76 BIOLOGY AND TECHNIQUE 



bromin. Iodine with a much heavier atomic weight than either of the 

 former is distinctly less bactericidal. 



Chloride of Lime. — Of the halogen compounds commonly used 

 in practice, the most important is what is popularly known as 

 chloride of lime or bleaching powder. As to the composition of this 

 substance, there is some difference of opinion. It was formerly be- 

 lieved to be a mixture of calcium hypochlorite, Ca(C102), and of 

 calcium chloride, CaCU. The fact that the substance is not deliques- 

 cent, however, speaks against the presence of calcium chloride as such, 

 and it is probable that it consists of a single compound with the for- 

 mula CaOCU. The action of acids or even of atmospheric CO2 upon 

 this substance results in the liberation of chlorine. For instance, 



CaCClzO) + 2HC1 = CaCl2 + 2HC10. 

 2HC10 -I- 2HC1 = 2H2 + 2CI2. 



Bleaching powder is readily soluble in about twenty parts of water. 

 According to Nissen,^ solutions of 2 in 1,000 of this substance, destroy 

 vegetative forms of bacteria in five to ten minutes. 



Teechloride of iodine (ICI3), another halogen derivative, is an 

 extremely strong disinfectant, being efficient for vegetative forms in 

 solutions of 0.1 per cent in one minute and a 1 per cent solution de- 

 stroying spores within a few minutes.^ 



Surgeons have found that painting with tincture of iodine (10 per 

 cent) is a simple and reliable method of sterilizing the skin. It is now 

 used in many clinics as the sole disinfecting agent in sterilizing the field 

 of operation. 



The oxidizing agents most commonly employed are peroxide of hy- 

 drogen (H2O2) and permanganate of potassium (KMn04). 



Peroxide of hydrogen is formed by the action of dilute sulphuric 

 acid upon peroxide of barium. It readily gives up oxygen and acts 

 upon bacteria probably by virtue of the liberation of nascent oxygen. 

 In the presence of organic matter such as blood, pus, etc., associated 

 with bacteria, H2O2 is quickly reduced and weakened. It is important 

 that the H2O2 come in immediate contact with the bacteria. In prac- 

 tice, therefore, blood and pus should be removed from wounds when 

 applying the H2O2 or a large excess of H2O2 should be used. 



Permanganate op Potassium, acting probably in the same way, is 

 a powerful germicide. It also is readily reduced by many organic sub- 

 stances often associated with bacteria, being rendered weaker thereby. 



1 Nissen, Zeit. f. Hyg., viii, 1890. ■= v. Behring, Zeit. f. Hyg., ix, 1891. 



