THE DESTRUCTION OF BACTERIA 89 



such solutions, inasmuch as it diminishes the concentration of free 

 ions. In practice, however, NaCl or NH 4 C1 is added to bichloride 

 of mercury solutions, since these substances aid in holding in solu- 

 tion mercury compounds formed in the presence of alkaline al- 

 buminous material, blood serum, pus, etc. 



The principles underlying disinfection have been still further 

 elucidated by Chick, 36 who showed that the rate at which bacteria 

 were killed followed the definite mathematical expression for velocity 

 of simple chemical reactions of the monomolecular type. It was 

 found necessary to express the concentration of ionized antiseptics 

 in terms of concentration of the active ion (Hg) instead of total 

 molecular concentration, thus adducing further verification of the 

 work of Scheuerlen and Spiro, and Kronig and Paul. There was 

 also found to be an unusually high temperature coefficient for dis- 

 infectant action, in the case of phenol the reaction velocity increas- 

 ing eight times for a rise of 10 C., while with metallic salts, the 

 increase was about three-fold. The advantage to be gained by the 

 use of warm solutions is, therefore, evident. 



Halogens.* In regard to the halogens, Kronig and Paul have 

 shown that the germicidal power of this class of elements is inversely 

 proportionate to their atomic weights. Thus, chlorine with the 

 lowest atomic weight is the strongest disinfectant of the group. 

 Next, and almost equal to this, is bromine. Iodine with a much 

 heavier atomic weight than either of the former is distinctly less 

 bactericidal. 



CHLORIDE OF LIME. Of the halogen compounds used in practice, 

 the most important is chloride of lime or bleaching powder. As to 

 the composition of this substance, there is some difference of opinion. 

 It was formerly believed to be a mixture of calcium hypochlorite, 

 Ca(C10 2 ), and of calcium chloride, CaCl 2 . The fact that the sub- 

 stance is not deliquescent, however, speaks against the presence of 

 calcium chloride as such, and it is probable that it consists of a 

 single compound with the formula CaOCl 2 . The action of acids or 

 even of atmospheric C0 2 upon this substance results In the liberation 

 of chlorine. For instance : 



Ca(Cl 2 0) + 2HC1 = CaCl 2 + 2HC10. 

 _ 2HC10 + 2HC1 = 2H,0 + 2C1 2 . 



"Chick, Jour, of Hyg., 8, 1908, 92, and 10, 1910, 238. 



* For consideration of the uses of chlorin and bleaching powder for the treat- 

 ment of drinking water, see Chapter on "Water." 



