142 AGEICULTUEAL AND INDUSTRIAL BACTERIOLOGY 



Silver salts, particularly silver nitrate, possess high 

 germicidal power but are relatively unstable, especially in 

 the presence of light and organic matter. Colloidal solu- 

 tions of metallic silver are extensively used in medicine, the 

 high cost of silver preventing their extensive use for other 

 purposes. 



The soluble salts of mercury are high in disinfecting 

 power. This is particularly true of mercuric chloride and 

 mercuric iodide. Mercuric chloride in solutions of one to 

 one thousand destroys microorganisms rapidly. The soluble 

 salts of copper, particularly copper sulphate, are also high 

 in disinfecting value. It has found extensive use in the 

 purification of water for domestic use. Copper combines 

 very largely with the organic material present and settles 

 out in insoluble form so that very little copper appears in 

 the water when drawn from the tap. Within recent years, 

 however, its use in water supplies has been very largely 

 [ displaced by chlorine. Copper apparently has a particularly 

 ihigh power of destroying algae, that is, it is an algicide. 

 Solutions of copper sulphate as dilute as one part of copper 

 to a million parts of water are effective for such purposes. 

 Copper is the most important constituent of most fungi- 

 cides, as Bordeaux mixture. 



The Non-electrolytes. Of the alcohols, ethyl alcohol 

 (C 2 H 5 OH) is most commonly used. Its power of disinfec- 

 tion, however, is usually greatly overestimated. It is most 

 effective in a concentration of 70 per cent. Either higher or 

 lower concentrations will fall off rapidly in disinfecting 

 power. 



Formaldehyde (HCHO) is the most commonly used 

 aldehyde and is most efficient as a disinfectant. It is 

 usually sold on the market as formalin, the commercial or 

 trade name for a 40 per cent solution of formaldehyde in 

 water. Formalin of standard strength should not be below 

 37 per cent formaldehyde gas. Formaldehyde may be used 



