42 PRINCIPLES OF GENERAL PHYSIOLOGY 



taking place against resistance. This being so, a formula similar in form to that 

 of Ohm's law in electricity must hold. Thus : 



chemical force 



Velocity of reaction = : = : . 



chemical resistance 



Cl>emical force is a function of the free energy ; very little is definitely known 

 as to chemical resistance, except that it is greatly diminished by rise of 

 temperature. 



All chemical reactions are, therefore, increased in rate by rise of temperature. 

 Some confusion is apt to arise with respect to endothermic reactions, on account of 

 the effect of temperature on the equilibrium point, to be described presently. 

 Endothennic reactions require to be supplied with energy from their surroundings, 

 since the products have a greater store of potential energy than the bodies from 

 which they are produced ; but it must not be forgotten that they progress of 

 themselves. A chemical reaction takes place, in fact, when the intensity factor of 

 the energy associated with the original mixture is greater than that of the final 

 system (see Mellor's book, 1904, p. 25), whether the reaction be endo- or 

 exothermic. 



From the standpoint of the kinetic theory of heat, it is easy to see why all 

 processes conditioned by rate of molecular movement are accelerated by rise of 

 temperature. But, as Nernst points out (1911, p. 680), it is not so easy to see 

 why the acceleration of chemical reactions is as great as it is. A rise of 10 C. 

 usually doubles or trebles this rate (Law of van't Hoff), whereas " the velocity of 

 molecular movement in gases, and in all probability in liquids also, is proportional 

 to the square root of _the absolute temperature." So that, if it has a value of 

 100 at 20, it will only increase to 101 '7 at 30, instead of to 200; Goldschmidt 

 (1909, p. 206), however, has shown that only those molecules react whose velocity 

 exceeds a certain high value, so that the difficulty disappears. 



Conclusions are sometimes drawn as to the nature of a particular process from 

 the value of the temperature coefficient. This quantity varies so much, not only 

 according to the position on the scale of temperature at which the reaction happens 

 to take place, but also in individual cases, that, on this ground alone, caution 

 must be exercised. 



For example, the saponification of ethyl butyrate by barium hydroxide between 50 and 

 60 has the low value for a chemical reaction of 1'33 for 10 (Trautz and Volkmann, 1908, 

 p. 79), whereas diffusion, a physical process, has a value nearly as high, viz., 1"28 (Nernst, 

 1888, p. 624). Chick and Martin (1910, p. 415) find that the heat coagulation of haemoglobin 

 has the extraordinarily high temperature coefficient of 13'8 for 10, while that of albumin is 

 even higher. It is of interest that P. von Schroeder (1903, p. 88) finds that gelatine solution, 

 in a particular condition, has a viscosity at 21 represented by 13'76, whereas at 31 it is only 

 1 "42 ; that is about ten times less for 10 rise of temperature. As will be seen later, colloids 

 of the type of gelatine play a large part in vital processes. The temperature coefficient of the 

 rate of absorption of water by the seeds of barley has recently been shown by Adrian Brown 

 and Worley (1912, pp. 546-553) to be of the order of that usually regarded as characteristic of 

 chemical reactions. They also find that the rate is an exponential function of the temperature. 

 This is, as Mellor points out (1904, p. 394), very rare for a physical process. The increase of 

 the vapour pressure of a liquid is one of these rare cases, and, in fact, the value of the 

 exponent in Brown and Worley's experiments is the same as that of the vapour pressure of 

 water. The bearing of this fact on the effect of temperature on chemical reaction in general 

 will be found in Chapter VIII. 



The impossibility of forming conclusions as to the physical or chemical nature 

 of a process from the temperature coefficient of its velocity is well shown by the 

 work of Knowlton and Starling (1912, p. 206), on the effect of temperature on the 

 rate of the heart-beat in the isolated heart-lung preparation. This rate is a linear 

 function of the temperature, as shown by Fig. 28. In other words, a given rise of 

 temperature produces the same increase at different points of the scale. But such 

 a relationship is what we find in the simplest physical process, such as the 

 expansion of a gas. Therefore, if the temperature coefficient is any index, the 

 heart-beat is a purely physical process. This is obviously an absurd conclusion. 

 We know that rise of temperature accelerates the chemical changes in the heart 

 muscle, as evidenced by the increase in the oxygen consumption (Lovatt Evans, 

 1912, p. 231), and, in fact, it is very interesting to find that this increased 



