THE DYNAMICS OF DISINFECTION 



137 



450^ 



With regard to animal oils, Harris, Bunker, and Milas (1932) find that some, 

 such as seal oil and tuna oil, give off vapours which are germicidal, while others, 

 such as cod-liver oil and sardine oil, become germicidal only after exposure to 

 sunlight or ultra-violet light. It is possible that HgOj is given off by the animal 

 oils, and that its rate of evolution is accelerated by irradiation. 



Sulphonamides and Mould Products. — These two groups of substances, whose 

 chief interest lies in their ability to control infection in the body, will be more 

 conveniently discussed in Chapter 6. 



The Dynamics of Disinfection 



Reaction Velocity. — The figures obtained by Kronig and Paul (1897) in their 

 work on the disinfection of anthrax spores by HgClj were submitted by Madsen 

 and Nyman (1907) to a mathematical analysis, with the result that the reaction 

 velocity of disinfection was found to be similar to that obtaining in a unimolecular 

 reaction. Madsen and Nyman themselves made fresh experiments, using the 

 garnet method, and were able to confirm the findings of Kjonig and Paul. In 

 the following year Chick (1908), working independently, reached the same 

 conclusions with regard to the analogy 

 between disinfection and a unimolecular 

 reaction (Fig. 21). 



In a unimolecular reaction only one 

 of the reacting substances need be re- 

 garded as undergoing change, the rate of 

 change being proportional to the con- 

 centration of this substance. Examples 

 in chemistry are the inversion of cane 

 sugar by acids, the decomposition of AsHg 

 into As and Hj, and the disintegration 

 of radio-active substances. When only 

 one of the reacting substances is under- 

 going change, the velocity of this change 

 according to the Law of Mass Action 

 will depend upon the concentration of 

 this substance at any given moment, 

 the temperature and other conditions 

 remaining constant. This statement may 

 be expressed by the relation 



V=C.A; 



Fig. 21. 



Disinfection of anthrax spores with 5 per 

 cent, phenol at 33-3° C. The curve is 

 drawn through a series of calculated 

 points ; the circles represent the experi- 

 mental observations. 



(After Chick.) 



in which V represents the velocity of 



the reaction, C the concentration of the 



substance, and k a constant depending on the nature of the substance. The 



dx 

 velocity may be expressed by — in which x represents the amount of sub- 



dt, 



stance changed in time t ; if the original amount of substance is designated by a, 



then a—x will represent the amount remaining after time t. The equation may 



now be written : 



— = k [a —X) 

 dt 



