S. C. BROOKS 171 



But diffusion is a process which is accelerated to a characteristic 

 degree by an increase of temperature, and its presence as a limiting 

 factor can be detected by conducting the reaction at different temper- 

 atures and noting how much its velocity is affected. The temperature 

 coefficient, Qio, for diffusion is 1.22 to 1.28, for *'dark" chemical reac- 

 tions usually 2.0 to 3.0 or above, and for light reactions 1.0 to 1.1, 

 Some light reactions, notably those in which light does no work but 

 acts merely as a catalyst, are exceptional in that they have higher 

 coefficients. 



Experiments were carried out early in the course of these studies 

 to determine the temperature coefficient of the process of photo- 

 inactivation. A sample of complement was diluted to 2 per cent 

 and divided into portions of 20 cc. each. These were exposed for 

 different lengths of time at each of several different temperatures 

 (0, 10, 20, 30, or 40°C.). The exposure was made in the manner 

 previously described^ except that the complement was maintained at 

 the temperatures above 0° by partial immersion of the container in 

 a water bath which was kept within 1° of the desired temperature. 

 The calculation of reaction velocities is based on the assumption 

 that the reaction is monomolecular. It will be apparent from data 

 given further on in this paper that this is the most nearly accurate 

 simple assumption which is possible (Table I) .^ 



^ Brooks, S. C, /. Med. Research, 1918, xxxviii, 345. 



^ The samples exposed at 30 and 40° are injured by heat as well as by light. 

 The efficiencies given in Table I have been corrected as follows: the average rate 

 of heat inactivation was calculated from the injury suffered by duplicate samples 

 kept in the same water bath with each radiated sample for the corresponding 

 length of time. Both photoinactivation (Section V of this paper), and thermo- 

 inactivation (Madsen, T., and Watabiki, T., Overs, kong. danske Videnskah. Sel- 

 skabs Forhandl., 1915, 125) follow the course of a monomolecular reaction and 

 their observed velocities can be expressed as the velocity constant, k, of the mono- 

 molecular reaction isotherm, ^ = - log . Bv subtracting from ko, the observed 



t a—x 



velocity when light and heat are both acting, the value kn, observed when only 

 heat is acting, we arrive at a value, ki, which we may consider to be the com- 

 ponent of the velocity due to the light alone, kg was found to be 0.010 at 40 

 and 0.005 at 30°C. These values were subtracted from the ko for each expo- 

 sure, and from these values of k^ there were obtained (by the use of the reaction 

 isotherm) corrected values of the efficiencies to be expected if light alone were to 



