question will be discussed further in Chapter XIV. 



THE EFFECT OF TEMPERATURE ON EQUILIBRIUM 



The confusion that is sometimes made between the effect of heat in increa>inu r 

 the rate of a change, and its effect on the position of equilibrium in a reversible 

 reaction, has been already alluded to. We have seen that the rate of any reaction, 

 exothermic or endothermic, is accelerated by rise of temperature. On the position 

 of equilibrium, its effect may differ in individual cases, as may be seen theoretically 

 from the consideration that, of the two balanced opposing reactions, either one 

 may be accelerated more than the other. If, for example, the synthetic reaction 

 in the case of alcohol, acid, ester, and water were accelerated more than the 

 hydrolytic one, the equilibrium would be moved in such a direction that more ester 

 would be present and less alcohol and acid, and conversely. 



In actual fact, the effect in question differs in direction in the case of exothermic 

 and endothermic reactions. The law expressing this relationship was deduced 

 thermodynamically by van't Hoff (1884, pp. 161-176). For the reasoning adopted, 

 the reader may consult Mellor's "Chemiial Statics and Dynamics" (pp. 395-401). 

 The " Principle of Mobile Equilibrium," introduced by van't Hoff, may be 

 expressed briefly as follows : Any change of the temperature of a system in 

 equilibrium is followed by a reverse thermal change within the system. By 

 taking separately the three possible cases, the meaning will be made more 

 intelligible. 



1. Suppose that a reaction has taken place by which a substance B has been 

 formed from another substance A. If this reaction has been accompanied by the 

 evolution of heat, a rise of temperature will cause an increase in the quantity of A. 

 In other words, the reaction is partially reversed. Since the law holds for physical 

 as well as chemical phenomena, it may easily be remembered by consideration of 

 the condensation of water vapour (A) to liquid (B), which is accompanied by 

 evolution of heat. The law tells us that raising the temperature will increase the 

 quantity of steam (A), as every one knows. 



2. If the reaction is endothermic, accompanied by absorption of heat, rise of 

 temperature will cause decrease in the quantity of A, that is, the reaction will go 

 on further. One may say that, as the reaction requires heat to progress, an extra 

 supply will help it on. An illustration, merely to assist the memory, is the case of 

 ether (A). By evaporation spontaneously to vapour (B) it cools, and, if prevented 

 from absorbing heat from its surroundings, it may become so cold that evaporation 

 practically ceases. If heat be supplied, more vapour (B) will be formed, and the 

 liquid phase (A) will diminish. 



3. The third case is that of a reaction in which no thermal, change occurs. 

 Here a rise of temperature will have no effect on the relative amounts of A and B. 



An instructive case to consider in this connection is that of the taking up of a dye by a 

 substance which is stained by it, say paper, or tissue in the process of histological staining. 



