DISINFECTION AND DISINFECTANTS 253 



suggested in America for the destruction - of typhoid 

 bacilli and cholera vibrios in drinking-water. 



When the metal is very finely subdivided, as it is in 

 the colloidal state, its disinfectant powers increase. 

 Colloidal silver and colloidal mercury are credited with 

 amazing efficiency. 



Inorganic Salts. Solutions of mercury salts are strong 

 disinfectants, efficiency depending on the proportion of 

 mercury in dissociation. As pointed out by Sommerville 

 and Ainslie Walker, mercuric chloride has been assigned a 

 much greater disinfectant value than is justified, as a 

 result of workers failing to recognise that when the 

 medicated organisms or spores are transferred to the sub- 

 culture tube a small quantity of mercuric chloride is 

 carried over, which exerts an inhibitory action on the 

 organisms, and the subculture tube shows no growth. But 

 if a drop of sulphuretted hydrogen water be added, the 

 mercury is converted into an inert sulphide, and permits 

 the growth of organisms that otherwise would not take 

 place within the time limits of the experiments. Mercuric 

 chloride is poisonous, and with proteins or soaps forms 

 compounds which have no germicidal action. The 

 addition of hydrochloric acid (as in the L.G.B. formula) 

 largely counteracts this depreciation. Cheatle (Medical 

 Annual, 1915) appears to regard this combination with 

 protein as a point in favour of the salt: ' Mercuric chloride 

 is adsorbed by skin, and will remain in a soluble condition 

 for a long time. In wounds it is adsorbed by its own 

 precipitated albuminate, and is given up into solution 

 slowly. Therefore it possesses a marked and most 

 desirable depot action.' The binoxide is strongly dis- 

 infectant in a potassium iodide solution, and is not affected 

 by proteins to the same degree (see also p. 251). Soluble 

 silver salts, chiefly used in ophthalmic work and for 

 cauterising dog-bites, are powerful disinfectants, weaker 

 than perchloride, but far less sensitive to proteins. They 

 are incompatible with chlorides, except in certain organic 

 combinations, from which silver chloride is only partially 

 precipitated. 



Iron and zinc salts have been credited with useful dis- 

 infectant action, but their value is of no practical account. 

 Zinc chloride, copper sulphate, sodium fluoride, and zinc 

 fluoride are used for preserving timber. 



